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LIBALIAS(3)	       FreeBSD Library Functions Manual		   LIBALIAS(3)

     libalias -- packet	aliasing library for masquerading and network address

     #include <sys/types.h>
     #include <netinet/in.h>
     #include <alias.h>

     Function prototypes are given in the main body of the text.

     The libalias library is a collection of functions for aliasing and	de-
     aliasing of IP packets, intended for masquerading and network address
     translation (NAT).

     This library is a moderately portable set of functions designed to	assist
     in	the process of IP masquerading and network address translation.	 Out-
     going packets from	a local	network	with unregistered IP addresses can be
     aliased to	appear as if they came from an accessible IP address.  Incom-
     ing packets are then de-aliased so	that they are sent to the correct ma-
     chine on the local	network.

     A certain amount of flexibility is	built into the packet aliasing engine.
     In	the simplest mode of operation,	a many-to-one address mapping takes
     place between the local network and the packet aliasing host.  This is
     known as IP masquerading.	In addition, one-to-one	mappings between local
     and public	addresses can also be implemented, which is known as static
     NAT.  In between these extremes, different	groups of private addresses
     can be linked to different	public addresses, comprising several distinct
     many-to-one mappings.  Also, a given public address and port can be stat-
     ically redirected to a private address/port.

     One special function, LibAliasInit(), must	always be called before	any
     packet handling may be performed, and the returned	instance pointer must
     be	passed to all the other	functions.  Normally, the LibAliasSetAddress()
     function is called	afterwards, to set the default aliasing	address.  In
     addition, the operating mode of the packet	aliasing engine	can be custom-
     ized by calling LibAliasSetMode().

     struct libalias * LibAliasInit(struct libalias *)

	   This	function is used to initialize internal	data structures.  When
	   called the first time, a NULL pointer should	be passed as an	argu-
	   ment.  The following	mode bits are always set after calling
	   LibAliasInit().  See	the description	of LibAliasSetMode() below for
	   the meaning of these	mode bits.


	   This	function will always return the	packet aliasing	engine to the
	   same	initial	state.	The LibAliasSetAddress() function is normally
	   called afterwards, and any desired changes from the default mode
	   bits	listed above require a call to LibAliasSetMode().

	   It is mandatory that	this function be called	at the beginning of a
	   program prior to any	packet handling.

     void LibAliasUninit(struct	libalias *)

	   This	function has no	return value and is used to clear any re-
	   sources attached to internal	data structures.

	   This	function should	be called when a program stops using the
	   aliasing engine; amongst other things, it clears out	any firewall
	   holes.  To provide backwards	compatibility and extra	security, it
	   is added to the atexit(3) chain by LibAliasInit().

     void LibAliasSetAddress(struct libalias *,	struct in_addr addr)

	   This	function sets the source address to which outgoing packets
	   from	the local area network are aliased.  All outgoing packets are
	   re-mapped to	this address unless overridden by a static address
	   mapping established by LibAliasRedirectAddr().  If this function
	   has not been	called,	and no static rules match, an outgoing packet
	   retains its source address.

	   If the PKT_ALIAS_RESET_ON_ADDR_CHANGE mode bit is set (the default
	   mode	of operation), then the	internal aliasing link tables will be
	   reset any time the aliasing address changes.	 This is useful	for
	   interfaces such as ppp(8), where the	IP address may or may not
	   change on successive	dial-up	attempts.

	   If the PKT_ALIAS_RESET_ON_ADDR_CHANGE mode bit is set to zero, this
	   function can	also be	used to	dynamically change the aliasing	ad-
	   dress on a packet-to-packet basis (it is a low overhead call).

	   It is mandatory that	this function be called	prior to any packet

     unsigned int LibAliasSetMode(struct libalias *, unsigned int flags,
     unsigned int mask)

	   This	function sets or clears	mode bits according to the value of
	   flags.  Only	bits marked in mask are	affected.  The following mode
	   bits	are defined in <alias.h>:

		   Enables logging into	/var/log/alias.log.  Each time an
		   aliasing link is created or deleted,	the log	file is	ap-
		   pended to with the current number of	ICMP, TCP and UDP
		   links.  Mainly useful for debugging when the	log file is
		   viewed continuously with tail(1).

		   If this mode	bit is set, all	incoming packets associated
		   with	new TCP	connections or new UDP transactions will be
		   marked for being ignored (LibAliasIn() returns
		   PKT_ALIAS_IGNORED code) by the calling program.  Response
		   packets to connections or transactions initiated from the
		   packet aliasing host	or local network will be unaffected.
		   This	mode bit is useful for implementing a one-way fire-

		   If this mode	bit is set, the	packet-aliasing	engine will
		   attempt to leave the	alias port numbers unchanged from the
		   actual local	port numbers.  This can	be done	as long	as the
		   quintuple (proto, alias addr, alias port, remote addr, re-
		   mote	port) is unique.  If a conflict	exists,	a new aliasing
		   port	number is chosen even if this mode bit is set.

