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dhcpd.conf(5)		      File Formats Manual		 dhcpd.conf(5)

       dhcpd.conf - dhcpd configuration	file

       The  dhcpd.conf	file contains configuration information	for dhcpd, the
       Internet	Systems	Consortium DHCP	Server.

       The dhcpd.conf file is a	free-form ASCII	text file.  It	is  parsed  by
       the  recursive-descent  parser  built into dhcpd.  The file may contain
       extra tabs and newlines for formatting purposes.	 Keywords in the  file
       are  case-insensitive.  Comments	may be placed anywhere within the file
       (except within quotes).	Comments begin with the	# character and	end at
       the end of the line.

       The file	essentially consists of	a list of statements.  Statements fall
       into two	broad categories - parameters and declarations.

       Parameter statements either say how to do something (e.g., how  long  a
       lease  to  offer),  whether to do something (e.g., should dhcpd provide
       addresses to unknown clients), or what parameters  to  provide  to  the
       client (e.g., use gateway

       Declarations  are  used to describe the topology	of the network,	to de-
       scribe clients on the network, to provide addresses  that  can  be  as-
       signed to clients, or to	apply a	group of parameters to a group of dec-
       larations.  In any group	of parameters and declarations,	all parameters
       must be specified before	any declarations which depend on those parame-
       ters may	be specified.

       Declarations about network topology include the shared-network and  the
       subnet  declarations.   If  clients  on a subnet	are to be assigned ad-
       dresses dynamically, a range declaration	must appear within the	subnet
       declaration.   For  clients  with statically assigned addresses,	or for
       installations where only	known clients will be served, each such	client
       must  have  a  host  declaration.  If parameters	are to be applied to a
       group of	declarations which are not related strictly  on	 a  per-subnet
       basis, the group	declaration can	be used.

       For  every  subnet  which will be served, and for every subnet to which
       the dhcp	server is connected, there must	 be  one  subnet  declaration,
       which  tells  dhcpd how to recognize that an address is on that subnet.
       A subnet	declaration is required	for each subnet	even if	 no  addresses
       will be dynamically allocated on	that subnet.

       Some  installations  have  physical  networks on	which more than	one IP
       subnet operates.	 For example, if there is a site-wide requirement that
       8-bit  subnet  masks  be	 used, but a department	with a single physical
       ethernet	network	expands	to the point where it has more than 254	nodes,
       it may be necessary to run two 8-bit subnets on the same	ethernet until
       such time as a new physical network can be added.  In  this  case,  the
       subnet  declarations  for  these	 two  networks	must  be enclosed in a
       shared-network declaration.

       Note that even when the shared-network declaration is absent, an	 empty
       one  is created by the server to	contain	the subnet (and	any scoped pa-
       rameters	included in the	subnet).  For practical	purposes,  this	 means
       that  "stateless"  DHCP	clients,  which	are not	tied to	addresses (and
       therefore subnets) will receive	the  same  configuration  as  stateful

       Some  sites  may	 have  departments which have clients on more than one
       subnet, but it may be desirable to offer	those clients a	uniform	set of
       parameters  which  are  different than what would be offered to clients
       from other departments on the same subnet.  For clients which  will  be
       declared	 explicitly  with host declarations, these declarations	can be
       enclosed	in a group declaration along with  the	parameters  which  are
       common to that department.  For clients whose addresses will be dynami-
       cally assigned, class declarations and conditional declarations may  be
       used  to	 group	parameter  assignments based on	information the	client

       When a client is	to be booted, its boot parameters  are	determined  by
       consulting that client's	host declaration (if any), and then consulting
       any class declarations matching the client, followed by the pool,  sub-
       net  and	shared-network declarations for	the IP address assigned	to the
       client.	Each of	these declarations itself  appears  within  a  lexical
       scope,  and  all	 declarations at less specific lexical scopes are also
       consulted for client option declarations.  Scopes are never  considered
       twice,  and  if parameters are declared in more than one	scope, the pa-
       rameter declared	in the most specific scope is the one that is used.

       When dhcpd tries	to find	a host declaration  for	 a  client,  it	 first
       looks for a host	declaration which has a	fixed-address declaration that
       lists an	IP address that	is valid for the subnet	or shared  network  on
       which  the  client  is  booting.	 If it doesn't find any	such entry, it
       tries to	find an	entry which has	no fixed-address declaration.

       A typical dhcpd.conf file will look something like this:

       global parameters...

       subnet netmask {
	 subnet-specific parameters...

       subnet netmask {
	 subnet-specific parameters...

       subnet netmask {
	 subnet-specific parameters...

       group {
	 group-specific	parameters...
	 host {
	   host-specific parameters...
	 host {
	   host-specific parameters...
	 host {
	   host-specific parameters...

				      Figure 1

       Notice that at the beginning of the file, there's a  place  for	global
       parameters.  These might	be things like the organization's domain name,
       the addresses of	the name servers (if they are common to	the entire or-
       ganization), and	so on.	So, for	example:

	    option domain-name "";
	    option domain-name-servers,;

				      Figure 2

       As  you	can see	in Figure 2, you can specify host addresses in parame-
       ters using their	domain names rather than their numeric	IP  addresses.
       If  a given hostname resolves to	more than one IP address (for example,
       if that host has	two ethernet interfaces), then	where  possible,  both
       addresses are supplied to the client.

       The  most obvious reason	for having subnet-specific parameters as shown
       in Figure 1 is that each	subnet,	of necessity, has its own router.   So
       for the first subnet, for example, there	should be something like:

	    option routers;

       Note  that  the address here is specified numerically.  This is not re-
       quired -	if you have a different	domain name for	each interface on your
       router,	it's  perfectly	legitimate to use the domain name for that in-
       terface instead of the numeric address.	However, in many  cases	 there
       may  be only one	domain name for	all of a router's IP addresses,	and it
       would not be appropriate	to use that name here.

       In Figure 1 there is also a group statement, which provides common  pa-
       rameters	for a set of three hosts - zappo, beppo	and harpo.  As you can
       see, these hosts	are all	in the domain, so it  might  make
       sense  for  a group-specific parameter to override the domain name sup-
       plied to	these hosts:

	    option domain-name "";

       Also, given the domain they're in, these	are  probably  test  machines.
       If we wanted to test the	DHCP leasing mechanism,	we might set the lease
       timeout somewhat	shorter	than the default:

	    max-lease-time 120;
	    default-lease-time 120;

       You may have noticed that while some parameters start with  the	option
       keyword,	some do	not.  Parameters starting with the option keyword cor-
       respond to actual DHCP options, while parameters	that do	not start with
       the  option  keyword  either  control  the  behavior of the DHCP	server
       (e.g., how long a lease dhcpd will give out), or	specify	client parame-
       ters  that  are not optional in the DHCP	protocol (for example, server-
       name and	filename).

       In Figure 1, each host had host-specific	parameters.  These  could  in-
       clude  such things as the hostname option, the name of a	file to	upload
       (the filename parameter)	and the	address	of the server  from  which  to
       upload the file (the next-server	parameter).  In	general, any parameter
       can appear anywhere that	parameters are allowed,	and  will  be  applied
       according to the	scope in which the parameter appears.

       Imagine that you	have a site with a lot of NCD X-Terminals.  These ter-
       minals come in a	variety	of models, and you want	to  specify  the  boot
       files  for each model.  One way to do this would	be to have host	decla-
       rations for each	server and group them by model:

       group {
	 filename "Xncd19r";
	 next-server ncd-booter;

	 host ncd1 { hardware ethernet 0:c0:c3:49:2b:57; }
	 host ncd4 { hardware ethernet 0:c0:c3:80:fc:32; }
	 host ncd8 { hardware ethernet 0:c0:c3:22:46:81; }

       group {
	 filename "Xncd19c";
	 next-server ncd-booter;

	 host ncd2 { hardware ethernet 0:c0:c3:88:2d:81; }
	 host ncd3 { hardware ethernet 0:c0:c3:00:14:11; }

       group {
	 filename "XncdHMX";
	 next-server ncd-booter;

	 host ncd1 { hardware ethernet 0:c0:c3:11:90:23; }
	 host ncd4 { hardware ethernet 0:c0:c3:91:a7:8;	}
	 host ncd8 { hardware ethernet 0:c0:c3:cc:a:8f;	}

       The pool	and pool6 declarations can be used to specify a	 pool  of  ad-
       dresses	that  will  be	treated	 differently  than another pool	of ad-
       dresses,	even on	the same network segment or subnet.  For example,  you
       may  want  to  provide a	large set of addresses that can	be assigned to
       DHCP clients that are registered	to your	DHCP server, while providing a
       smaller	set  of	 addresses,  possibly with short lease times, that are
       available for unknown clients.  If you have a firewall, you may be able
       to  arrange for addresses from one pool to be allowed access to the In-
       ternet, while addresses in another pool are not,	thus encouraging users
       to register their DHCP clients.	To do this, you	would set up a pair of
       pool declarations:

       subnet netmask {
	 option	routers;

	 # Unknown clients get this pool.
	 pool {
	   option domain-name-servers;
	   max-lease-time 300;
	   allow unknown-clients;

	 # Known clients get this pool.
	 pool {
	   option domain-name-servers,;
	   max-lease-time 28800;
	   deny	unknown-clients;

       It is also possible to set up entirely different	subnets	for known  and
       unknown	clients	- address pools	exist at the level of shared networks,
       so address ranges within	pool declarations can be on different subnets.

       As you can see in the preceding example,	pools can  have	 permit	 lists
       that  control  which  clients  are allowed access to the	pool and which
       aren't.	Each entry in a	pool's permit list is introduced with the  al-
       low  or	deny  keyword.	 If  a pool has	a permit list, then only those
       clients that match specific entries on the permit list will be eligible
       to  be  assigned	 addresses  from the pool.  If a pool has a deny list,
       then only those clients that do not match any entries on	the deny  list
       will  be	 eligible.    If  both permit and deny lists exist for a pool,
       then only clients that match the	permit list and	do not match the  deny
       list will be allowed access.

       The pool6 declaration is	similar	to the pool declaration.  Currently it
       is only allowed within a	subnet6	declaration, and may not  be  included
       directly	 in  a	shared network declaration.  In	addition to the	range6
       statement it allows the prefix6 statement to be included.  You may  in-
       clude range6 statements for both	NA and TA and prefixy6 statements in a
       single pool6 statement.

       Address allocation is actually only done	when a client is in  the  INIT
       state and has sent a DHCPDISCOVER message.  If the client thinks	it has
       a valid lease and sends a DHCPREQUEST to	initiate or renew that	lease,
       the server has only three choices - it can ignore the DHCPREQUEST, send
       a DHCPNAK to tell the client it should stop using the address, or  send
       a  DHCPACK,  telling  the  client to go ahead and use the address for a

       If the server finds the address the client is requesting, and that  ad-
       dress  is  available to the client, the server will send	a DHCPACK.  If
       the address is no longer	available, or the client  isn't	 permitted  to
       have  it,  the server will send a DHCPNAK.  If the server knows nothing
       about the address, it will remain silent, unless	the address is	incor-
       rect  for the network segment to	which the client has been attached and
       the server is authoritative for that network segment, in	which case the
       server  will  send  a DHCPNAK even though it doesn't know about the ad-

       There may be a host declaration matching	the  client's  identification.
       If  that	 host  declaration  contains  a	fixed-address declaration that
       lists an	IP address that	is valid for the network segment to which  the
       client  is  connected.  In this case, the DHCP server will never	do dy-
       namic address allocation.  In this case,	the client is required to take
       the  address  specified in the host declaration.	 If the	client sends a
       DHCPREQUEST for some other address, the	server	will  respond  with  a

       When  the  DHCP	server allocates a new address for a client (remember,
       this only happens if the	client has  sent  a  DHCPDISCOVER),  it	 first
       looks  to see if	the client already has a valid lease on	an IP address,
       or if there is an old IP	address	the client had before that hasn't  yet
       been  reassigned.   In that case, the server will take that address and
       check it	to see if the client is	still permitted	to  use	 it.   If  the
       client  is  no  longer  permitted  to use it, the lease is freed	if the
       server thought it was still in use - the	fact that the client has  sent
       a  DHCPDISCOVER proves to the server that the client is no longer using
       the lease.

       If no existing lease is found, or if the	client is forbidden to receive
       the  existing  lease,  then the server will look	in the list of address
       pools for the network segment to	which the client  is  attached	for  a
       lease  that is not in use and that the client is	permitted to have.  It
       looks through each pool declaration in sequence (all range declarations
       that appear outside of pool declarations	are grouped into a single pool
       with no permit list).  If the permit  list  for	the  pool  allows  the
       client  to be allocated an address from that pool, the pool is examined
       to see if there is an address available.	 If so,	 then  the  client  is
       tentatively assigned that address.  Otherwise, the next pool is tested.
       If no addresses are found that can be assigned to the  client,  no  re-
       sponse is sent to the client.

       If  an  address is found	that the client	is permitted to	have, and that
       has never been assigned to any client before, the  address  is  immedi-
       ately allocated to the client.  If the address is available for alloca-
       tion but	has been previously assigned to	a different client, the	server
       will  keep looking in hopes of finding an address that has never	before
       been assigned to	a client.

       The DHCP	server generates the list of available	IP  addresses  from  a
       hash  table.   This means that the addresses are	not sorted in any par-
       ticular order, and so it	is not possible	to predict the order in	 which
       the DHCP	server will allocate IP	addresses.  Users of previous versions
       of the ISC DHCP server may have become accustomed to  the  DHCP	server
       allocating  IP addresses	in ascending order, but	this is	no longer pos-
       sible, and there	is no way to configure this behavior with version 3 of
       the ISC DHCP server.

       The  DHCP  server  checks IP addresses to see if	they are in use	before
       allocating them to clients.  It does this by sending an ICMP  Echo  re-
       quest message to	the IP address being allocated.	 If no ICMP Echo reply
       is received within a second, the	address	is assumed to be  free.	  This
       is  only	 done for leases that have been	specified in range statements,
       and only	when the lease is thought by the DHCP  server  to  be  free  -
       i.e.,  the DHCP server or its failover peer has not listed the lease as
       in use.

