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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.

       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 index;

	 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 255,  of	 which
	 the most reasonable is	128.

       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 addded 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 from either the fqdn option (if present)	or the
       hostname	option (if present).  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  example	above,	"jschmoe"  instead  of

       If  the defaults	for choosing the host name are not appropriate you can
       write your own statement	to set the ddns-hostname variable as you wish.

       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 he	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

       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 scope in	which to provide configuration
       information  about a specific client, and also provides a way to	assign
       a client	a fixed	address.  The host declaration provides	a way for  the
       DHCP  server  to	identify a DHCP	or BOOTP client, and also a way	to as-
       sign the	client a static	IP address.

       If it is	desirable to be	able to	boot a DHCP or BOOTP  client  on  more
       than  one  subnet  with	fixed  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.

       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
       reclaim these leases, but while the client is running through the pool,
       it may cause serious thrashing in the DNS, and it will also  cause  the
       DHCP server to forget old DHCP client address allocations.

       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 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.   This  is
       only  relevant  when  doing interim DNS updates.	 See the documentation
       under the heading THE INTERIM 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 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).

	 Please	 note  that  as	delayed-ack is currently experimental, the de-
	 layed-ack feature is not compiled in by default, but must be  enabled
	 at compile time with './configure --enable-delayed-ack'.

       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  with  no
	 modifications	except	for CLTT (Client Last Transmission Time) which
	 does not require disk operations. This	feature	applies	to IPv4	only.

       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 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 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 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

       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.

	    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 the renew+rebind	timers,	or 3/4
	    the	default	lease time if none were	specified.

	 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-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

	    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 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.  This option only
	    affects the	behavior of the	interim	DNS update scheme, and has  no
	    effect  on the ad-hoc DNS update scheme.  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.  This can only work
	    with the interim DNS update	scheme.	 It is not recommended 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 re-
	    news its lease, which could	have a significant performance	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";

	    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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