		   This	bit should be set when the packet aliasing host	origi-
		   nates network traffic as well as forwards it.  When the
		   packet aliasing host	is waiting for a connection from an
		   unknown host	address	or unknown port	number (e.g. an	FTP
		   data	connection), this mode bit specifies that a socket be
		   allocated as	a place	holder to prevent port conflicts.
		   Once	a connection is	established, usually within a minute
		   or so, the socket is	closed.

		   If this mode	bit is set, traffic on the local network which
		   does	not originate from unregistered	address	spaces will be
		   ignored.  Standard Class A, B and C unregistered addresses
		   are:	->   (Class A subnet)	->   (Class B subnets)	->  (Class C subnets)

		   This	option is useful in the	case that the packet aliasing
		   host	has both registered and	unregistered subnets on	dif-
		   ferent interfaces.  The registered subnet is	fully accessi-
		   ble to the outside world, so	traffic	from it	does not need
		   to be passed	through	the packet aliasing engine.

		   Like	PKT_ALIAS_UNREGISTERED_ONLY, but includes the RFC 6598
		   (Carrier Grade NAT) subnet as follows:	->  (RFC 6598 subnet)

		   When	this mode bit is set and LibAliasSetAddress() is
		   called to change the	aliasing address, the internal link
		   table of the	packet aliasing	engine will be cleared.	 This
		   operating mode is useful for	ppp(8) links where the inter-
		   face	address	can sometimes change or	remain the same	be-
		   tween dial-up attempts.  If this mode bit is	not set, the
		   link	table will never be reset in the event of an address

		   This	option makes libalias "punch holes" in an
		   ipfirewall(4) - based firewall for FTP/IRC DCC connections.
		   The holes punched are bound by from/to IP address and port;
		   it will not be possible to use a hole for another connec-
		   tion.  A hole is removed when the connection	that uses it
		   dies.  To cater to unexpected death of a program using
		   libalias (e.g. kill -9), changing the state of the flag
		   will	clear the entire firewall range	allocated for holes.
		   This	clearing will also happen on the initial call to
		   LibAliasSetFWBase(),	which must happen prior	to setting
		   this	flag.

		   This	option makes libalias reverse the way it handles in-
		   coming and outgoing packets,	allowing it to be fed with
		   data	that passes through the	internal interface rather than
		   the external	one.

		   This	option tells libalias to obey transparent proxy	rules
		   only.  Normal packet	aliasing is not	performed.  See
		   LibAliasProxyRule() below for details.

		   This	option is used by ipfw_nat only.  Specifying it	as a
		   flag	to LibAliasSetMode() has no effect.  See section
		   NETWORK ADDRESS TRANSLATION in ipfw(8) for more details.

     void LibAliasSetFWBase(struct libalias *, unsigned	int base, unsigned int

	   Set the firewall range allocated for	punching firewall holes	(with
	   the PKT_ALIAS_PUNCH_FW flag).  The range is cleared for all rules
	   on initialization.

     void LibAliasSkinnyPort(struct libalias *,	unsigned int port)

	   Set the TCP port used by the	Skinny Station protocol.  Skinny is
	   used	by Cisco IP phones to communicate with Cisco Call Managers to
	   set up voice	over IP	calls.	If this	is not set, Skinny aliasing
	   will	not be done.  The typical port used by Skinny is 2000.

     The packet	handling functions are used to modify incoming (remote to lo-
     cal) and outgoing (local to remote) packets.  The calling program is re-
     sponsible for receiving and sending packets via network interfaces.

     Along with	LibAliasInit() and LibAliasSetAddress(), the two packet	han-
     dling functions, LibAliasIn() and LibAliasOut(), comprise the minimal set
     of	functions needed for a basic IP	masquerading implementation.

     int LibAliasIn(struct libalias *, void *buffer, int maxpacketsize)

	   An incoming packet coming from a remote machine to the local	net-
	   work	is de-aliased by this function.	 The IP	packet is pointed to
	   by buffer, and maxpacketsize	indicates the size of the data struc-
	   ture	containing the packet and should be at least as	large as the
	   actual packet size.

	   Return codes:

		   The packet aliasing process was successful.

		   The packet was ignored and not de-aliased.  This can	happen
		   if the protocol is unrecognized, as for an ICMP message
		   type	that is	not handled, or	if incoming packets for	new
		   connections are being ignored (if the
		   PKT_ALIAS_DENY_INCOMING mode	bit was	set using

		   This	is returned when a fragment cannot be resolved because
		   the header fragment has not been sent yet.  In this situa-
		   tion, fragments must	be saved with LibAliasSaveFragment()
		   until a header fragment is found.