       If a response is	received to an ICMP Echo request, the DHCP server  as-
       sumes that there	is a configuration error - the IP address is in	use by
       some host on the	network	that is	not a DHCP client.  It marks  the  ad-
       dress  as  abandoned, and will not assign it to clients.	The lease will
       remain abandoned	for a minimum of abandon-lease-time seconds.

       If a DHCP client	tries to get an	IP address, but	 none  are  available,
       but there are abandoned IP addresses, then the DHCP server will attempt
       to reclaim an abandoned IP address.  It marks one IP address  as	 free,
       and  then  does	the same ICMP Echo request check described previously.
       If there	is no answer to	the ICMP Echo request, the address is assigned
       to the client.

       The  DHCP  server  does not cycle through abandoned IP addresses	if the
       first IP	address	it tries to reclaim is free.  Rather,  when  the  next
       DHCPDISCOVER comes in from the client, it will attempt a	new allocation
       using the same method described here, and will typically	try a  new  IP

       This version of the ISC DHCP server supports the	DHCP failover protocol
       as documented in	draft-ietf-dhc-failover-12.txt.	 This is not  a	 final
       protocol	 document,  and	we have	not done interoperability testing with
       other vendors' implementations of this protocol,	so you must not	assume
       that  this implementation conforms to the standard.  If you wish	to use
       the failover protocol, make sure	that both failover peers  are  running
       the same	version	of the ISC DHCP	server.

       The failover protocol allows two	DHCP servers (and no more than two) to
       share a common address pool.  Each server will have about half  of  the
       available  IP  addresses	 in the	pool at	any given time for allocation.
       If one server fails, the	other server will continue to renew leases out
       of the pool, and	will allocate new addresses out	of the roughly half of
       available addresses that	it had	when  communications  with  the	 other
       server were lost.

       It  is possible during a	prolonged failure to tell the remaining	server
       that the	other server is	down, in which case the	remaining server  will
       (over  time)  reclaim  all the addresses	the other server had available
       for allocation, and begin to reuse them.	 This is  called  putting  the
       server into the PARTNER-DOWN state.

       You  can	put the	server into the	PARTNER-DOWN state either by using the
       omshell (1) command  or	by  stopping  the  server,  editing  the  last
       failover	 state	declaration  in	 the  lease  file,  and	restarting the
       server.	If you use this	last method, change the	"my state" line	to:

       failover	peer name state	{
       my state	partner-down;.
       peer state state	at date;

       It is only required to change "my state"	as shown above.

       When the	other server comes back	online,	it should automatically	detect
       that  it	has been offline and request a complete	update from the	server
       that was	running	in the PARTNER-DOWN state, and then both servers  will
       resume processing together.

       It is possible to get into a dangerous situation: if you	put one	server
       into the	PARTNER-DOWN state, and	then *that* server goes	down, and  the
       other  server  comes  back  up, the other server	will not know that the
       first server was	in the PARTNER-DOWN state,  and	 may  issue  addresses
       previously  issued  by the other	server to different clients, resulting
       in IP address conflicts.	 Before	putting	 a  server  into  PARTNER-DOWN
       state,  therefore, make sure that the other server will not restart au-

       The failover protocol defines a primary server  role  and  a  secondary
       server  role.   There  are some differences in how primaries and	secon-
       daries act, but most of the differences simply have to do with  provid-
       ing  a  way for each peer to behave in the opposite way from the	other.
       So one server must be configured	as primary, and	the other must be con-
       figured	as  secondary,	and  it	 doesn't  matter too much which	one is

       When a server starts that has  not  previously  communicated  with  its
       failover	 peer, it must establish communications	with its failover peer
       and synchronize with it before it can serve clients.  This  can	happen
       either  because	you  have just configured your DHCP servers to perform
       failover	for the	first time, or because one of  your  failover  servers
       has failed catastrophically and lost its	database.

       The  initial  recovery  process	is  designed  to  ensure that when one
       failover	peer loses its database	and then  resynchronizes,  any	leases
       that the	failed server gave out before it failed	will be	honored.  When
       the failed server starts	up, it notices that it has no  saved  failover
       state, and attempts to contact its peer.

       When  it	 has established contact, it asks the peer for a complete copy
       its peer's lease	database.  The peer then sends its complete  database,
       and sends a message indicating that it is done.	The failed server then
       waits until MCLT	has passed, and	once MCLT has passed both servers make
       the transition back into	normal operation.  This	waiting	period ensures
       that any	leases the failed server may have given	out while out of  con-
       tact with its partner will have expired.

       While the failed	server is recovering, its partner remains in the part-
       ner-down	state, which means that	it is serving all clients.  The	failed
       server provides no service at all to DHCP clients until it has made the
       transition into normal operation.

       In the case where both servers detect that they have never before  com-
       municated  with their partner, they both	come up	in this	recovery state
       and follow the procedure	we have	just described.	 In this case, no ser-
       vice will be provided to	DHCP clients until MCLT	has expired.

       In  order  to  configure	failover, you need to write a peer declaration
       that configures the failover protocol, and you need to write peer  ref-
       erences	in  each  pool	declaration for	which you want to do failover.
       You do not have to do failover for all pools on a  given	 network  seg-
       ment.	You must not tell one server it's doing	failover on a particu-
       lar address pool	and tell the other it is not.  You must	not  have  any
       common  address pools on	which you are not doing	failover.  A pool dec-
       laration	that utilizes failover would look like this:

       pool {
	    failover peer "foo";
	    pool specific parameters

       The  server currently  does very	 little	 sanity	checking,  so if   you
       configure  it wrong, it will just  fail in odd ways.  I would recommend
       therefore that you either do  failover or don't do failover, but	 don't
       do  any mixed pools.  Also,  use	the same master	configuration file for
       both  servers,  and  have  a  separate file  that  contains  the	  peer
       declaration  and	includes the master file.  This	will help you to avoid
       configuration  mismatches.  As our  implementation evolves,  this  will
       become	less of	 a  problem.  A	 basic	sample dhcpd.conf  file	for  a
       primary server might look like this:

       failover	peer "foo" {
	 port 519;
	 peer address;
	 peer port 520;
	 max-response-delay 60;
	 max-unacked-updates 10;
	 mclt 3600;
	 split 128;
	 load balance max seconds 3;

       include "/etc/dhcpd.master";

       The statements in the peer declaration are as follows:

       The primary and secondary statements

	 [ primary | secondary ];

	 This determines whether the server is primary or  secondary,  as  de-
	 scribed earlier under DHCP FAILOVER.

       The address statement

	 address address;

	 The  address  statement  declares the IP address or DNS name on which
	 the server should listen for connections from its failover peer,  and
	 also  the  value to use for the DHCP Failover Protocol	server identi-
	 fier.	Because	this value is used as an identifier,  it  may  not  be

       The peer	address	statement

	 peer address address;

	 The  peer  address  statement	declares the IP	address	or DNS name to
	 which the server should  connect  to  reach  its  failover  peer  for
	 failover messages.

       The port	statement

	 port port-number;

	 The  port  statement declares the TCP port on which the server	should
	 listen	for connections	from its failover peer.	 This statement	may be
	 omitted, in which case	the IANA assigned port number 647 will be used
	 by default.

       The peer	port statement

	 peer port port-number;

	 The peer port statement declares the TCP port	to  which  the	server
	 should	 connect  to  reach  its  failover peer	for failover messages.
	 This statement	may be omitted,	in which case the IANA	assigned  port
	 number	647 will be used by default.

       The max-response-delay statement

	 max-response-delay seconds;

	 The  max-response-delay statement tells the DHCP server how many sec-
	 onds may pass without receiving a message from	its failover peer  be-
	 fore  it  assumes  that connection has	failed.	 This number should be
	 small enough that a transient network failure that breaks the connec-
	 tion  will not	result in the servers being out	of communication for a
	 long time, but	large enough that the server isn't  constantly	making
	 and breaking connections.  This parameter must	be specified.

       The max-unacked-updates statement

	 max-unacked-updates count;

	 The  max-unacked-updates  statement  tells the	remote DHCP server how
	 many BNDUPD messages it can send before it receives a BNDACK from the
	 local	system.	  We  don't  have enough operational experience	to say
	 what a	good value for this is,	but 10 seems to	work.  This  parameter
	 must be specified.

       The mclt	statement

	 mclt seconds;

	 The  mclt statement defines the Maximum Client	Lead Time.  It must be
	 specified on the primary, and may not be specified on the  secondary.
	 This is the length of time for	which a	lease may be renewed by	either
	 failover peer without contacting the other.  The longer you set this,
	 the  longer  it  will	take  for the running server to	recover	IP ad-
	 dresses after moving into PARTNER-DOWN	state.	The  shorter  you  set
	 it, the more load your	servers	will experience	when they are not com-
	 municating.  A	value of something like	3600 is	 probably  reasonable,
	 but  again  bear  in mind that	we have	no real	operational experience
	 with this.

       The split statement

	 split bits;

	 The split statement specifies the split between the primary and  sec-
	 ondary	for the	purposes of load balancing.  Whenever a	client makes a
	 DHCP request, the DHCP	server runs a hash on the  client  identifica-
	 tion,	resulting  in  value  from 0 to	255.  This is used as an index
	 into a	256 bit	field.	If the bit at that index is set,  the  primary
	 is  responsible.   If the bit at that index is	not set, the secondary
	 is responsible.  The split value determines how many of  the  leading
	 bits are set to one.  So, in practice,	higher split values will cause
	 the primary to	serve more clients than	the  secondary.	  Lower	 split
	 values,  the converse.	 Legal values are between 0 and	256 inclusive,
	 of which the most reasonable is 128.  Note that a value  of  0	 makes
	 the  secondary	 responsible  for all clients and a value of 256 makes
	 the primary responsible for all clients.

       The hba statement

	 hba colon-separated-hex-list;

	 The hba statement specifies the split between the  primary  and  sec-
	 ondary	 as  a bitmap rather than a cutoff, which theoretically	allows
	 for finer-grained control.  In	practice, there	is  probably  no  need
	 for such fine-grained control,	however.  An example hba statement:

	   hba ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:ff:

	 This  is  equivalent  to  a split 128;	statement, and identical.  The
	 following two examples	are also equivalent to a split of 128, but are
	 not identical:

	   hba aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:aa:

	   hba 55:55:55:55:55:55:55:55:55:55:55:55:55:55:55:55:

	 They are equivalent, because half the bits are	set to 0, half are set
	 to 1 (0xa and 0x5 are 1010 and	0101 binary respectively)  and	conse-
	 quently  this	would  roughly	divide the clients equally between the
	 servers.  They	are not	identical, because the actual peers this would
	 load balance to each server are different for each example.

	 You must only have split or hba defined, never	both.  For most	cases,
	 the fine-grained control that hba offers isn't	necessary,  and	 split
	 should	be used.

       The load	balance	max seconds statement

	 load balance max seconds seconds;

	 This statement	allows you to configure	a cutoff after which load bal-
	 ancing	is disabled.  The cutoff is based on  the  number  of  seconds
	 since	the client sent	its first DHCPDISCOVER or DHCPREQUEST message,
	 and only works	with clients that correctly implement the secs field -
	 fortunately  most clients do.	We recommend setting this to something
	 like 3	or 5.  The effect of this is that if one of the	failover peers
	 gets into a state where it is responding to failover messages but not
	 responding to some client requests, the other failover	peer will take
	 over its client load automatically as the clients retry.

       The auto-partner-down statement

	 auto-partner-down seconds;

	 This  statement  instructs  the server	to initiate a timed delay upon
	 entering the communications-interrupted state (any situation of being
	 out-of-contact	 with the remote failover peer).  At the conclusion of
	 the timer, the	 server	 will  automatically  enter  the  partner-down
	 state.	 This permits the server to allocate leases from the partner's
	 free lease pool after an STOS+MCLT timer expires, which can  be  dan-
	 gerous	 if  the  partner  is  in  fact	operating at the time (the two
	 servers will give conflicting bindings).

	 Think very carefully before enabling this feature.  The  partner-down
	 and  communications-interrupted  states  are intentionally segregated
	 because there do exist	situations where a failover server can fail to
	 communicate  with  its	peer, but still	has the	ability	to receive and
	 reply to requests from	DHCP clients.  In general, this	feature	should
	 only  be used in those	deployments where the failover servers are di-
	 rectly	connected to one another, such as  by  a  dedicated  hardwired
	 link ("a heartbeat cable").

	 A  zero  value	 disables  the auto-partner-down feature (also the de-
	 fault), and any positive value	indicates the time in seconds to  wait
	 before	automatically entering partner-down.

       The Failover pool balance statements.

	  max-lease-misbalance percentage;
	  max-lease-ownership percentage;
	  min-balance seconds;
	  max-balance seconds;

	 This version of the DHCP Server evaluates pool	balance	on a schedule,
	 rather	than on	demand as leases are allocated.	 The  latter  approach
	 proved	 to be slightly	klunky when pool misbalanced reach total satu-
	 ration	-- when	any server ran out of leases to	assign,	it  also  lost
	 its ability to	notice it had run dry.

	 In  order  to understand pool balance,	some elements of its operation
	 first need to be defined.   First,  there  are	 'free'	 and  'backup'
	 leases.   Both	 of  these  are	 referred  to  as 'free	state leases'.
	 'free'	and 'backup' are 'the free states' for	the  purpose  of  this
	 document.   The difference is that only the primary may allocate from
	 'free'	leases unless under special circumstances, and only  the  sec-
	 ondary	may allocate 'backup' leases.

	 When  pool balance is performed, the only plausible expectation is to
	 provide a 50/50 split of  the	free  state  leases  between  the  two
	 servers.   This is because no one can predict which server will fail,
	 regardless of the relative load placed	upon the two servers, so  giv-
	 ing each server half the leases gives both servers the	same amount of
	 'failure endurance'.  Therefore, there	is no  way  to	configure  any
	 different  behaviour,	outside	of some	very small windows we will de-
	 scribe	shortly.