		   The packet aliasing process was successful, and a header
		   fragment was	found.	This is	a signal to retrieve any unre-
		   solved fragments with LibAliasGetFragment() and de-alias
		   them	with LibAliasFragmentIn().

		   An internal error within the	packet aliasing	engine oc-

     int LibAliasOut(struct libalias *,	void *buffer, int maxpacketsize)

	   An outgoing packet coming from the local network to a remote	ma-
	   chine is aliased by this function.  The IP packet is	pointed	to by
	   buffer, and maxpacketsize indicates the maximum packet size permis-
	   sible should	the packet length be changed.  IP encoding protocols
	   place address and port information in the encapsulated data stream
	   which has to	be modified and	can account for	changes	in packet
	   length.  Well known examples	of such	protocols are FTP and IRC DCC.

	   Return codes:

		   The packet aliasing process was successful.

		   The packet was ignored and not aliased.  This can happen if
		   the protocol	is unrecognized, or possibly an	ICMP message
		   type	is not handled.

		   An internal error within the	packet aliasing	engine oc-

     The functions described in	this section allow machines on the local net-
     work to be	accessible in some degree to new incoming connections from the
     external network.	Individual ports can be	re-mapped or static network
     address translations can be designated.

     struct alias_link * LibAliasRedirectPort(struct libalias *,
     struct in_addr local_addr,	u_short	local_port,
     struct in_addr remote_addr, u_short remote_port,
     struct in_addr alias_addr,	u_short	alias_port, u_char proto)

	   This	function specifies that	traffic	from a given remote ad-
	   dress/port to an alias address/port be redirected to	a specified
	   local address/port.	The parameter proto can	be either IPPROTO_TCP
	   or IPPROTO_UDP, as defined in <netinet/in.h>.

	   If local_addr or alias_addr is zero,	this indicates that the	packet
	   aliasing address as established by LibAliasSetAddress() is to be
	   used.  Even if LibAliasSetAddress() is called to change the address
	   after LibAliasRedirectPort()	is called, a zero reference will track
	   this	change.

	   If the link is further set up to operate with load sharing, then
	   local_addr and local_port are ignored, and are selected dynamically
	   from	the server pool, as described in LibAliasAddServer() below.

	   If remote_addr is zero, this	indicates to redirect packets from any
	   remote address.  Likewise, if remote_port is	zero, this indicates
	   to redirect packets originating from	any remote port	number.	 The
	   remote port specification will almost always	be zero, but non-zero
	   remote addresses can	sometimes be useful for	firewalling.  If two
	   calls to LibAliasRedirectPort() overlap in their address/port spec-
	   ifications, then the	most recent call will have precedence.

	   This	function returns a pointer which can subsequently be used by
	   LibAliasRedirectDelete().  If NULL is returned, then	the function
	   call	did not	complete successfully.

	   All port numbers should be in network address byte order, so	it is
	   necessary to	use htons(3) to	convert	these parameters from inter-
	   nally readable numbers to network byte order.  Addresses are	also
	   in network byte order, which	is implicit in the use of the struct
	   in_addr data	type.

     struct alias_link * LibAliasRedirectAddr(struct libalias *,
     struct in_addr local_addr,	struct in_addr alias_addr)

	   This	function designates that all incoming traffic to alias_addr be
	   redirected to local_addr.  Similarly, all outgoing traffic from
	   local_addr is aliased to alias_addr.

	   If local_addr or alias_addr is zero,	this indicates that the	packet
	   aliasing address as established by LibAliasSetAddress() is to be
	   used.  Even if LibAliasSetAddress() is called to change the address
	   after LibAliasRedirectAddr()	is called, a zero reference will track
	   this	change.

	   If the link is further set up to operate with load sharing, then
	   the local_addr argument is ignored, and is selected dynamically
	   from	the server pool, as described in LibAliasAddServer() below.

	   If subsequent calls to LibAliasRedirectAddr() use the same aliasing
	   address, all	new incoming traffic to	this aliasing address will be
	   redirected to the local address made	in the last function call.
	   New traffic generated by any	of the local machines, designated in
	   the several function	calls, will be aliased to the same address.
	   Consider the	following example:

	   LibAliasRedirectAddr(la, inet_aton(""),
	   LibAliasRedirectAddr(la, inet_aton(""),
	   LibAliasRedirectAddr(la, inet_aton(""),

	   Any outgoing	connections such as telnet(1) or ftp(1)	from, and will appear	to come	from  Any incoming connections to will
	   be directed to

	   Any calls to	LibAliasRedirectPort() will have precedence over ad-
	   dress mappings designated by	LibAliasRedirectAddr().