	 The first thing calculated on any pool	balance	run  is	 a  value  re-
	 ferred	 to  as	'lts', or "Leases To Send".  This, simply, is the dif-
	 ference in the	count of free and backup leases, divided by two.   For
	 the  secondary,  it  is the difference	in the backup and free leases,
	 divided by two.  The resulting	value is signed: if  it	 is  positive,
	 the  local  server  is	 expected to hand out leases to	retain a 50/50
	 balance.  If it is negative, the remote server	 would	need  to  send
	 leases	 to  balance  the  pool.  Once the lts value reaches zero, the
	 pool is perfectly balanced (give or take one lease in the case	of  an
	 odd number of total free state	leases).

	 The  current  approach	 is still something of a hybrid	of the old ap-
	 proach, marked	by the presence	of the max-lease-misbalance statement.
	 This parameter	configures what	used to	be a 10% fixed value in	previ-
	 ous versions: if lts is less than free+backup *  max-lease-misbalance
	 percent,  then	 the server will skip balancing	a given	pool (it won't
	 bother	moving any leases, even	if some	 leases	 "should"  be  moved).
	 The  meaning  of  this	value is also somewhat overloaded, however, in
	 that it also governs the estimation of	when to	attempt	to balance the
	 pool (which may then also be skipped over).  The oldest leases	in the
	 free and backup states	are examined.  The time	they have  resided  in
	 their	respective  queues is used as an estimate to indicate how much
	 time it is probable it	would take before the leases at	the top	of the
	 list  would  be consumed (and thus, how long it would take to use all
	 leases	in that	state).	 This percentage  is  directly	multiplied  by
	 this  time, and fit into the schedule if it falls within the min-bal-
	 ance and max-balance configured values.   The	scheduled  pool	 check
	 time  is  only	moved in a downwards direction,	it is never increased.
	 Lastly, if the	lts is more than double	this number  in	 the  negative
	 direction, the	local server will 'panic' and transmit a Failover pro-
	 tocol POOLREQ message,	in the hopes that the remote  system  will  be
	 woken up into action.

	 Once the lts value exceeds the	max-lease-misbalance percentage	of to-
	 tal free state	leases as described above, leases are moved to the re-
	 mote server.  This is done in two passes.

	 In  the  first	pass, only leases whose	most recent bound client would
	 have been served by the remote	server - according to the Load Balance
	 Algorithm  (see  above	 split and hba configuration statements) - are
	 given away to the peer.  This first pass  will	 happily  continue  to
	 give  away  leases, decrementing the lts value	by one for each, until
	 the lts value has reached the negative	of the total number of	leases
	 multiplied  by	 the max-lease-ownership percentage.  So it is through
	 this value that you can permit	a small	misbalance of the lease	 pools
	 -  for	 the  purpose  of  giving  the peer more than a	50/50 share of
	 leases	in the hopes that their	clients	might some day return  and  be
	 allocated by the peer (operating normally).  This process is referred
	 to as 'MAC Address Affinity', but this	is somewhat misnamed:  it  ap-
	 plies	equally	 to  DHCP  Client  Identifier options.	Note also that
	 affinity is applied to	leases when they enter the state  'free'  from
	 'expired' or 'released'.  In this case	also, leases will not be moved
	 from free to backup if	the secondary already has more than its	share.

	 The second pass is only entered into if the first pass	fails  to  re-
	 duce  the lts underneath the total number of free state leases	multi-
	 plied by the max-lease-ownership percentage.  In this pass, the  old-
	 est  leases  are  given over to the peer without second thought about
	 the Load Balance Algorithm, and this continues	until  the  lts	 falls
	 under	this  value.   In this way, the	local server will also happily
	 keep a	small percentage of the	leases that would normally  load  bal-
	 ance to itself.

	 So,  the  max-lease-misbalance	 value	acts  as  a  behavioural gate.
	 Smaller values	will cause more	leases to transition states to balance
	 the pools over	time, higher values will decrease the amount of	change
	 (but may lead to pool starvation if there's a run on leases).

	 The max-lease-ownership value permits a small	(percentage)  skew  in
	 the  lease  balance of	a percentage of	the total number of free state

	 Finally, the min-balance and max-balance make certain that  a	sched-
	 uled rebalance	event happens within a reasonable timeframe (not to be
	 thrown	off by,	for example, a 7 year old free lease).

	 Plausible values for the percentages lie between 0  and  100,	inclu-
	 sive, but values over 50 are indistinguishable	from one another (once
	 lts exceeds 50% of the	free state leases, one server  must  therefore
	 have  100% of the leases in its respective free state).  It is	recom-
	 mended	to select a max-lease-ownership	value that is lower  than  the
	 value	selected for the max-lease-misbalance value.  max-lease-owner-
	 ship defaults to 10, and max-lease-misbalance defaults	to 15.

	 Plausible values for the min-balance and max-balance times also range
	 from 0	to (2^32)-1 (or	the limit of your local	time_t value), but de-
	 fault to values 60 and	3600 respectively (to place balance events be-
	 tween 1 minute	and 1 hour).

       Clients	can be separated into classes, and treated differently depend-
       ing on what class they are in.  This separation can be done either with
       a  conditional  statement,  or  with a match statement within the class
       declaration.  It	is possible to specify a limit on the total number  of
       clients	within	a particular class or subclass that may	hold leases at
       one time, and it	is possible to specify automatic subclassing based  on
       the contents of the client packet.

       Classing	support	for DHCPv6 clients was added in	4.3.0.	It follows the
       same rules as for DHCPv4	except that support for	 billing  classes  has
       not been	added yet.

       To  add	clients	 to  classes  based on conditional evaluation, you can
       specify a matching expression in	the class statement:

       class "ras-clients" {
	 match if substring (option dhcp-client-identifier, 1, 3) = "RAS";

       Note that whether you use matching expressions or  add  statements  (or
       both)  to  classify  clients, you must always write a class declaration
       for any class that you use.  If there will be no	match statement	and no
       in-scope	statements for a class,	the declaration	should look like this:

       class "ras-clients" {

       In  addition  to	classes, it is possible	to declare subclasses.	A sub-
       class is	a class	with the same name as a	regular	class, but with	a spe-
       cific  submatch expression which	is hashed for quick matching.  This is
       essentially a speed hack	- the main  difference	between	 five  classes
       with  match  expressions	 and one class with five subclasses is that it
       will be quicker to find the subclasses.	Subclasses work	as follows:

       class "allocation-class-1" {
	 match pick-first-value	(option	dhcp-client-identifier,	hardware);

       class "allocation-class-2" {
	 match pick-first-value	(option	dhcp-client-identifier,	hardware);

       subclass	"allocation-class-1" 1:8:0:2b:4c:39:ad;
       subclass	"allocation-class-2" 1:8:0:2b:a9:cc:e3;
       subclass	"allocation-class-1" 1:0:0:c4:aa:29:44;

       subnet netmask {
	 pool {
	   allow members of "allocation-class-1";
	 pool {
	   allow members of "allocation-class-2";

       The data	following the class name in the	subclass declaration is	a con-
       stant  value  to	 use  in  matching the match expression	for the	class.
       When class matching is done, the	server will evaluate the match expres-
       sion  and  then	look  the  result up in	the hash table.	 If it finds a
       match, the client is considered a member	of both	the class and the sub-

       Subclasses  can	be declared with or without scope.  In the above exam-
       ple, the	sole purpose of	the subclass is	to allow some  clients	access
       to  one address pool, while other clients are given access to the other
       pool, so	these subclasses are declared without scopes.  If part of  the
       purpose	of  the	subclass were to define	different parameter values for
       some clients, you might want to declare some subclasses with scopes.

       In the above example, if	you had	a single client	that needed some  con-
       figuration parameters, while most didn't, you might write the following
       subclass	declaration for	that client:

       subclass	"allocation-class-2" 1:08:00:2b:a1:11:31 {
	 option	root-path "samsara:/var/diskless/alphapc";
	 filename "/tftpboot/netbsd.alphapc-diskless";

       In this example,	we've used subclassing as a way	to control address al-
       location	 on  a	per-client  basis.  However, it's also possible	to use
       subclassing in ways that	are not	specific to clients - for example,  to
       use  the	 value of the vendor-class-identifier option to	determine what
       values to send in the vendor-encapsulated-options option.   An  example
       of  this	 is  shown  under  the VENDOR ENCAPSULATED OPTIONS head	in the
       dhcp-options(5) manual page.

       You may specify a limit to the number of	clients	in a class that	can be
       assigned	leases.	 The effect of this will be to make it difficult for a
       new client in a class to	get an address.	 Once  a  class	 with  such  a
       limit  has  reached  its	limit, the only	way a new client in that class
       can get a lease is for an existing client to relinquish its lease,  ei-
       ther by letting it expire, or by	sending	a DHCPRELEASE packet.  Classes
       with lease limits are specified as follows:

       class "limited-1" {
	 lease limit 4;

       This will produce a class in which a maximum of four members may	hold a
       lease at	one time.

       It  is  possible	 to  declare  a	spawning class.	 A spawning class is a
       class that automatically	produces subclasses based on what  the	client
       sends.	The  reason  that spawning classes were	created	was to make it
       possible	to create lease-limited	classes	on the	fly.   The  envisioned
       application  is	a cable-modem environment where	the ISP	wishes to pro-
       vide clients at a particular site with more than	one  IP	 address,  but
       does  not  wish to provide such clients with their own subnet, nor give
       them an unlimited number	of IP addresses	from the  network  segment  to
       which they are connected.

       Many  cable  modem  head-end  systems  can be configured	to add a Relay
       Agent Information option	to DHCP	packets	when relaying them to the DHCP
       server.	 These	systems	typically add a	circuit	ID or remote ID	option
       that uniquely identifies	the customer site.  To take advantage of this,
       you can write a class declaration as follows:

       class "customer"	{
	 spawn with option agent.circuit-id;
	 lease limit 4;

       Now  whenever  a	 request comes in from a customer site,	the circuit ID
       option will be checked against the class's hash table.  If  a  subclass
       is  found that matches the circuit ID, the client will be classified in
       that subclass and treated accordingly.  If no subclass is found	match-
       ing  the	 circuit  ID,  a  new  one  will  be created and logged	in the
       dhcpd.leases file, and the client will be classified in this new	class.
       Once  the  client  has been classified, it will be treated according to
       the rules of the	class, including, in this case,	being subject  to  the
       per-site	limit of four leases.

       The  use	 of the	subclass spawning mechanism is not restricted to relay
       agent options - this particular example is given	only because it	 is  a
       fairly straightforward one.

       In  some	 cases,	 it  may  be  useful to	use one	expression to assign a
       client to a particular class, and a second expression to	put it into  a
       subclass	of that	class.	This can be done by combining the match	if and
       spawn with statements, or the match if and match	statements.  For exam-

       class "jr-cable-modems" {
	 match if option dhcp-vendor-identifier	= "jrcm";
	 spawn with option agent.circuit-id;
	 lease limit 4;

       class "dv-dsl-modems" {
	 match if option dhcp-vendor-identifier	= "dvdsl";
	 spawn with option agent.circuit-id;
	 lease limit 16;

       This  allows you	to have	two classes that both have the same spawn with
       expression without getting the clients in the two classes confused with
       each other.

       The  DHCP  server has the ability to dynamically	update the Domain Name
       System.	Within the configuration files,	you can	define	how  you  want
       the  Domain Name	System to be updated.  These updates are RFC 2136 com-
       pliant so any DNS server	supporting RFC 2136 should be able  to	accept
       updates from the	DHCP server.

       There  are two DNS schemes implemented.	The interim option is based on
       draft revisions of the DDNS documents  while  the  standard  option  is
       based on	the RFCs for DHCP-DNS interaction and DHCIDs.  A third option,
       ad-hoc, was deprecated and has now been removed	from  the  code	 base.
       The DHCP	server must be configured to use one of	the two	currently-sup-
       ported methods, or not to do DNS	updates.

       New installations should	use the	standard option.  Older	 installations
       may want	to continue using the interim option for backwards compatibil-
       ity with	the DNS	database until the database can	be updated.  This  can
       be done with the	ddns-update-style configuration	parameter.

       the interim and standard	DNS update schemes operate mostly according to
       work from the IETF.  The	interim	version	was based  on  the  drafts  in
       progress	at the time while the standard is based	on the completed RFCs.
       The standard RFCs are:

			    RFC	4701 (updated by RF5494)
				      RFC 4702
				      RFC 4703

       And the corresponding drafts were:


       The basic framework for the two schemes is similar with the main	 mate-
       rial  difference	 being that a DHCID RR is used in the standard version
       while the interim versions uses a TXT RR.  The format of	the TXT	record
       bears  a	 resemblance  to the DHCID RR but it is	not equivalent (MD5 vs
       SHA2, field length differences etc).

       In these	two schemes the	DHCP server does not necessarily always	update
       both the	A and the PTR records.	The FQDN option	includes a flag	which,
       when sent by the	client,	indicates that the client wishes to update its
       own  A  record.	 In  that case,	the server can be configured either to
       honor the client's intentions or	ignore them.  This is  done  with  the
       statement  allow	 client-updates;  or  the  statement ignore client-up-
       dates;.	By default, client updates are allowed.

       If the server is	configured to allow client updates, then if the	client
       sends a fully-qualified domain name in the FQDN option, the server will
       use that	name the client	sent in	the FQDN  option  to  update  the  PTR
       record.	 For example, let us say that the client is a visitor from the
       "" domain, whose hostname is "jschmoe".  The  server  is  for
       the "" domain.  The DHCP client indicates in the FQDN	option
       that its	FQDN is	"".	 It  also  indicates  that  it
       wants  to  update its own A record.  The	DHCP server therefore does not
       attempt to set up an A record for the client, but does  set  up	a  PTR
       record  for  the	 IP  address  that  it assigns the client, pointing at  Once the DHCP client has an	IP address, it can up-
       date  its  own A	record,	assuming that the "" DNS server will
       allow it	to do so.

       If the server is	configured not to allow	 client	 updates,  or  if  the
       client doesn't want to do its own update, the server will simply	choose
       a name for the client. By default, the server will choose from the fol-
       lowing three values:

	    1. fqdn option (if present)
	    2. hostname	option (if present)
	    3. Configured hostname option (if defined).

       If  these  defaults  for	choosing the host name are not appropriate you
       can write your own statement to set the ddns-hostname variable  as  you
       wish.  If none of the above are found the server	will use the host dec-
       laration	name (if one) and use-host-decl-names is on.

       It will use its own domain name for the client.	It  will  then	update
       both the	A and PTR record, using	the name that it chose for the client.
       If the client sends a fully-qualified domain name in the	 fqdn  option,
       the  server uses	only the leftmost part of the domain name - in the ex-
       ample above, "jschmoe" instead of "".

       Further,	if the ignore client-updates;  directive  is  used,  then  the
       server  will  in	addition send a	response in the	DHCP packet, using the
       FQDN Option, that implies to the	client that it should perform its  own
       updates	if it chooses to do so.	 With deny client-updates;, a response
       is sent which indicates the client may not perform updates.

       Both the	standard and interim options also include a  method  to	 allow
       more  than  one DHCP server to update the DNS database without acciden-
       tally deleting A	records	that shouldn't be deleted nor failing to add A
       records that should be added.  For the standard option the method works
       as follows:

       When the	DHCP server issues a client a new lease,  it  creates  a  text
       string  that  is	an SHA hash over the DHCP client's identification (see
       RFCs 4701 & 4702	for details).  The update attempts to add an A	record
       with the	name the server	chose and a DHCID record containing the	hashed
       identifier string (hashid).  If this update  succeeds,  the  server  is

       If  the update fails because the	A record already exists, then the DHCP
       server attempts to add the A record with	the  prerequisite  that	 there
       must  be	 a DHCID record	in the same name as the	new A record, and that
       DHCID record's contents must be equal to	hashid.	 If this  update  suc-
       ceeds,  then  the client	has its	A record and PTR record.  If it	fails,
       then the	name the client	has been assigned (or requested)  is  in  use,
       and  can't  be used by the client.  At this point the DHCP server gives
       up trying to do a DNS update for	the client until the client chooses  a
       new name.

       The  server  also  does not update very aggressively.  Because each DNS
       update involves a round trip to the DNS server, there is	a cost associ-
       ated  with  doing  updates  even	if they	do not actually	modify the DNS
       database.  So the DHCP server tracks whether or not it has updated  the
       record  in  the past (this information is stored	on the lease) and does
       not attempt to update records that it thinks it has already updated.

       This can	lead to	cases where the	DHCP server adds a  record,  and  then
       the  record  is	deleted	 through  some other mechanism,	but the	server
       never again updates the DNS because  it	thinks	the  data  is  already
       there.  In this case the	data can be removed from the lease through op-
       erator intervention, and	once this has been done, the DNS will  be  up-
       dated the next time the client renews.

       The  interim  DNS update	scheme was written before the RFCs were	final-
       ized and	does not quite follow them.  The RFCs call  for	 a  new	 DHCID
       RRtype while the	interim	DNS update scheme uses a TXT record.  In addi-
       tion the	ddns-resolution	draft called for the  DHCP  server  to	put  a
       DHCID  RR  on the PTR record, but the interim update method does	not do
       this.  In the final RFC this requirement	was relaxed such that a	server
       may add a DHCID RR to the PTR record.

       When  you set your DNS server up	to allow updates from the DHCP server,
       you may be exposing it to unauthorized updates.	 To  avoid  this,  you
       should  use TSIG	signatures - a method of cryptographically signing up-
       dates using a shared secret key.	 As long as you	protect	the secrecy of
       this  key, your updates should also be secure.  Note, however, that the
       DHCP protocol itself provides no	security, and that clients can	there-
       fore  provide information to the	DHCP server which the DHCP server will
       then use	in its updates,	with the constraints described previously.

       The DNS server must be configured to allow updates for  any  zone  that
       the DHCP	server will be updating.  For example, let us say that clients
       in  the	domain	will  be  assigned  addresses  on  the  subnet.	In  that case, you will	need a key declaration
       for the TSIG key	you will be using, and also two	 zone  declarations  -
       one  for	the zone containing A records that will	be updates and one for
       the zone	containing PTR records - for ISC BIND, something like this:

       key DHCP_UPDATER	{
	 algorithm HMAC-MD5.SIG-ALG.REG.INT;
	 secret	pRP5FapFoJ95JEL06sv4PQ==;

       zone "" {
	    type master;
	    file "";
	    allow-update { key DHCP_UPDATER; };

       zone "" {
	    type master;
	    file "10.10.17.db";
	    allow-update { key DHCP_UPDATER; };

       You will	also have to configure your DHCP server	to do updates to these
       zones.	To  do	so,  you  need	to  add	 something  like  this to your
       dhcpd.conf file:

       key DHCP_UPDATER	{
	 algorithm HMAC-MD5.SIG-ALG.REG.INT;
	 secret	pRP5FapFoJ95JEL06sv4PQ==;

       zone EXAMPLE.ORG. {

       zone {

       The primary statement specifies the IP address of the name server whose
       zone  information  is to	be updated.  In	addition to the	primary	state-
       ment there are also the primary6	, secondary and	secondary6 statements.
       The  primary6  statement	specifies an IPv6 address for the name server.
       The secondaries provide for additional addresses	for name servers to be
       used  if	 the primary does not respond.	The number of name servers the
       DDNS code will attempt to use before giving up is limited and  is  cur-
       rently set to three.

       Note that the zone declarations have to correspond to authority records
       in your name server - in	the above example, there must be an SOA	record
       for  ""  and	for "".  For example, if
       there were a subdomain ""	 with  no  separate  SOA,  you
       could not write a zone declaration for ""  Also keep in
       mind that zone names in your DHCP configuration should end  in  a  ".";
       this  is	 the  preferred	syntax.	 If you	do not end your	zone name in a
       ".", the	DHCP server will figure	it out.	 Also note that	 in  the  DHCP
       configuration,  zone  names  are	not encapsulated in quotes where there
       are in the DNS configuration.

       You should choose your own secret key, of course.  The ISC BIND 9  dis-
       tribution  comes	 with  a  program  for	generating  secret keys	called
       dnssec-keygen.  If you are using	BIND 9's dnssec-keygen,	the above  key
       would be	created	as follows:

	    dnssec-keygen -a HMAC-MD5 -b 128 -n	USER DHCP_UPDATER

       The  key	 name, algorithm, and secret must match	that being used	by the
       DNS server. The DHCP server  currently  supports	 the  following	 algo-


       You  may	 wish to enable	logging	of DNS updates on your DNS server.  To
       do so, you might	write a	logging	statement like the following:

       logging {
	    channel update_debug {
		 file "/var/log/update-debug.log";
		 severity  debug 3;
		 print-category	yes;
		 print-severity	yes;
		 print-time	yes;
	    channel security_info    {
		 file "/var/log/";
		 severity  info;
		 print-category	yes;
		 print-severity	yes;
		 print-time	yes;

	    category update { update_debug; };
	    category security {	security_info; };

       You must	create the  /var/log/  and  /var/log/update-de-
       bug.log files before starting the name server.  For more	information on
       configuring ISC BIND, consult the documentation that accompanies	it.

       There are three kinds of	events that can	happen regarding a lease,  and
       it  is  possible	 to  declare  statements  that occur when any of these
       events happen.  These events are	the commit event, when the server  has
       made  a	commitment  of a certain lease to a client, the	release	event,
       when the	client has released the	server from its	 commitment,  and  the
       expiry event, when the commitment expires.

       To  declare  a  set of statements to execute when an event happens, you
       must use	the on statement, followed by the name of the event,  followed
       by  a  series of	statements to execute when the event happens, enclosed
       in braces.

       The include statement

	include	"filename";

       The include statement is	used to	read in	a named	file, and process  the
       contents	of that	file as	though it were entered in place	of the include

       The shared-network statement

	shared-network name {
	  [ parameters ]
	  [ declarations ]

       The shared-network statement is used to inform  the  DHCP  server  that
       some  IP	subnets	actually share the same	physical network.  Any subnets
       in a shared network should be declared within a	shared-network	state-
       ment.   Parameters  specified  in  the shared-network statement will be
       used when booting clients on those subnets unless  parameters  provided
       at  the	subnet or host level override them.  If	any subnet in a	shared
       network has addresses available for dynamic allocation, those addresses
       are  collected  into a common pool for that shared network and assigned
       to clients as needed.  There is no way to distinguish on	 which	subnet
       of a shared network a client should boot.

       Name  should be the name	of the shared network.	This name is used when
       printing	debugging messages, so it should be descriptive	for the	shared
       network.	 The name may have the syntax of a valid domain	name (although
       it will never be	used as	such), or it may be any	 arbitrary  name,  en-
       closed in quotes.

       The subnet statement

	subnet subnet-number netmask netmask {
	  [ parameters ]
	  [ declarations ]

       The  subnet  statement is used to provide dhcpd with enough information
       to tell whether or not an IP address is on that subnet.	It may also be
       used  to	 provide  subnet-specific  parameters  and to specify what ad-
       dresses may be dynamically allocated to clients booting on that subnet.
       Such addresses are specified using the range declaration.

       The subnet-number should	be an IP address or domain name	which resolves
       to the subnet number of the subnet being	described.  The	netmask	should
       be  an  IP  address or domain name which	resolves to the	subnet mask of
       the subnet being	described.  The	subnet number, together	with the  net-
       mask,  are  sufficient  to determine whether any	given IP address is on
       the specified subnet.

       Although	a netmask must be given	with every subnet declaration,	it  is
       recommended  that if there is any variance in subnet masks at a site, a
       subnet-mask option statement be used in each subnet declaration to  set
       the  desired  subnet  mask, since any subnet-mask option	statement will
       override	the subnet mask	declared in the	subnet statement.

       The subnet6 statement

	subnet6	subnet6-number {
	  [ parameters ]
	  [ declarations ]

       The subnet6 statement is	used to	provide	dhcpd with enough  information
       to tell whether or not an IPv6 address is on that subnet6.  It may also
       be used to provide subnet-specific parameters and to specify  what  ad-
       dresses may be dynamically allocated to clients booting on that subnet.

       The  subnet6-number  should be an IPv6 network identifier, specified as

       The range statement

       range [ dynamic-bootp ] low-address [ high-address];

       For any subnet on which addresses will be assigned  dynamically,	 there
       must  be	 at  least one range statement.	 The range statement gives the
       lowest and highest IP addresses in a range.  All	IP  addresses  in  the
       range should be in the subnet in	which the range	statement is declared.
       The dynamic-bootp flag may be specified if addresses in	the  specified
       range  may  be  dynamically  assigned  to BOOTP clients as well as DHCP
       clients.	 When specifying a single address, high-address	can  be	 omit-

       The range6 statement

       range6 low-address high-address;
       range6 subnet6-number;
       range6 subnet6-number temporary;
       range6 address temporary;

       For  any	 IPv6 subnet6 on which addresses will be assigned dynamically,
       there must be at	least one range6 statement. The	range6	statement  can
       either  be  the	lowest	and highest IPv6 addresses in a	range6,	or use
       CIDR notation, specified	as ip6-address/bits. All IP addresses  in  the
       range6  should  be  in the subnet6 in which the range6 statement	is de-

       The temporary variant makes the prefix (by default on 64	 bits)	avail-
       able  for  temporary  (RFC 4941)	addresses. A new address per prefix in
       the shared network is computed at each request with  an	IA_TA  option.
       Release and Confirm ignores temporary addresses.

       Any IPv6	addresses given	to hosts with fixed-address6 are excluded from
       the range6, as are IPv6 addresses on the	server itself.

       The prefix6 statement

       prefix6 low-address high-address	/ bits;

       The prefix6 is the range6 equivalent for	Prefix Delegation (RFC	3633).
       Prefixes	 of  bits length are assigned between low-address and high-ad-

       Any IPv6	prefixes given to static entries  (hosts)  with	 fixed-prefix6
       are excluded from the prefix6.

       This  statement is currently global but it should have a	shared-network

       The host	statement

	host hostname {
	  [ parameters ]
	  [ declarations ]

       The host	declaration provides a way for the DHCP	server to  identify  a
       DHCP  or	BOOTP client.  This allows the server to provide configuration
       information including fixed addresses or, in DHCPv6, fixed prefixes for
       a specific client.

       If  it  is  desirable to	be able	to boot	a DHCP or BOOTP	client on more
       than one	subnet with fixed v4 addresses,	more than one address  may  be
       specified  in  the  fixed-address  declaration,	or  more than one host
       statement may be	specified matching the same client.

       The fixed-address6 declaration is used for v6 addresses.	 At this  time
       it  only	 works	with a single address.	For multiple addresses specify
       multiple	host statements.

       If client-specific boot parameters must change based on the network  to
       which the client	is attached, then multiple host	declarations should be
       used.  The host declarations will only match a client if	one  of	 their
       fixed-address  statements  is  viable on	the subnet (or shared network)
       where the client	is attached.  Conversely, for a	 host  declaration  to
       match  a	client being allocated a dynamic address, it must not have any
       fixed-address statements.  You may therefore need  a  mixture  of  host
       declarations  for  any  given client...some having fixed-address	state-
       ments, others without.

       hostname	should be a name identifying the host.	If a  hostname	option
       is not specified	for the	host, hostname is used.

       Host declarations are matched to	actual DHCP or BOOTP clients by	match-
       ing the dhcp-client-identifier option specified in the host declaration
       to  the	one supplied by	the client, or,	if the host declaration	or the
       client does not provide a dhcp-client-identifier	 option,  by  matching
       the  hardware parameter in the host declaration to the network hardware
       address supplied	by the client.	BOOTP clients do not normally  provide
       a  dhcp-client-identifier, so the hardware address must be used for all
       clients that may	boot using the BOOTP protocol.

       DHCPv6 servers can use the host-identifier option parameter in the host
       declaration,  and  specify  any	option	with a fixed value to identify

       Please be aware that only the  dhcp-client-identifier  option  and  the
       hardware	 address can be	used to	match a	host declaration, or the host-
       identifier option parameter for DHCPv6 servers.	For example, it	is not
       possible	 to  match  a host declaration to a host-name option.  This is
       because the host-name option cannot be guaranteed to be unique for  any
       given client, whereas both the hardware address and dhcp-client-identi-
       fier option are at least	theoretically guaranteed to  be	 unique	 to  a
       given client.