	   This	function returns a pointer which can subsequently be used by
	   LibAliasRedirectDelete().  If NULL is returned, then	the function
	   call	did not	complete successfully.

     int LibAliasAddServer(struct libalias *, struct alias_link	*link,
     struct in_addr addr, u_short port)

	   This	function sets the link up for Load Sharing using IP Network
	   Address Translation (RFC 2391, LSNAT).  LSNAT operates as follows.
	   A client attempts to	access a server	by using the server virtual
	   address.  The LSNAT router transparently redirects the request to
	   one of the hosts in the server pool,	using a	real-time load sharing
	   algorithm.  Multiple	sessions may be	initiated from the same
	   client, and each session could be directed to a different host
	   based on the	load balance across server pool	hosts when the ses-
	   sions are initiated.	 If load sharing is desired for	just a few
	   specific services, the configuration	on LSNAT could be defined to
	   restrict load sharing to just the services desired.

	   Currently, only the simplest	selection algorithm is implemented,
	   where a host	is selected on a round-robin basis only, without re-
	   gard	to load	on the host.

	   First, the link is created by either	LibAliasRedirectPort() or
	   LibAliasRedirectAddr().  Then, LibAliasAddServer() is called	multi-
	   ple times to	add entries to the link's server pool.

	   For links created with LibAliasRedirectAddr(), the port argument is
	   ignored and could have any value, e.g. htons(~0).

	   This	function returns 0 on success, -1 otherwise.

     int LibAliasRedirectDynamic(struct	libalias *, struct alias_link *link)

	   This	function marks the specified static redirect rule entered by
	   LibAliasRedirectPort() as dynamic.  This can	be used	to e.g.	dynam-
	   ically redirect a single TCP	connection, after which	the rule is
	   removed.  Only fully	specified links	can be made dynamic.  (See the
	   sections below for a	definition of static vs. dynamic, and par-
	   tially vs. fully specified links.)

	   This	function returns 0 on success, -1 otherwise.

     void LibAliasRedirectDelete(struct	libalias *, struct alias_link *link)

	   This	function will delete a specific	static redirect	rule entered
	   by LibAliasRedirectPort() or	LibAliasRedirectAddr().	 The parameter
	   link	is the pointer returned	by either of the redirection func-
	   tions.  If an invalid pointer is passed to
	   LibAliasRedirectDelete(), then a program crash or unpredictable op-
	   eration could result, so care is needed when	using this function.

     int LibAliasProxyRule(struct libalias *, const char *cmd)

	   The passed cmd string consists of one or more pairs of words.  The
	   first word in each pair is a	token and the second is	the value that
	   should be applied for that token.  Tokens and their argument	types
	   are as follows:

	   type	encode_ip_hdr |	encode_tcp_stream | no_encode
		   In order to support transparent proxying, it	is necessary
		   to somehow pass the original	address	and port information
		   into	the new	destination server.  If	encode_ip_hdr is spec-
		   ified, the original destination address and port are	passed
		   as an extra IP option.  If encode_tcp_stream	is specified,
		   the original	destination address and	port are passed	as the
		   first piece of data in the TCP stream in the	format "DEST
		   IP port".

	   port	portnum
		   Only	packets	with the destination port portnum are proxied.

	   server host[:portnum]
		   This	specifies the host and portnum that the	data is	to be
		   redirected to.  host	must be	an IP address rather than a
		   DNS host name.  If portnum is not specified,	the destina-
		   tion	port number is not changed.

		   The server specification is mandatory unless	the delete
		   command is being used.

	   rule	index
		   Normally, each call to LibAliasProxyRule() inserts the next
		   rule	at the start of	a linear list of rules.	 If an index
		   is specified, the new rule will be checked after all	rules
		   with	lower indices.	Calls to LibAliasProxyRule() that do
		   not specify a rule are assigned rule	0.

	   delete index
		   This	token and its argument MUST NOT	be used	with any other
		   tokens.  When used, all existing rules with the given index
		   are deleted.

	   proto tcp | udp
		   If specified, only packets of the given protocol type are

	   src IP[/bits]
		   If specified, only packets with a source address matching
		   the given IP	are matched.  If bits is also specified, then
		   the first bits bits of IP are taken as a network specifica-
		   tion, and all IP addresses from that	network	will be

	   dst IP[/bits]
		   If specified, only packets with a destination address
		   matching the	given IP are matched.  If bits is also speci-
		   fied, then the first	bits bits of IP	are taken as a network
		   specification, and all IP addresses from that network will
		   be matched.