       The group statement

	group {
	  [ parameters ]
	  [ declarations ]

       The group statement is used simply to apply one or more parameters to a
       group of	declarations.  It can be used to group hosts, shared networks,
       subnets,	or even	other groups.

       The  allow  and	deny statements	can be used to control the response of
       the DHCP	server to various sorts	of requests.  The allow	and deny  key-
       words  actually have different meanings depending on the	context.  In a
       pool context, these keywords can	be used	to set up access lists for ad-
       dress allocation	pools.	In other contexts, the keywords	simply control
       general server behavior with respect to clients based on	scope.	 In  a
       non-pool	 context,  the ignore keyword can be used in place of the deny
       keyword to prevent logging of denied requests.

       The following usages of allow and deny will work	in any scope, although
       it is not recommended that they be used in pool declarations.

       The unknown-clients keyword

	allow unknown-clients;
	deny unknown-clients;
	ignore unknown-clients;

       The unknown-clients flag	is used	to tell	dhcpd whether or not to	dynam-
       ically assign addresses to unknown clients.  Dynamic address assignment
       to  unknown clients is allowed by default.  An unknown client is	simply
       a client	that has no host declaration.

       The use of this option is now deprecated.  If you  are  trying  to  re-
       strict access on	your network to	known clients, you should use deny un-
       known-clients; inside of	your address  pool,  as	 described  under  the

       The bootp keyword

	allow bootp;
	deny bootp;
	ignore bootp;

       The bootp flag is used to tell dhcpd whether or not to respond to bootp
       queries.	 Bootp queries are allowed by default.

       The booting keyword

	allow booting;
	deny booting;
	ignore booting;

       The booting flag	is used	to tell	dhcpd whether or  not  to  respond  to
       queries	from  a	particular client.  This keyword only has meaning when
       it appears in a host declaration.  By default, booting is allowed,  but
       if it is	disabled for a particular client, then that client will	not be
       able to get an address from the DHCP server.

       The duplicates keyword

	allow duplicates;
	deny duplicates;

       Host declarations can match client messages based on  the  DHCP	Client
       Identifier  option  or  based on	the client's network hardware type and
       MAC address.  If	the MAC	address	is used,  the  host  declaration  will
       match  any  client  with	that MAC address - even	clients	with different
       client identifiers.  This doesn't normally happen, but is possible when
       one  computer  has more than one	operating system installed on it - for
       example,	Microsoft Windows and NetBSD or	Linux.

       The duplicates flag tells the DHCP server that if a request is received
       from  a	client that matches the	MAC address of a host declaration, any
       other leases matching that MAC  address	should	be  discarded  by  the
       server,	even  if  the UID is not the same.  This is a violation	of the
       DHCP protocol, but can prevent clients whose client identifiers	change
       regularly  from	holding	many leases at the same	time.  By default, du-
       plicates	are allowed.

       The declines keyword

	allow declines;
	deny declines;
	ignore declines;

       The DHCPDECLINE message is used by DHCP clients to  indicate  that  the
       lease  the server has offered is	not valid.  When the server receives a
       DHCPDECLINE for a particular address, it	 normally  abandons  that  ad-
       dress,  assuming	 that  some unauthorized system	is using it.  Unfortu-
       nately, a malicious or buggy client can,	 using	DHCPDECLINE  messages,
       completely  exhaust the DHCP server's allocation	pool.  The server will
       eventually reclaim these	leases,	but not	while the  client  is  running
       through	the  pool. This	may cause serious thrashing in the DNS,	and it
       will also cause the DHCP	server to forget old DHCP client address allo-

       The declines flag tells the DHCP	server whether or not to honor DHCPDE-
       CLINE messages.	If it is set to	deny or	ignore in a particular	scope,
       the DHCP	server will not	respond	to DHCPDECLINE messages.

       The declines flag is only supported by DHCPv4 servers.  Given the large
       IPv6 address space and the internal limits imposed by the server's  ad-
       dress  generation  mechanism  we	don't think it is necessary for	DHCPv6
       servers at this time.

       Currently, abandoned IPv6 addresses are reclaimed in one	of two ways:
	   a) Client renews a specific address:
	   If a	client using a given DUID submits a DHCP REQUEST containing
	   the last address abandoned by that DUID, the	address	will be
	   reassigned to that client.

	   b) Upon the second restart following	an address abandonment.	 When
	   an address is abandoned it is both recorded as such in the lease
	   file	and retained as	abandoned in server memory until the server
	   is restarted. Upon restart, the server will process the lease file
	   and all addresses whose last	known state is abandoned will be
	   retained as such in memory but not rewritten	to the lease file.
	   This	means that a subsequent	restart	of the server will not see the
	   abandoned addresses in the lease file and therefore have no record
	   of them as abandoned	in memory and as such perceive them as free
	   for assignment.

       The total number	addresses in a pool, available for a given DUID	value,
       is internally limited by	the server's address generation	mechanism.  If
       through mistaken	configuration, multiple	clients	 are  using  the  same
       DUID  they  will	competing for the same addresses causing the server to
       reach this internal limit rather	quickly.  The internal limit  isolates
       this  type  of  activity	 such  that address range is not exhausted for
       other DUID values.  The appearance of the following error log,  can  be
       an indication of	this condition:

	   "Best match for DUID	<XX> is	an abandoned address, This may be a
	    result of multiple clients attempting to use this DUID"

	   where <XX> is an actual DUID	value depicted as colon	separated
	   string of bytes in hexadecimal values.

       The client-updates keyword

	allow client-updates;
	deny client-updates;

       The  client-updates  flag tells the DHCP	server whether or not to honor
       the client's intention to do its	own update of its A record.   See  the
       documentation under the heading THE DNS UPDATE SCHEME for details.

       The leasequery keyword

	allow leasequery;
	deny leasequery;

       The leasequery flag tells the DHCP server whether or not	to answer DHC-
       PLEASEQUERY packets. The	answer to a DHCPLEASEQUERY packet includes in-
       formation  about	 a specific lease, such	as when	it was issued and when
       it will expire. By default, the server will not respond to these	 pack-

       The  uses  of the allow and deny	keywords shown in the previous section
       work pretty much	the same way whether the client	is sending a  DHCPDIS-
       COVER  or  a  DHCPREQUEST message - an address will be allocated	to the
       client (either the old address it's requesting, or a new	 address)  and
       then  that address will be tested to see	if it's	okay to	let the	client
       have it.	 If the	client requested it, and it's  not  okay,  the	server
       will send a DHCPNAK message.  Otherwise,	the server will	simply not re-
       spond to	the client.  If	it is okay to give the address to the  client,
       the server will send a DHCPACK message.

       The  primary motivation behind pool declarations	is to have address al-
       location	pools whose allocation policies	are different.	A  client  may
       be denied access	to one pool, but allowed access	to another pool	on the
       same network segment.  In order for this	to work, access	control	has to
       be  done	 during	 address  allocation,  not after address allocation is

       When a DHCPREQUEST message is processed,	address	allocation simply con-
       sists  of looking up the	address	the client is requesting and seeing if
       it's still available for	the client.  If	it is, then  the  DHCP	server
       checks both the address pool permit lists and the relevant in-scope al-
       low and deny statements to see if it's okay to give the	lease  to  the
       client.	 In the	case of	a DHCPDISCOVER message,	the allocation process
       is done as described previously in the ADDRESS ALLOCATION section.

       When declaring permit lists for address allocation pools, the following
       syntaxes	are recognized following the allow or deny keywords:


       If  specified, this statement either allows or prevents allocation from
       this pool to any	client that has	a host declaration (i.e.,  is  known).
       A  client  is known if it has a host declaration	in any scope, not just
       the current scope.


       If specified, this statement either allows or prevents allocation  from
       this  pool  to  any  client  that has no	host declaration (i.e.,	is not

	members	of "class";

       If specified, this statement either allows or prevents allocation  from
       this pool to any	client that is a member	of the named class.

	dynamic	bootp clients;

       If  specified, this statement either allows or prevents allocation from
       this pool to any	bootp client.

	authenticated clients;

       If specified, this statement either allows or prevents allocation  from
       this  pool to any client	that has been authenticated using the DHCP au-
       thentication protocol.  This is not yet supported.

	unauthenticated	clients;

       If specified, this statement either allows or prevents allocation  from
       this  pool to any client	that has not been authenticated	using the DHCP
       authentication protocol.	 This is not yet supported.

	all clients;

       If specified, this statement either allows or prevents allocation  from
       this  pool  to  all clients.  This can be used when you want to write a
       pool declaration	for some reason, but hold it in	reserve, or  when  you
       want  to	 renumber  your	 network  quickly, and thus want the server to
       force all clients that have been	allocated addresses from this pool  to
       obtain new addresses immediately	when they next renew.

	after time;

       If  specified, this statement either allows or prevents allocation from
       this pool after a given date. This can be used when you	want  to  move
       clients	from one pool to another. The server adjusts the regular lease
       time so that the	latest expiry time is  at  the	given  time+min-lease-
       time.   A short min-lease-time enforces a step change, whereas a	longer
       min-lease-time allows for a gradual  change.   time  is	either	second
       since  epoch,  or  a  UTC  time string e.g.  4 2007/08/24 09:14:32 or a
       string with time	zone offset in	seconds	 e.g.  4  2007/08/24  11:14:32

       The abandon-lease-time statement

	 adandon-lease-time time;

	 Time  should be the maximum amount of time (in	seconds) that an aban-
	 doned IPv4 lease remains unavailable  for  assignment	to  a  client.
	 Abandoned leases will only be offered to clients if there are no free
	 leases.  If not defined, the default abandon lease time is 86400 sec-
	 onds  (24 hours).  Note the abandoned lease time for a	given lease is
	 preserved across server restarts.  The	parameter may only be  set  at
	 the global scope and is evaluated only	once during server startup.

	 Values	 less  than sixty seconds are not recommended as this is below
	 the ping check	threshold and can  cause  leases  once	abandoned  but
	 since	returned  to  the free state to	not be pinged before being of-
	 fered.	 If the	requested time is larger than 0x7FFFFFFF -  1  or  the
	 sum  of  the  current	time  plus  the	 abandoned time	isgreater than
	 0x7FFFFFFF it is treated as infinite.

       The adaptive-lease-time-threshold statement

	 adaptive-lease-time-threshold percentage;

	 When the number of allocated leases within a  pool  rises  above  the
	 percentage  given  in	this  statement, the DHCP server decreases the
	 lease length for new clients within this pool to min-lease-time  sec-
	 onds.	Clients	 renewing  an already valid (long) leases get at least
	 the remaining time from the current lease. Since  the	leases	expire
	 faster,  the server may either	recover	more quickly or	avoid pool ex-
	 haustion entirely.  Once the number of	allocated  leases  drop	 below
	 the  threshold, the server reverts back to normal lease times.	 Valid
	 percentages are between 1 and 99.

       The always-broadcast statement

	 always-broadcast flag;

	 The DHCP and BOOTP protocols both require DHCP	and BOOTP  clients  to
	 set the broadcast bit in the flags field of the BOOTP message header.
	 Unfortunately,	some DHCP and BOOTP clients do not do this, and	there-
	 fore may not receive responses	from the DHCP server.  The DHCP	server
	 can be	made to	always broadcast its responses to clients  by  setting
	 this  flag  to	 'on' for the relevant scope; relevant scopes would be
	 inside	a conditional statement, as a parameter	for a class, or	 as  a
	 parameter for a host declaration.  To avoid creating excess broadcast
	 traffic on your network, we recommend that you	restrict  the  use  of
	 this  option  to as few clients as possible.  For example, the	Micro-
	 soft DHCP client is known not to have this problem, as	are the	 Open-
	 Transport and ISC DHCP	clients.

       The always-reply-rfc1048	statement

	 always-reply-rfc1048 flag;

	 Some  BOOTP clients expect RFC1048-style responses, but do not	follow
	 RFC1048 when sending their requests.  You can tell that a  client  is
	 having	this problem if	it is not getting the options you have config-
	 ured for it and if you	see in	the  server  log  the  message	"(non-
	 rfc1048)" printed with	each BOOTREQUEST that is logged.

	 If you	want to	send rfc1048 options to	such a client, you can set the
	 always-reply-rfc1048 option in	that client's  host  declaration,  and
	 the  DHCP  server  will respond with an RFC-1048-style	vendor options
	 field.	 This flag can be set  in  any	scope,	and  will  affect  all
	 clients covered by that scope.

       The authoritative statement


	 not authoritative;

	 The  DHCP server will normally	assume that the	configuration informa-
	 tion about a given network segment is not known to be correct and  is
	 not  authoritative.   This is so that if a naive user installs	a DHCP
	 server	not fully understanding	how to configure it, it	does not  send
	 spurious  DHCPNAK  messages  to  clients that have obtained addresses
	 from a	legitimate DHCP	server on the network.

	 Network administrators	setting	 up  authoritative  DHCP  servers  for
	 their networks	should always write authoritative; at the top of their
	 configuration file to indicate	that the DHCP server should send DHCP-
	 NAK  messages to misconfigured	clients.  If this is not done, clients
	 will be unable	to get a correct IP address after changing subnets un-
	 til their old lease has expired, which	could take quite a long	time.

	 Usually,  writing  authoritative; at the top level of the file	should
	 be sufficient.	 However, if a DHCP server is to be set	up so that  it
	 is aware of some networks for which it	is authoritative and some net-
	 works for which it is not, it may be more appropriate to declare  au-
	 thority on a per-network-segment basis.

	 Note  that the	most specific scope for	which the concept of authority
	 makes any sense is the	physical network segment -  either  a  shared-
	 network  statement or a subnet	statement that is not contained	within
	 a shared-network statement.  It is not	meaningful to specify that the
	 server	is authoritative for some subnets within a shared network, but
	 not authoritative for others, nor is it meaningful  to	 specify  that
	 the  server  is authoritative for some	host declarations and not oth-

       The boot-unknown-clients	statement

	 boot-unknown-clients flag;

	 If the	boot-unknown-clients statement is present and has a  value  of
	 false	or  off,  then	clients	for which there	is no host declaration
	 will not be allowed to	obtain IP addresses.  If this statement	is not
	 present  or has a value of true or on,	then clients without host dec-
	 larations will	be allowed to obtain IP	addresses, as  long  as	 those
	 addresses  are	 not  restricted  by  allow and	deny statements	within
	 their pool declarations.

       The db-time-format statement

	 db-time-format	[ default | local ] ;

	 The DHCP server software  outputs  several  timestamps	 when  writing
	 leases	 to  persistent	storage.  This configuration parameter selects
	 one of	two output formats.  The default format	prints the day,	 date,
	 and  time  in	UTC, while the local format prints the system seconds-
	 since-epoch, and helpfully provides the day and time  in  the	system
	 timezone  in  a comment.  The time formats are	described in detail in
	 the dhcpd.leases(5) manpage.

       The ddns-hostname statement

	 ddns-hostname name;

	 The name parameter should be the hostname that	will be	used  in  set-
	 ting up the client's A	and PTR	records.  If no	ddns-hostname is spec-
	 ified in scope, then the server will derive  the  hostname  automati-
	 cally,	 using	an algorithm that varies for each of the different up-
	 date methods.

       The ddns-domainname statement

	 ddns-domainname name;

	 The name parameter should be the domain name that will	be appended to
	 the client's hostname to form a fully-qualified domain-name (FQDN).

       The dns-local-address4 and dns-local-address6 statements

	 ddns-local-address4 address;

	 ddns-local-address6 address;

	 The  address  parameter  should be the	local IPv4 or IPv6 address the
	 server	should use as the from address when sending  DDNS  update  re-

       The ddns-rev-domainname statement

	 ddns-rev-domainname name;

	 The name parameter should be the domain name that will	be appended to
	 the client's reversed IP address to produce a name  for  use  in  the
	 client's  PTR	record.	  By default, this is "", but the
	 default can be	overridden here.

	 The reversed IP address to which this domain name is appended is  al-
	 ways  the IP address of the client, in	dotted quad notation, reversed
	 -  for	 example,  if  the  IP	address	 assigned  to  the  client  is,  then  the  reversed  IP	address	 is  So a
	 client	with that IP address would, by default,	be given a PTR	record

       The ddns-update-style parameter

	 ddns-update-style style;

	 The  style  parameter	must be	one of standard, interim or none.  The
	 ddns-update-style statement is	only meaningful	in the outer  scope  -
	 it  is	 evaluated once	after reading the dhcpd.conf file, rather than
	 each time a client is assigned	an IP address, so there	is no  way  to
	 use different DNS update styles for different clients.	The default is

       The ddns-updates	statement

	  ddns-updates flag;

	 The ddns-updates parameter controls whether or	not  the  server  will
	 attempt  to  do  a DNS	update when a lease is confirmed.  Set this to
	 off if	the server should not attempt to do updates within  a  certain
	 scope.	  The ddns-updates parameter is	on by default.	To disable DNS
	 updates in all	scopes,	it is preferable to use	the  ddns-update-style
	 statement, setting the	style to none.

       The default-lease-time statement

	 default-lease-time time;

	 Time should be	the length in seconds that will	be assigned to a lease
	 if the	client requesting the lease does not ask for a specific	 expi-
	 ration	 time.	 This is used for both DHCPv4 and DHCPv6 leases	(it is
	 also known as the "valid lifetime" in DHCPv6).	 The default is	 43200

       The delayed-ack and max-ack-delay statements

	 delayed-ack count;

	 max-ack-delay microseconds;

	 Count should be an integer value from zero to 2^16-1, and defaults to
	 28.  The count	represents how many DHCPv4  replies  maximum  will  be
	 queued	 pending transmission until after a database commit event.  If
	 this number is	reached, a database commit event  (commonly  resulting
	 in  fsync() and representing a	performance penalty) will be made, and
	 the reply packets will	be transmitted in a  batch  afterwards.	  This
	 preserves  the	 RFC2131  direction  that  "stable storage" be updated
	 prior to replying to clients.	Should the  DHCPv4  sockets  "go  dry"
	 (select()  returns  immediately  with no read sockets), the commit is
	 made and any queued packets are transmitted.

	 Similarly, microseconds indicates how many microseconds are permitted
	 to  pass  inbetween queuing a packet pending an fsync,	and performing
	 the fsync.  Valid values range	from 0	to  2^32-1,  and  defaults  to
	 250,000 (1/4 of a second).

	 The  delayed-ack  feature  is not compiled in by default, but must be
	 enabled at  compile  time  with  './configure	--enable-delayed-ack'.
	 While	we no longer consider it experimental and we don't know	of any
	 issues	with it, in order to minimize problems with existing  configu-
	 ration	files we have left it disabled by default.

       The dhcp-cache-threshold	statement

	 dhcp-cache-threshold percentage;

	 The  dhcp-cache-threshold  statement takes one	integer	parameter with
	 allowed values	between	0 and 100. The default value is	25 (25%	of the
	 lease	time).	This  parameter	 expresses the percentage of the total
	 lease time, measured from the beginning, during which a client's  at-
	 tempt	to renew its lease will	result in getting the already assigned
	 lease,	rather than an extended	lease.

	 Clients that attempt renewal frequently can cause the server  to  up-
	 date and write	the database frequently	resulting in a performance im-
	 pact on the server.  The dhcp-cache-threshold statement instructs the
	 DHCP  server  to  avoid  updating leases too frequently thus avoiding
	 this behavior.	 Instead the  server  assigns  the  same  lease	 (i.e.
	 reuses	 it) with no modifications except for CLTT (Client Last	Trans-
	 mission Time) which does not require disk  operations.	 This  feature
	 applies to IPv4 only.

	 When  an  existing  lease is matched to a renewing client, it will be
	 reused	if all of the following	conditions are true:
	     1.	The dhcp-cache-threshold is larger than	zero
	     2.	The current lease is active
	     3.	The percentage of the lease time that has elapsed is less than
	     4.	The client information provided	in the renewal does not	alter
	     any of the	following:
		a. DNS information and DNS updates are enabled
		b. Billing class to which the lease is associated
		c. The host declaration	associated with	the lease
		d. The client id - this	may happen if a	client boots without
		a client id and	then starts using one in subsequent requests.

	 Note that the lease can be reused if the options the client or	 relay
	 agent	sends  are changed.  These changes will	not be recorded	in the
	 in-memory or on-disk databases	until  the  client  renews  after  the
	 threshold time	is reached.

       The do-forward-updates statement

	 do-forward-updates flag;

	 The  do-forward-updates  statement  instructs	the  DHCP server as to
	 whether it should attempt to update a DHCP client's A record when the
	 client	 acquires or renews a lease.  This statement has no effect un-
	 less DNS updates are enabled.	Forward	updates	 are  enabled  by  de-
	 fault.	  If  this  statement  is used to disable forward updates, the
	 DHCP server will never	attempt	to update the client's A  record,  and
	 will  only  ever  attempt  to	update	the client's PTR record	if the
	 client	supplies an FQDN that should be	placed in the PTR record using
	 the  fqdn  option.   If  forward updates are enabled, the DHCP	server
	 will still honor the setting of the client-updates flag.

       The dont-use-fsync statement

	 dont-use-fsync	flag;

	 The dont-use-fsync statement instructs	the DHCP server	if  it	should
	 call  fsync()	when writing leases to the lease file.	By default and
	 if the	flag is	set to false the server	will call fsync().   Suppress-
	 ing  the  call	 to fsync() may	increase the performance of the	server
	 but it	also adds a risk that a	lease will not be properly written  to
	 the  disk  after it has been issued to	a client and before the	server
	 stops.	 This can lead to duplicate leases being issued	 to  different
	 clients.  Using this option is	not recommended.

       The dynamic-bootp-lease-cutoff statement

	 dynamic-bootp-lease-cutoff date;

	 The dynamic-bootp-lease-cutoff	statement sets the ending time for all
	 leases	assigned dynamically to	BOOTP clients.	Because	BOOTP  clients
	 do  not  have	any  way of renewing leases, and don't know that their
	 leases	could expire, by default dhcpd assigns infinite	leases to  all
	 BOOTP	clients.  However, it may make sense in	some situations	to set
	 a cutoff date for all BOOTP leases - for example, the end of a	school
	 term, or the time at night when a facility is closed and all machines
	 are required to be powered off.

	 Date should be	the date on which all assigned BOOTP leases will  end.
	 The date is specified in the form:


	 W  is the day of the week expressed as	a number from zero (Sunday) to
	 six (Saturday).  YYYY is the year, including the century.  MM is  the
	 month	expressed  as  a  number  from	1 to 12.  DD is	the day	of the
	 month,	counting from 1.  HH is	the hour, from zero to 23.  MM is  the
	 minute	 and SS	is the second.	The time is always in Coordinated Uni-
	 versal	Time (UTC), not	local time.

       The dynamic-bootp-lease-length statement

	 dynamic-bootp-lease-length length;

	 The dynamic-bootp-lease-length	statement is used to set the length of
	 leases	 dynamically assigned to BOOTP clients.	 At some sites,	it may
	 be possible to	assume that a lease is no longer in use	if its	holder
	 has  not  used	BOOTP or DHCP to get its address within	a certain time
	 period.  The period is	specified in length as a  number  of  seconds.
	 If  a client reboots using BOOTP during the timeout period, the lease
	 duration is reset to length, so a BOOTP client	that boots  frequently
	 enough	 will  never  lose its lease.  Needless	to say,	this parameter
	 should	be adjusted with extreme caution.

       The echo-client-id statement

	 echo-client-id	flag;

	 The echo-client-id statement is used to enable	or  disable  RFC  6842
	 compliant  behavior.	If the echo-client-id statement	is present and
	 has a value of	true or	on, and	a DHCP DISCOVER	or REQUEST is received
	 which	contains  the  client  identifier option (Option code 61), the
	 server	will copy the option into its response (DHCP ACK or  NAK)  per
	 RFC  6842.  In	other words if the client sends	the option it will re-
	 ceive it back.	By default, this flag is off  and  client  identifiers
	 will not echoed back to the client.

       The filename statement

	 filename "filename";

	 The filename statement	can be used to specify the name	of the initial
	 boot file which is to be loaded by a client.  The filename should  be
	 a filename recognizable to whatever file transfer protocol the	client
	 can be	expected to use	to load	the file.

       The fixed-address declaration

	 fixed-address address [, address ... ];

	 The fixed-address declaration is used to assign one or	more fixed  IP
	 addresses  to a client.  It should only appear	in a host declaration.
	 If more than one address is supplied, then when the client boots,  it
	 will be assigned the address that corresponds to the network on which
	 it is booting.	 If none of the	addresses in the fixed-address	state-
	 ment are valid	for the	network	to which the client is connected, that
	 client	will not match the host	declaration containing that  fixed-ad-
	 dress	declaration.   Each  address  in the fixed-address declaration
	 should	be either an IP	address	or a domain name that resolves to  one
	 or more IP addresses.

       The fixed-address6 declaration

	 fixed-address6	ip6-address ;

	 The  fixed-address6  declaration  is  used to assign a	fixed IPv6 ad-
	 dresses to a client.  It should only appear in	a host declaration.

       The fixed-prefix6 declaration

	 fixed-prefix6 low-address / bits;

	 The fixed-prefix6 declaration is used to assign a fixed  IPv6	prefix
	 to a client.  It should only appear in	a host declaration, but	multi-
	 ple fixed-prefix6 statements may appear in a single host declaration.

	 The low-address specifies the start of	the prefix and the bits	speci-
	 fies the size of the prefix in	bits.

	 If there are multiple prefixes	for a given host entry the server will
	 choose	one that matches the requested prefix size or, if none	match,
	 the first one.

	 If there are multiple host declarations the server will try to	choose
	 a declaration where the fixed-address6	matches	the  client's  subnet.
	 If  none  match it will choose	one that doesn't have a	fixed-address6

	 Note Well: Unlike the fixed address the fixed prefix does not need to
	 match	a  subnet in order to be served.  This allows you to provide a
	 prefix	to a client that is outside of the subnet on which the	client
	 makes the request to the the server.

       The get-lease-hostnames statement

	 get-lease-hostnames flag;

	 The  get-lease-hostnames  statement  is used to tell dhcpd whether or
	 not to	look up	the domain name	corresponding to  the  IP  address  of
	 each  address	in  the	 lease	pool and use that address for the DHCP
	 hostname option.  If flag is true, then this lookup is	done  for  all
	 addresses  in the current scope.  By default, or if flag is false, no
	 lookups are done.

       The hardware statement

	 hardware hardware-type	hardware-address;

	 In order for a	BOOTP client to	be recognized,	its  network  hardware
	 address  must	be declared using a hardware clause in the host	state-
	 ment.	hardware-type must be the name of a physical  hardware	inter-
	 face  type.   Currently,  only	 the ethernet and token-ring types are
	 recognized, although support for a fddi hardware  type	 (and  others)
	 would	also  be  desirable.   The hardware-address should be a	set of
	 hexadecimal octets (numbers from 0 through ff)	separated  by  colons.
	 The hardware statement	may also be used for DHCP clients.

       The host-identifier option statement

	 host-identifier option	option-name option-data;


	 host-identifier v6relopt number option-name option-data;

	 This  identifies a DHCPv6 client in a host statement.	option-name is
	 any option, and option-data is	the value  for	the  option  that  the
	 client	 will  send. The option-data must be a constant	value.	In the
	 v6relopts case	the additional number is the relay to examine for  the
	 specified  option name	and value.  The	values are the same as for the
	 v6relay option.  0 is a no-op,	1 is the relay closest to the  client,
	 2 the next one	in and so on.  Values that are larger than the maximum
	 number	of relays (currently 32) indicate the  relay  closest  to  the
	 server	independent of number.

       The ignore-client-uids statement

	 ignore-client-uids flag;

	 If  the  ignore-client-uids  statement	 is present and	has a value of
	 true or on, the UID for clients will not be recorded.	If this	state-
	 ment  is not present or has a value of	false or off, then client UIDs
	 will be recorded.