	   This	function is usually used to redirect outgoing connections for
	   internal machines that are not permitted certain types of internet
	   access, or to restrict access to certain external machines.

     struct alias_link * LibAliasRedirectProto(struct libalias *,
     struct in_addr local_addr,	struct in_addr remote_addr,
     struct in_addr alias_addr,	u_char proto)

	   This	function specifies that	any IP packet with protocol number of
	   proto from a	given remote address to	an alias address will be redi-
	   rected to a specified local address.

	   If local_addr or alias_addr is zero,	this indicates that the	packet
	   aliasing address as established by LibAliasSetAddress() is to be
	   used.  Even if LibAliasSetAddress() is called to change the address
	   after LibAliasRedirectProto() is called, a zero reference will
	   track this change.

	   If remote_addr is zero, this	indicates to redirect packets from any
	   remote address.  Non-zero remote addresses can sometimes be useful
	   for firewalling.

	   If two calls	to LibAliasRedirectProto() overlap in their address
	   specifications, then	the most recent	call will have precedence.

	   This	function returns a pointer which can subsequently be used by
	   LibAliasRedirectDelete().  If NULL is returned, then	the function
	   call	did not	complete successfully.

     The functions in this section are used to deal with incoming fragments.

     Outgoing fragments	are handled within LibAliasOut() by changing the ad-
     dress according to	any applicable mapping set by LibAliasRedirectAddr(),
     or	the default aliasing address set by LibAliasSetAddress().

     Incoming fragments	are handled in one of two ways.	 If the	header of a
     fragmented	IP packet has already been seen, then all subsequent fragments
     will be re-mapped in the same manner the header fragment was.  Fragments
     which arrive before the header are	saved and then retrieved once the
     header fragment has been resolved.

     int LibAliasSaveFragment(struct libalias *, void *ptr)

	   When	LibAliasIn() returns PKT_ALIAS_UNRESOLVED_FRAGMENT, this func-
	   tion	can be used to save the	pointer	to the unresolved fragment.

	   It is implicitly assumed that ptr points to a block of memory allo-
	   cated by malloc(3).	If the fragment	is never resolved, the packet
	   aliasing engine will	automatically free the memory after a timeout
	   period.  [Eventually	this function should be	modified so that a
	   callback function for freeing memory	is passed as an	argument.]

	   This	function returns PKT_ALIAS_OK if it was	successful and
	   PKT_ALIAS_ERROR if there was	an error.

     void * LibAliasGetFragment(struct libalias	*, void	*buffer)

	   This	function can be	used to	retrieve fragment pointers saved by
	   LibAliasSaveFragment().  The	IP header fragment pointed to by
	   buffer is the header	fragment indicated when	LibAliasIn() returns
	   PKT_ALIAS_FOUND_HEADER_FRAGMENT.  Once a fragment pointer is	re-
	   trieved, it becomes the calling program's responsibility to free
	   the dynamically allocated memory for	the fragment.

	   The LibAliasGetFragment() function can be called sequentially until
	   there are no	more fragments available, at which time	it returns

     void LibAliasFragmentIn(struct libalias *,	void *header, void *fragment)

	   When	a fragment is retrieved	with LibAliasGetFragment(), it can
	   then	be de-aliased with a call to LibAliasFragmentIn().  The	header
	   argument is the pointer to a	header fragment	used as	a template,
	   and fragment	is the pointer to the packet to	be de-aliased.

     struct alias_link * AddLink(struct	libalias *, struct in_addr src_addr,
     struct in_addr dst_addr, struct in_addr alias_addr, u_short src_port,
     u_short dst_port, int alias_param,	int link_type)

	   This	function adds new state	to the instance	hash table.  The
	   dst_address and/or dst_port may be given as zero, which introduces
	   some	dynamic	character into the link, since LibAliasSetAddress can
	   change the address that is used.  However, in the current implemen-
	   tation, such	links can only be used for inbound (ext	-> int)	traf-

     void LibAliasSetTarget(struct libalias *, struct in_addr addr)

	   When	an incoming packet not associated with any pre-existing	alias-
	   ing link arrives at the host	machine, it will be sent to the	ad-
	   dress indicated by a	call to	LibAliasSetTarget().

	   If this function is called with an INADDR_NONE address argument,
	   then	all new	incoming packets go to the address set by

	   If this function is not called, or is called	with an	INADDR_ANY ad-
	   dress argument, then	all new	incoming packets go to the address
	   specified in	the packet.  This allows external machines to talk di-
	   rectly to internal machines if they can route packets to the	ma-
	   chine in question.

     u_short LibAliasInternetChecksum(struct libalias *, u_short *buffer, int

	   This	is a utility function that does	not seem to be available else-
	   where and is	included as a convenience.  It computes	the internet
	   checksum, which is used in both IP and protocol-specific headers
	   (TCP, UDP, ICMP).