       The infinite-is-reserved	statement

	 infinite-is-reserved flag;

	 ISC DHCP now supports 'reserved' leases.  See the section on RESERVED
	 LEASES	 below.	 If this flag is on, the server	will automatically re-
	 serve	leases	allocated  to  clients	which  requested  an  infinite
	 (0xffffffff) lease-time.

	 The default is	off.

       The lease-file-name statement

	 lease-file-name name;

	 Name should be	the name of the	DHCP server's lease file.  By default,
	 this is DBDIR/dhcpd.leases.  This statement must appear in the	 outer
	 scope	of the configuration file - if it appears in some other	scope,
	 it will have no effect.  Furthermore, it has no effect	if  overridden
	 by the	-lf flag or the	PATH_DHCPD_DB environment variable.

       The limit-addrs-per-ia statement

	 limit-addrs-per-ia number;

	 By default, the DHCPv6	server will limit clients to one IAADDR	per IA
	 option, meaning one address.  If you wish to permit clients  to  hang
	 onto multiple addresses at a time, configure a	larger number here.

	 Note  that  there  is	no  present  method to configure the server to
	 forcibly configure the	client with one	IP address per each subnet  on
	 a shared network.  This is left to future work.

       The dhcpv6-lease-file-name statement

	 dhcpv6-lease-file-name	name;

	 Name  is  the name of the lease file to use if	and only if the	server
	 is running in DHCPv6 mode.  By	default, this is  DBDIR/dhcpd6.leases.
	 This  statement, like lease-file-name,	must appear in the outer scope
	 of the	configuration file.  It	has no effect if overridden by the -lf
	 flag  or  the	PATH_DHCPD6_DB environment variable.  If dhcpv6-lease-
	 file-name is not specified, but lease-file-name is, the latter	 value
	 will be used.

       The lease-id-format parameter

	 lease-id-format format;

	 The  format  parameter	 must  be either octal or hex.	This parameter
	 governs the format used to write certain values to lease files.  With
	 the  default  format,	octal, values are written as quoted strings in
	 which non-printable characters	are represented	as octal escapes  -  a
	 backslash  character  followed	 by  three octal digits.  When the hex
	 format	is specified, values are written  as  an  unquoted  series  of
	 pairs of hexadecimal digits, separated	by colons.

	 Currently,  the  values  written out based on lease-id-format are the
	 server-duid, the uid  (DHCPv4	leases),  and  the  IAID_DUID  (DHCPv6
	 leases).   Note  the  server automatically reads the values in	either

       The local-port statement

	 local-port port;

	 This statement	causes the DHCP	server to listen for DHCP requests  on
	 the UDP port specified	in port, rather	than on	port 67.

       The local-address statement

	 local-address address;

	 This  statement  causes  the  DHCP server to listen for DHCP requests
	 sent to the specified address,	rather than requests sent to  all  ad-
	 dresses.   Since  serving directly attached DHCP clients implies that
	 the server must respond to requests sent to the all-ones IP  address,
	 this  option  cannot be used if clients are on	directly attached net-
	 works;	it is only  realistically  useful  for	a  server  whose  only
	 clients are reached via unicasts, such	as via DHCP relay agents.

	 Note:	 This  statement  is only effective if the server was compiled
	 using the USE_SOCKETS #define statement, which	is default on a	 small
	 number	 of  operating	systems, and must be explicitly	chosen at com-
	 pile-time for all others.  You	can be sure if your server is compiled
	 with USE_SOCKETS if you see lines of this format at startup:

	  Listening on Socket/eth0

	 Note  also  that since	this bind()s all DHCP sockets to the specified
	 address, that only one	address	may be supported  in  a	 daemon	 at  a
	 given time.

       The log-facility	statement

	 log-facility facility;

	 This statement	causes the DHCP	server to do all of its	logging	on the
	 specified log facility	once the dhcpd.conf file has  been  read.   By
	 default  the  DHCP  server logs to the	daemon facility.  Possible log
	 facilities include auth, authpriv,  cron,  daemon,  ftp,  kern,  lpr,
	 mail,	mark,  news,  ntp,  security,  syslog,	user, uucp, and	local0
	 through local7.  Not all of these facilities  are  available  on  all
	 systems,  and	there  may be other facilities available on other sys-

	 In addition to	setting	this value, you	may need to modify  your  sys-
	 log.conf  file	to configure logging of	the DHCP server.  For example,
	 you might add a line like this:

	      local7.debug /var/log/dhcpd.log

	 The syntax of the syslog.conf file may	be different on	some operating
	 systems  -  consult  the  syslog.conf manual page to be sure.	To get
	 syslog	to start logging to the	new file, you must  first  create  the
	 file  with correct ownership and permissions (usually,	the same owner
	 and permissions of your /var/log/messages or  /usr/adm/messages  file
	 should	 be  fine) and send a SIGHUP to	syslogd.  Some systems support
	 log rollover using a shell script  or	program	 called	 newsyslog  or
	 logrotate, and	you may	be able	to configure this as well so that your
	 log file doesn't grow uncontrollably.

	 Because the log-facility setting  is  controlled  by  the  dhcpd.conf
	 file,	log  messages printed while parsing the	dhcpd.conf file	or be-
	 fore parsing it are logged to the default log facility.   To  prevent
	 this,	see the	README file included with this distribution, which de-
	 scribes BUG: where is that mentioned in README?  how  to  change  the
	 default  log  facility.  When this parameter is used, the DHCP	server
	 prints	its startup message a second time after	parsing	the configura-
	 tion file, so that the	log will be as complete	as possible.

       The log-threshold-high and log-threshold-low statements

	 log-threshold-high percentage;

	 log-threshold-low percentage;

	 The  log-threshold-low	 and log-threshold-high	statements are used to
	 control when a	message	is output about	pool  usage.   The  value  for
	 both  of  them	 is  the  percentage  of the pool in use.  If the high
	 threshold is 0	or has not been	specified, no messages	will  be  pro-
	 duced.	  If  a	 high threshold	is given, a message is output once the
	 pool usage passes that	level.	After that, no more messages  will  be
	 output	 until	the  pool usage	falls below the	low threshold.	If the
	 low threshold is not given, it	default	to a value of zero.

	 A special case	occurs when the	low threshold is set to	be higer  than
	 the  high  threshold.	In this	case, a	message	will be	generated each
	 time a	lease is acknowledged when the pool usage is  above  the  high

	 Note that threshold logging will be automatically disabled for	shared
	 subnets whose total number of addresses is larger than	(2^64)-1.  The
	 server	will emit a log	statement at startup when threshold logging is
	 disabled as shown below:

	     "Threshold	logging	disabled for shared  subnet  of	 ranges:  <ad-

	 This  is  likely  to have no practical	runtime	effect as CPUs are un-
	 likely	to support a server actually reaching such a large  number  of

       The max-lease-time statement

	 max-lease-time	time;

	 Time should be	the maximum length in seconds that will	be assigned to
	 a lease.  If not defined, the default maximum lease  time  is	86400.
	 The only exception to this is that Dynamic BOOTP lease	lengths, which
	 are not specified by the client, are not limited by this maximum.

       The min-lease-time statement

	 min-lease-time	time;

	 Time should be	the minimum length in seconds that will	be assigned to
	 a  lease.   The  default  is the minimum of 300 seconds or max-lease-

       The min-secs statement

	 min-secs seconds;

	 Seconds should	be the minimum number of seconds since a client	 began
	 trying	 to acquire a new lease	before the DHCP	server will respond to
	 its request.  The number of seconds is	based on what the  client  re-
	 ports,	 and  the maximum value	that the client	can report is 255 sec-
	 onds.	Generally, setting this	to one will result in the DHCP	server
	 not  responding  to the client's first	request, but always responding
	 to its	second request.

	 This can be used to set up a secondary	DHCP server which never	offers
	 an  address  to  a  client  until the primary server has been given a
	 chance	to do so.  If the primary server is down, the client will bind
	 to  the secondary server, but otherwise clients should	always bind to
	 the primary.  Note that this does not,	by itself,  permit  a  primary
	 server	and a secondary	server to share	a pool of dynamically-allocat-
	 able addresses.

       The next-server statement

	 next-server server-name;

	 The next-server statement is used to specify the host address of  the
	 server	 from  which  the initial boot file (specified in the filename
	 statement) is to be loaded.  Server-name should be a numeric  IP  ad-
	 dress or a domain name.

       The omapi-port statement

	 omapi-port port;

	 The  omapi-port  statement causes the DHCP server to listen for OMAPI
	 connections on	the specified port.  This statement is required	to en-
	 able  the  OMAPI  protocol,  which  is	used to	examine	and modify the
	 state of the DHCP server as it	is running.

       The one-lease-per-client	statement

	 one-lease-per-client flag;

	 If this flag is enabled, whenever a client sends a DHCPREQUEST	for  a
	 particular lease, the server will automatically free any other	leases
	 the client holds.  This presumes that when the	client sends a DHCPRE-
	 QUEST,	 it has	forgotten any lease not	mentioned in the DHCPREQUEST -
	 i.e., the client has only a single network interface and it does  not
	 remember leases it's holding on networks to which it is not currently
	 attached.  Neither of these assumptions are guaranteed	 or  provable,
	 so we urge caution in the use of this statement.

       The pid-file-name statement

	 pid-file-name name;

	 Name  should  be the name of the DHCP server's	process	ID file.  This
	 is the	file in	which the DHCP server's	process	ID is stored when  the
	 server	 starts.   By  default,	 this  is  RUNDIR/  Like the
	 lease-file-name statement, this statement must	appear	in  the	 outer
	 scope	of  the	configuration file.  It	has no effect if overridden by
	 the -pf flag or the PATH_DHCPD_PID environment	variable.

	 The dhcpv6-pid-file-name statement

	    dhcpv6-pid-file-name name;

	    Name is the	name of	the pid	file to	use if and only	if the	server
	    is	running	in DHCPv6 mode.	 By default, this is DBDIR/
	    This statement, like pid-file-name,	must appear in the outer scope
	    of	the configuration file.	 It has	no effect if overridden	by the
	    -pf	 flag  or  the	PATH_DHCPD6_PID	 environment   variable.    If
	    dhcpv6-pid-file-name  is  not specified, but pid-file-name is, the
	    latter value will be used.

	 The ping-check	statement

	    ping-check flag;

	    When the DHCP server is considering	dynamically allocating	an  IP
	    address  to	a client, it first sends an ICMP Echo request (a ping)
	    to the address being assigned.  It waits for a second, and	if  no
	    ICMP  Echo	response has been heard, it assigns the	address.  If a
	    response is	heard, the lease is abandoned, and the server does not
	    respond to the client.  The	lease will remain abandoned for	a min-
	    imum of abandon-lease-time seconds.

	    If a there are no free addressses but there	are abandoned  IP  ad-
	    dresses,  the  DHCP	server will attempt to reclaim an abandoned IP
	    address regardless of the value of abandon-lease-time.

	    This ping check introduces a default one-second delay in  respond-
	    ing	 to  DHCPDISCOVER  messages,  which  can be a problem for some
	    clients.  The default delay	of one second may be configured	 using
	    the	 ping-timeout parameter.  The ping-check configuration parame-
	    ter	can be used to control checking	- if its value	is  false,  no
	    ping check is done.

	 The ping-timeout statement

	    ping-timeout seconds;

	    If	the DHCP server	determined it should send an ICMP echo request
	    (a ping) because the ping-check statement  is  true,  ping-timeout
	    allows  you	 to  configure how many	seconds	the DHCP server	should
	    wait for an	ICMP Echo response to be heard,	if no  ICMP  Echo  re-
	    sponse  has	 been  received	before the timeout expires, it assigns
	    the	address.  If a response	is heard, the lease is abandoned,  and
	    the	 server	 does  not respond to the client.  If no value is set,
	    ping-timeout defaults to 1 second.

	 The preferred-lifetime	statement

	    preferred-lifetime seconds;

	    IPv6 addresses have	'valid'	and 'preferred'	lifetimes.  The	 valid
	    lifetime  determines  at what point	at lease might be said to have
	    expired, and is no longer useable.	A preferred lifetime is	an ad-
	    visory  condition to help applications move	off of the address and
	    onto currently valid addresses (should there still be any open TCP
	    sockets or similar).

	    The	preferred lifetime defaults to 5/8 the default lease time.

	 The prefix-length-mode	statement

	    prefix-length-mode mode;

	    According to RFC 3633, DHCPv6 clients may specify preferences when
	    soliciting prefixes	by including an	IA_PD Prefix option within the
	    IA_PD  option.  Among  the preferences that	may be conveyed	is the
	    "prefix-length". When non-zero it  indicates  a  client's  desired
	    length  for	 offered  prefixes.   The RFC states that servers "MAY
	    choose to use the select prefix(es)" but does not
	    specify  any particular rules for doing so.	The prefix-length-mode
	    statement can be used to set the prefix selection  rules  employed
	    by	the  server, when clients send a non-zero prefix-length	value.
	    The	mode parameter must be one of ignore, prefer, exact,  minimum,
	    or maximum where:

	    1. ignore -	The requested length is	ignored. The server will offer
	    the	first available	prefix.

	    2. prefer -	The server will	offer the first	available prefix  with
	    the	 same  length as the requested length.	If none	are found then
	    it will offer the first available prefix of	any length.

	    3. exact - The server will offer the first available  prefix  with
	    the	 same  length  as the requested	length.	 If none are found, it
	    will return	a status indicating no prefixes	 available.   This  is
	    the	default	behavior.

	    4. minimum - The server will offer the first available prefix with
	    the	same length as the requested length.  If none  are  found,  it
	    will  return  the  first  available	prefix whose length is greater
	    than (e.g. longer than), the requested value.  If  none  of	 those
	    are	 found,	 it will return	a status indicating no prefixes	avail-
	    able.  For example,	if client requests a length of	/60,  and  the
	    server  has	available prefixes of lengths /56 and /64, it will of-
	    fer	prefix of length /64.

	    5. maximum - The server will offer the first available prefix with
	    the	 same  length  as the requested	length.	 If none are found, it
	    will return	the first available prefix whose length	is  less  than
	    (e.g.  shorter  than),  the	requested value.  If none of those are
	    found, it will return a status indicating no  prefixes  available.
	    For	 example,  if  client requests a length	of /60,	and the	server
	    has	available prefixes of lengths /56 and /64,  it	will  offer  a
	    prefix of length /56.

	    In	general	 "first	available" is determined by the	order in which
	    pools are defined in the server's configuration.  For example,  if
	    a subnet is	defined	with three prefix pools	A,B, and C:

	    subnet 3000::/64 {
		 # pool	A
		 pool6 {
		 # pool	B
		 pool6 {
		 # pool	C
		 pool6 {

	    then  the  pools  will  be checked in the order A, B, C. For modes
	    prefer, minimum, and maximum this may mean checking	the  pools  in
	    that  order	 twice.	  A  first pass	through	is made	looking	for an
	    available prefix of	exactly	the preferred  length.	 If  none  are
	    found,  then  a  second pass is performed starting with pool A but
	    with appropriately adjusted	length criteria.

	 The remote-port statement

	    remote-port	port;

	    This statement causes the DHCP server to transmit  DHCP  responses
	    to	DHCP  clients upon the UDP port	specified in port, rather than
	    on port 68.	 In the	event that the UDP response is transmitted  to
	    a  DHCP Relay, the server generally	uses the local-port configura-
	    tion value.	 Should	the DHCP  Relay	 happen	 to  be	 addressed  as,	however, the DHCP Server transmits its response	to the
	    remote-port	configuration value.  This is  generally  only	useful
	    for	 testing  purposes, and	this configuration value should	gener-
	    ally not be	used.

	 The server-identifier statement

	    server-identifier hostname;

	    The	server-identifier statement can	be used	to  define  the	 value
	    that  is  sent  in	the  DHCP Server Identifier option for a given
	    scope.  The	value specified	must be	an IP  address	for  the  DHCP
	    server,  and must be reachable by all clients served by a particu-
	    lar	scope.

	    The	use of the server-identifier statement is  not	recommended  -
	    the	 only  reason to use it	is to force a value other than the de-
	    fault value	to be sent on occasions	where the default value	 would
	    be	incorrect.   The default value is the first IP address associ-
	    ated with the physical network interface on	which the request  ar-

	    The	 usual	case where the server-identifier statement needs to be
	    sent is when a physical interface has more than  one  IP  address,
	    and	 the  one  being sent by default isn't appropriate for some or
	    all	clients	served by that interface.  Another common case is when
	    an	alias is defined for the purpose of having a consistent	IP ad-
	    dress for the DHCP server, and it is desired that the clients  use
	    this IP address when contacting the	server.

	    Supplying a	value for the dhcp-server-identifier option is equiva-
	    lent to using the server-identifier	statement.

	 The server-id-check statement

	    server-id-check flag;

	    The	server-id-check	statement is used to control whether or	not  a
	    server,  participating in failover,	verifies that the value	of the
	    dhcp-server-identifier option in received DHCP REQUESTs match  the
	    server's  id  before processing the	request. Server	id checking is
	    disabled by	default.  Setting this flag enables  id	 checking  and
	    thereafter the server will only process requests that match.  Note
	    the	flag setting should be consistent between failover partners.

	    Unless overridden by use of	the server-identifier  statement,  the
	    value the server uses as its id will be the	first IP address asso-
	    ciated with	the physical network interface on  which  the  request

	    In	order  to reduce runtime overhead the server only checks for a
	    server id option in	the global  and	 subnet	 scopes.   Complicated
	    configurations  may	 result	in different server ids	for this check
	    and	when the server	id for a reply	packet	is  determined,	 which
	    would prohibit the server from responding.

	    The	 primary use for this option is	when a client broadcasts a re-
	    quest but requires that the	response come from a specific failover
	    peer.  An example of this would be when a client reboots while its
	    lease is still active - in this case both  servers	will  normally
	    respond.   Most  of	 the time the client won't check the server id
	    and	can use	either of the responses.  However if the  client  does
	    check the server id	it may reject the response if it came from the
	    wrong peer.	 If the	timing is such that the	"wrong"	peer  responds
	    first  most	of the time the	client may not get an address for some

	    Care should	be taken before	enabling this option.

	 The server-duid statement

	    server-duid	LLT [ hardware-type timestamp hardware-address ] ;

	    server-duid	EN enterprise-number enterprise-identifier ;

	    server-duid	LL [ hardware-type hardware-address ] ;

	    The	server-duid statement configures the server DUID. You may pick
	    either  LLT	(link local address plus time),	EN (enterprise), or LL
	    (link local).

	    If you choose LLT or LL, you may specify the exact contents	of the
	    DUID.   Otherwise the server will generate a DUID of the specified

	    If you choose EN, you must include the enterprise number  and  the

	    If there is	a server-duid statement	in the lease file it will take
	    precedence over the	server-duid statement from the config file and
	    a dhcp6.server-id option in	the config file	will override both.

	    The	default	server-duid type is LLT.

	 The server-name statement

	    server-name	name ;

	    The	 server-name statement can be used to inform the client	of the
	    name of the	server from which it is	booting.  Name should  be  the
	    name that will be provided to the client.

	 The dhcpv6-set-tee-times statement

	    dhcpv6-set-tee-times flag;

	    The	 dhcpv6-set-tee-times  statement  enables setting T1 and T2 to
	    the	values recommended in RFC 3315 (Section	22.4).	 When  setting
	    T1	and T2,	the server will	use dhcp-renewal-time and dhcp-rebind-
	    ing-time, respectively.  A value of	zero tells the client  it  may
	    choose its own value.

	    When those options are not defined then values will	be set to zero
	    unless the global dhcpv6-set-tee-timesis enabled.  When  this  op-
	    tion  is  enabled  the  times are calculated as recommended	by RFC
	    3315, Section 22.4:

		  T1 will be set to 0.5	times the shortest preferred lifetime
		  in the reply.	 If the	"shortest" preferred lifetime is
		  0xFFFFFFFF,  T1 will set to 0xFFFFFFFF.

		  T2 will be set to 0.8	times the shortest preferred lifetime
		  in the reply.	 If the	"shortest" preferred lifetime is
		  0xFFFFFFFF,  T2 will set to 0xFFFFFFFF.

	    Keep in mind that given sufficiently small	lease  lifetimes,  the
	    above  calculations	will result in the two values being equal. For
	    example, a 9 second	lease lifetime would yield T1 =	T2  =  4  sec-
	    onds,  which would cause clients to	issue rebinds only.  In	such a
	    case it would likely be better to explicitly define	the values.

	    Note that dhcpv6-set-tee-times is intended to be transitional  and
	    will  likely  be removed in	a future release. Once removed the be-
	    havior will	be to use the configured values	when present or	calcu-
	    late them per the RFC. If you want zeros, define them as zeros.

	 The site-option-space statement

	    site-option-space name ;

	    The	site-option-space statement can	be used	to determine from what
	    option space site-local options will be taken.  This can  be  used
	    in	much the same way as the vendor-option-space statement.	 Site-
	    local options in DHCP are those options whose  numeric  codes  are
	    greater  than  224.	  These	options	are intended for site-specific
	    uses, but are frequently used by vendors of	embedded hardware that
	    contains  DHCP  clients.   Because site-specific options are allo-
	    cated on an	ad hoc basis, it is quite possible that	 one  vendor's
	    DHCP  client  might	use the	same option code that another vendor's
	    client uses, for different purposes.  The site-option-space	option
	    can	be used	to assign a different set of site-specific options for
	    each such vendor, using conditional	evaluation (see	dhcp-eval  (5)
	    for	details).

	 The stash-agent-options statement

	    stash-agent-options	flag;

	    If	the  stash-agent-options parameter is true for a given client,
	    the	server will record the relay agent  information	 options  sent
	    during  the	 client's  initial DHCPREQUEST message when the	client
	    was	in the SELECTING state and behave as if	those options are  in-
	    cluded in all subsequent DHCPREQUEST messages sent in the RENEWING
	    state.  This works around a	problem	with relay  agent  information
	    options, which is that they	usually	not appear in DHCPREQUEST mes-
	    sages sent by the client in	the RENEWING state, because such  mes-
	    sages  are	unicast	 directly to the server	and not	sent through a
	    relay agent.

	 The update-conflict-detection statement

	    update-conflict-detection flag;

	    If the update-conflict-detection parameter	is  true,  the	server
	    will perform standard DHCID	multiple-client, one-name conflict de-
	    tection.  If the parameter has been	set  false,  the  server  will
	    skip this check and	instead	simply tear down any previous bindings
	    to install the new binding without question.  The default is true.

	 The update-optimization statement

	    update-optimization	flag;

	    If the update-optimization parameter is false for a	given  client,
	    the	server will attempt a DNS update for that client each time the
	    client renews its lease, rather than  only	attempting  an	update
	    when  it appears to	be necessary.  This will allow the DNS to heal
	    from database inconsistencies more easily, but the	cost  is  that
	    the	DHCP server must do many more DNS updates.  We recommend leav-
	    ing	this option enabled, which is the default. If  this  parameter
	    is	not  specified,	 or  is	true, the DHCP server will only	update
	    when the client information	changes, the client gets  a  different
	    lease, or the client's lease expires.

	 The update-static-leases statement

	    update-static-leases flag;

	    The	 update-static-leases flag, if enabled,	causes the DHCP	server
	    to do DNS updates for clients even if those	clients	are being  as-
	    signed their IP address using a fixed-address statement - that is,
	    the	client is being	given a	static assignment.  It is  not	recom-
	    mended  because the	DHCP server has	no way to tell that the	update
	    has	been done, and therefore will not delete the record when it is
	    not	 in  use.   Also, the server must attempt the update each time
	    the	client renews its lease, which could have a  significant  per-
	    formance  impact  in  environments that place heavy	demands	on the
	    DHCP server.

	 The use-host-decl-names statement

	    use-host-decl-names	flag;

	    If the use-host-decl-names parameter is true  in  a	 given	scope,
	    then  for  every host declaration within that scope, the name pro-
	    vided for the host declaration will	be supplied to the  client  as
	    its	hostname.  So, for example,

		group {
		  use-host-decl-names on;

		  host joe {
		    hardware ethernet 08:00:2b:4c:29:32;

	    is equivalent to

		  host joe {
		    hardware ethernet 08:00:2b:4c:29:32;
		    option host-name "joe";

	    Additionally, enabling use-host-decl-names instructs the server to
	    use	the host declaration name in the the forward DNS name,	if  no
	    other  values are available.  This value selection process is dis-
	    cussed in more detail under	DNS updates.

	    An option host-name	statement within a host	declaration will over-
	    ride the use of the	name in	the host declaration.

	    It	should	be noted here that most	DHCP clients completely	ignore
	    the	host-name option sent by the DHCP server, and there is no  way
	    to	configure them not to do this.	So you generally have a	choice
	    of either not having any hostname to  client  IP  address  mapping
	    that  the  client will recognize, or doing DNS updates.  It	is be-
	    yond the scope of this document to describe	how to make  this  de-

	 The use-lease-addr-for-default-route statement

	    use-lease-addr-for-default-route flag;

	    If	the  use-lease-addr-for-default-route  parameter  is true in a
	    given scope, then instead of sending the value  specified  in  the
	    routers option (or sending no value	at all), the IP	address	of the
	    lease being	assigned is  sent  to  the  client.   This  supposedly
	    causes  Win95  machines  to	ARP for	all IP addresses, which	can be
	    helpful if your router is configured for proxy ARP.	  The  use  of
	    this  feature  is  not recommended,	because	it won't work for many
	    DHCP clients.

	 The vendor-option-space statement

	    vendor-option-space	string;

	    The	vendor-option-space  parameter	determines  from  what	option
	    space vendor options are taken.  The use of	this configuration pa-
	    rameter is illustrated in the dhcp-options(5) manual page, in  the

       Sometimes it's helpful to be able to set	the value of a DHCP server pa-
       rameter based on	some value that	the client has sent.  To do this,  you
       can  use	expression evaluation.	The dhcp-eval(5) manual	page describes
       how to write expressions.  To assign the	result of an evaluation	to  an
       option, define the option as follows:

	 my-parameter =	expression ;

       For example:

	 ddns-hostname = binary-to-ascii (16, 8, "-",
					  substring (hardware, 1, 6));

       It's  often  useful to allocate a single	address	to a single client, in
       approximate perpetuity.	Host statements	with fixed-address clauses ex-
       ist  to a certain extent	to serve this purpose, but because host	state-
       ments are intended to approximate 'static configuration',  they	suffer
       from  not  being	referenced in a	littany	of other Server	Services, such
       as dynamic DNS, failover, 'on events' and so forth.

       If a standard dynamic lease, as from any	 range	statement,  is	marked
       'reserved', then	the server will	only allocate this lease to the	client
       it is identified	by (be that by client identifier or hardware address).

       In practice, this means that the	lease follows the normal state engine,
       enters  ACTIVE state when the client is bound to	it, expires, or	is re-
       leased, and any events or services that would normally be supplied dur-
       ing  these  events  are	processed  normally, as	with any other dynamic
       lease.  The only	difference is that  failover  servers  treat  reserved
       leases  as  special  when  they	enter the FREE or BACKUP states	- each
       server applies the lease	into the state it may allocate from - and  the
       leases  are  not	 placed	 on the	queue for allocation to	other clients.
       Instead they may	only be	'found'	by client  identity.   The  result  is
       that the	lease is only offered to the returning client.

       Care  should  probably  be taken	to ensure that the client only has one
       lease within a given subnet that	it is identified by.

       Leases may be set 'reserved' either through OMAPI, or through the  'in-
       finite-is-reserved' configuration option	(if this is applicable to your
       environment and mixture of clients).

       It should also be noted that leases marked 'reserved'  are  effectively
       treated the same	as leases marked 'bootp'.

       DHCP  option  statements	 are  documented in the	dhcp-options(5)	manual

       Expressions used	in DHCP	option statements and elsewhere	are documented
       in the dhcp-eval(5) manual page.

       dhcpd(8),   dhcpd.leases(5),  dhcp-options(5),  dhcp-eval(5),  RFC2132,

       dhcpd.conf(5) is	maintained by ISC.  Information	about Internet Systems
       Consortium can be found at



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