	   The buffer argument points to the data block	to be checksummed, and
	   nbytes is the number	of bytes.  The 16-bit checksum field should be
	   zeroed before computing the checksum.

	   Checksums can also be verified by operating on a block of data in-
	   cluding its checksum.  If the checksum is valid,
	   LibAliasInternetChecksum() will return zero.

     int LibAliasUnaliasOut(struct libalias *, void *buffer, int

	   An outgoing packet, which has already been aliased, has its private
	   address/port	information restored by	this function.	The IP packet
	   is pointed to by buffer, and	maxpacketsize is provided for error
	   checking purposes.  This function can be used if an already-aliased
	   packet needs	to have	its original IP	header restored	for further
	   processing (e.g. logging).

     This section is intended for those	who are	planning to modify the source
     code or want to create somewhat esoteric applications using the packet
     aliasing functions.

     The conceptual framework under which the packet aliasing engine operates
     is	described here.	 Central to the	discussion is the idea of an aliasing
     link which	describes the relationship for a given packet transaction be-
     tween the local machine, aliased identity and remote machine.  It is dis-
     cussed how	such links come	into existence and are destroyed.

     There is a	notion of an aliasing link, which is a 7-tuple describing a
     specific translation:

	   (local addr,	local port, alias addr,	alias port,
	    remote addr, remote	port, protocol)

     Outgoing packets have the local address and port number replaced with the
     alias address and port number.  Incoming packets undergo the reverse
     process.  The packet aliasing engine attempts to match packets against an
     internal table of aliasing	links to determine how to modify a given IP
     packet.  Both the IP header and protocol dependent	headers	are modified
     as	necessary.  Aliasing links are created and deleted as necessary	ac-
     cording to	network	traffic.

     Protocols can be TCP, UDP or even ICMP in certain circumstances.  (Some
     types of ICMP packets can be aliased according to sequence	or ID number
     which acts	as an equivalent port number for identifying how individual
     packets should be handled.)

     Each aliasing link	must have a unique combination of the following	five
     quantities: alias address/port, remote address/port and protocol.	This
     ensures that several machines on a	local network can share	the same
     aliasing IP address.  In cases where conflicts might arise, the aliasing
     port is chosen so that uniqueness is maintained.

     Aliasing links can	either be static or dynamic.  Static links persist in-
     definitely	and represent fixed rules for translating IP packets.  Dynamic
     links come	into existence for a specific TCP connection or	UDP transac-
     tion or ICMP ECHO sequence.  For the case of TCP, the connection can be
     monitored to see when the associated aliasing link	should be deleted.
     Aliasing links for	UDP transactions (and ICMP ECHO	and TIMESTAMP re-
     quests) work on a simple timeout rule.  When no activity is observed on a
     dynamic link for a	certain	amount of time it is automatically deleted.
     Timeout rules also	apply to TCP connections which do not open or close

     Aliasing links can	be partially specified,	meaning	that the remote	ad-
     dress and/or remote port are unknown.  In this case, when a packet	match-
     ing the incomplete	specification is found,	a fully	specified dynamic link
     is	created.  If the original partially specified link is dynamic, it will
     be	deleted	after the fully	specified link is created, otherwise it	will

     For instance, a partially specified link might be

	   (, 23,, 8066, 0, 0, tcp)

     The zeros denote unspecified components for the remote address and	port.
     If	this link were static it would have the	effect of redirecting all in-
     coming traffic from port 8066 of to port 23 (telnet) of
     machine on the	local network.	Each individual	telnet connec-
     tion would	initiate the creation of a distinct dynamic link.

     In	addition to aliasing links, there are also address mappings that can
     be	stored within the internal data	table of the packet aliasing mecha-

	   (local addr,	alias addr)

     Address mappings are searched when	creating new dynamic links.

     All outgoing packets from the local network automatically create a	dy-
     namic link	if they	do not match an	already	existing fully specified link.
     If	an address mapping exists for the outgoing packet, this	determines the
     alias address to be used.	If no mapping exists, then a default address,
     usually the address of the	packet aliasing	host, is used.	If necessary,
     this default address can be changed as often as each individual packet

     The aliasing port number is determined such that the new dynamic link
     does not conflict with any	existing links.	 In the	default	operating
     mode, the packet aliasing engine attempts to set the aliasing port	equal
     to	the local port number.	If this	results	in a conflict, then port num-
     bers are randomly chosen until a unique aliasing link can be established.
     In	an alternate operating mode, the first choice of an aliasing port is
     also random and unrelated to the local port number.

     One of the	latest improvements to libalias	was to make its	support	for
     new protocols independent from the	rest of	the library, giving it the
     ability to	load/unload support for	new protocols at run-time.  To achieve
     this feature, all the code	for protocol handling was moved	to a series of
     modules outside of	the main library.  These modules are compiled from the
     same sources but work in different	ways, depending	on whether they	are
     compiled to work inside a kernel or as part of the	userland library.

     When compiled for the kernel, libalias modules are	plain KLDs recogniz-
     able with the alias_ prefix.

     To	add support for	a new protocol,	load the corresponding module.	For

	   kldload alias_ftp

     When support for a	protocol is no longer needed, its module can be	un-

	   kldunload alias_ftp

     Due to the	differences between kernel and userland	(no KLD	mechanism,
     many different address spaces, etc.), we had to change a bit how to han-
     dle module	loading/tracking/unloading in userland.

     While compiled for	a userland libalias, all the modules are plain li-
     braries, residing in /usr/lib, and	recognizable with the libalias_	pre-

     There is a	configuration file, /etc/libalias.conf,	with the following
     contents (by default):


     This file contains	the paths to the modules that libalias will load.  To
     load/unload a new module, just add	its path to libalias.conf and call
     LibAliasRefreshModules() from the program.	 In case the application pro-
     vides a SIGHUP signal handler, add	a call to LibAliasRefreshModules() in-
     side the handler, and every time you want to refresh the loaded modules,
     send it the SIGHUP	signal:

	   kill	-HUP <process_pid>

     The modular architecture of libalias works	similar	whether	it is running
     inside the	kernel or in userland.	From alias_mod.c:

     /*	Protocol and userland module handlers chains. */
     LIST_HEAD(handler_chain, proto_handler) handler_chain ...
     SLIST_HEAD(dll_chain, dll)	dll_chain ...

     handler_chain keeps track of all the protocol handlers loaded, while
     ddl_chain tracks which userland modules are loaded.

     handler_chain is composed of struct proto_handler entries:

     struct proto_handler {
	     u_int pri;
	     int16_t dir;
	     uint8_t proto;
	     int (*fingerprint)(struct libalias	*la,
		      struct ip	*pip, struct alias_data	*ah);
	     int (*protohandler)(struct	libalias *la,
		      struct ip	*pip, struct alias_data	*ah);
	     TAILQ_ENTRY(proto_handler)	link;


     pri is the	priority assigned to a protocol	handler; lower priority	is

     dir is the	direction of packets: ingoing or outgoing.

     proto indicates to	which protocol this packet belongs: IP,	TCP or UDP.

     fingerprint points	to the fingerprint function while protohandler points
     to	the protocol handler function.

     The fingerprint function has the dual role	of checking if the incoming
     packet is found, and if it	belongs	to any categories that this module can

     The protohandler function actually	manipulates the	packet to make
     libalias correctly	NAT it.

     When a packet enters libalias, if it meets	a module hook, handler_chain
     is	searched to see	if there is an handler that matches this type of a
     packet (it	checks protocol	and direction of packet).  Then, if more than
     one handler is found, it starts with the module with the lowest priority
     number: it	calls the fingerprint function and interprets the result.

     If	the result value is equal to 0 then it calls the protocol handler of
     this handler and returns.	Otherwise, it proceeds to the next eligible
     module until the handler_chain is exhausted.

     Inside libalias, the module hook looks like this:

	   struct alias_data ad	= {
		   &ud->uh_sport,	   /* original source port */
		   &ud->uh_dport,	   /* original dest port */
		   256			   /* maxpacketsize */


	   /* walk out chain */
	   err = find_handler(IN, UDP, la, pip,	&ad);

     All data useful to	a module are gathered together in an alias_data	struc-
     ture, then	find_handler() is called.  The find_handler() function is re-
     sponsible for walking the handler chain; it receives as input parameters:

     IN	     direction

     UDP     working protocol

     la	     pointer to	this instance of libalias

     pip     pointer to	a struct ip

     ad	     pointer to	struct alias_data (see above)

     In	this case, find_handler() will search only for modules registered for
     supporting	INcoming UDP packets.

     As	was mentioned earlier, libalias	in userland is a bit different,	as
     care must be taken	in module handling as well (avoiding duplicate load of
     modules, avoiding modules with same name, etc.) so	dll_chain was intro-

     dll_chain contains	a list of all userland libalias	modules	loaded.

     When an application calls LibAliasRefreshModules(), libalias first	un-
     loads all the loaded modules, then	reloads	all the	modules	listed in
     /etc/libalias.conf: for every module loaded, a new	entry is added to

     dll_chain is composed of struct dll entries:

     struct dll	{
	     /*	name of	module */
	     char	     name[DLL_LEN];
	      *	ptr to shared obj obtained through
	      *	dlopen() - use this ptr	to get access
	      *	to any symbols from a loaded module
	      *	via dlsym()
	     void	     *handle;
	     struct dll	     *next;

     name is the name of the module.

     handle is a pointer to the	module obtained	through	dlopen(3).
     Whenever a	module is loaded in userland, an entry is added	to dll_chain,
     then every	protocol handler present in that module	is resolved and	regis-
     tered in handler_chain.

     There is a	module (called alias_dummy.[ch]) in libalias that can be used
     as	a skeleton for future work.  Here we analyse some parts	of that	mod-
     ule.  From	alias_dummy.c:

     struct proto_handler handlers[] = {
	     .pri = 666,
	     .dir = IN|OUT,
	     .proto = UDP|TCP,
	     .fingerprint = fingerprint,
	     .protohandler= protohandler,
	 { EOH }

     The variable handlers is the "most	important thing" in a module since it
     describes the handlers present and	lets the outside world use it in an
     opaque way.

     It	must ALWAYS be present in every	module,	and it MUST retain the name
     handlers, otherwise attempting to load a module in	userland will fail and
     complain about missing symbols: for more information about	module
     load/unload, please refer to LibAliasRefreshModules(),
     LibAliasLoadModule() and LibAliasUnloadModule() in	alias.c.

     handlers contains all the proto_handler structures	present	in a module.

     static int
     mod_handler(module_t mod, int type, void *data)
	     int error;

	     switch (type) {
	     case MOD_LOAD:
		     error = LibAliasAttachHandlers(handlers);
	     case MOD_UNLOAD:
		     error = LibAliasDetachHandlers(handlers);
		     error = EINVAL;
	     return (error);
     When running as KLD, mod_handler()	registers/deregisters the module using
     LibAliasAttachHandlers() and LibAliasDetachHandlers(), respectively.

     Every module must contain at least	2 functions: one fingerprint function
     and a protocol handler function.

     #ifdef _KERNEL
     fingerprint(struct	libalias *la, struct ip	*pip, struct alias_data	*ah)


     #ifdef _KERNEL
     protohandler(struct libalias *la, struct ip *pip,
		  struct alias_data *ah)

     and they must accept exactly these	input parameters.

     To	add module support into	an application that uses libalias, the follow-
     ing simple	steps can be followed.

     1.	  Find the main	file of	an application (let us call it main.c).

     2.	  Add this to the header section of main.c, if not already present:

		#include <signal.h>

	  and this just	after the header section:

		static void signal_handler(int);

     3.	  Add the following line to the	init function of an application	or, if
	  it does not have any init function, put it in	main():

		signal(SIGHUP, signal_handler);

	  and place the	signal_handler() function somewhere in main.c:

		static void
		signal_handler(int sig)


	  Otherwise, if	an application already traps the SIGHUP	signal,	just
	  add a	call to	LibAliasRefreshModules() in the	signal handler func-
     For example, to patch natd(8) to use libalias modules, just add the fol-
     lowing line to RefreshAddr(int sig	__unused):


     recompile and you are done.

     When working as KLD, libalias now has log support that happens on a buf-
     fer allocated inside struct libalias (from	alias_local.h):

     struct libalias {

	     /*	log descriptor	      */
     #ifdef  KERNEL_LOG
	     char	    *logDesc;	     /*
					      *	ptr to an auto-malloced
					      *	memory buffer when libalias
					      *	works as kld
	     FILE	    *logDesc;	     /*
					      *	ptr to /var/log/alias.log
					      *	when libalias runs as a
					      *	userland lib

     so	all applications using libalias	will be	able to	handle their own logs,
     if	they want, accessing logDesc.  Moreover, every change to a log buffer
     is	automatically added to syslog(3) with the LOG_SECURITY facility	and
     the LOG_INFO level.

     Charles Mott <>, versions 1.0 - 1.8,	2.0 - 2.4.
     Eivind Eklund <>, versions 1.8b,	1.9 and	2.5.  Added
     IRC DCC support as	well as	contributing a number of architectural im-
     provements; added the firewall bypass for FTP/IRC DCC.
     Erik Salander <> added support for	PPTP and RTSP.
     Junichi Satoh <> added support for RTSP/PNA.
     Ruslan Ermilov <> added support for PPTP and	LSNAT as well
     as	general	hacking.
     Gleb Smirnoff <> ported	the library to kernel space.
     Paolo Pisati <> made the library modular, moving support
     for all protocols (except for IP, TCP and UDP) to external	modules.

     Listed below, in approximate chronological	order, are individuals who
     have provided valuable comments and/or debugging assistance.

	   Gary	Roberts
	   Tom Torrance
	   Reto	Burkhalter
	   Martin Renters
	   Brian Somers
	   Paul	Traina
	   Ari Suutari
	   Dave	Remien
	   J. Fortes
	   Andrzej Bialecki
	   Gordon Burditt

FreeBSD	13.0			 May 31, 2021			  FreeBSD 13.0


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