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

NAME
     libalias -- packet	aliasing library for masquerading and network address
     translation

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

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

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

INTRODUCTION
     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
     machine 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 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 statically
     redirected	to a private address/port.

     The packet	aliasing engine	was designed to	operate	in user	space outside
     of	the kernel, without any	access to private kernel data structure, but
     the source	code can also be ported	to a kernel environment.

INITIALIZATION AND CONTROL
     One special function, PacketAliasInit(), must always be called before any
     packet handling may be performed.	Normally, the PacketAliasSetAddress()
     function is called	afterwards, to set the default aliasing	address.  In
     addition, the operating mode of the packet	aliasing engine	can be cus-
     tomized by	calling	PacketAliasSetMode().

     void PacketAliasInit(void)

	   This	function has no	arguments or return value and is used to ini-
	   tialize internal data structures.  The following mode bits are
	   always set after calling PacketAliasInit().	See the	description of
	   PacketAliasSetMode()	below for the meaning of these mode bits.

		 PKT_ALIAS_SAME_PORTS
		 PKT_ALIAS_USE_SOCKETS
		 PKT_ALIAS_RESET_ON_ADDR_CHANGE

	   This	function will always return the	packet aliasing	engine to the
	   same	initial	state.	The PacketAliasSetAddress() function is	nor-
	   mally called	afterwards, and	any desired changes from the default
	   mode	bits listed above require a call to PacketAliasSetMode().

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

     void PacketAliasUninit(void)

	   This	function has no	arguments or return value and is used to clear
	   any resources attached to internal data structures.

	   This	functions should be called when	a program stops	using the
	   aliasing engine; it does, amongst other things, clear out any fire-
	   wall	holes.	To provide backwards compatibility and extra security,
	   it is added to the atexit(3)	chain by PacketAliasInit().  Calling
	   it multiple times is	harmless.

     void PacketAliasSetAddress(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 PacketAliasRedirectAddr().  If this function
	   is not 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
	   address on a	packet to packet basis (it is a	low overhead call).

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

     unsigned int PacketAliasSetMode(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>:

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

	   PKT_ALIAS_DENY_INCOMING
		   If this mode	bit is set, all	incoming packets associated
		   with	new TCP	connections or new UDP transactions will be
		   marked for being ignored (PacketAliasIn() 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-
		   wall.

	   PKT_ALIAS_SAME_PORTS
		   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,
		   remote port)	is unique.  If a conflict exists, a new	alias-
		   ing port number is chosen even if this mode bit is set.

	   PKT_ALIAS_USE_SOCKETS
		   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.

	   PKT_ALIAS_UNREGISTERED_ONLY
		   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:

			 10.0.0.0     ->  10.255.255.255   (Class A subnet)
			 172.16.0.0   ->  172.31.255.255   (Class B subnets)
			 192.168.0.0  ->  192.168.255.255  (Class C subnets)

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

	   PKT_ALIAS_RESET_ON_ADDR_CHANGE
		   When	this mode bit is set and PacketAliasSetAddress() 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
		   between dial-up attempts.  If this mode bit is not set, the
		   link	table will never be reset in the event of an address
		   change.

	   PKT_ALIAS_PUNCH_FW
		   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	will also happen on the	initial	call to
		   PacketAliasSetFWBase().  This call must happen prior	to
		   setting this	flag.

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

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

     void PacketAliasSetFWBase(unsigned	int base, unsigned int num)

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

     void PacketAliasSkinnyPort(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.

PACKET HANDLING
     The packet	handling functions are used to modify incoming (remote to
     local) and	outgoing (local	to remote) packets.  The calling program is
     responsible for receiving and sending packets via network interfaces.

     Along with	PacketAliasInit() and PacketAliasSetAddress(), the two packet
     handling functions, PacketAliasIn() and PacketAliasOut(), comprise	mini-
     mal set of	functions needed for a basic IP	masquerading implementation.

     int PacketAliasIn(char *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:

	   PKT_ALIAS_OK
		   The packet aliasing process was successful.

	   PKT_ALIAS_IGNORED
		   The packet was ignored and not de-aliased.  This can	happen
		   if the protocol is unrecognized, possibly an	ICMP message
		   type	is not handled or if incoming packets for new connec-
		   tions are being ignored (if PKT_ALIAS_DENY_INCOMING mode
		   bit was set by PacketAliasSetMode()).

	   PKT_ALIAS_UNRESOLVED_FRAGMENT
		   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
		   PacketAliasSaveFragment() until a header fragment is	found.

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

	   PKT_ALIAS_ERROR
		   An internal error within the	packet aliasing	engine
		   occurred.

     int PacketAliasOut(char *buffer, int maxpacketsize)

	   An outgoing packet coming from the local network to a remote
	   machine is aliased by this function.	 The IP	packet is pointed to
	   by buffer, and maxpacketsize	indicates the maximum packet size per-
	   missible should the packet length be	changed.  IP encoding proto-
	   cols	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:

	   PKT_ALIAS_OK
		   The packet aliasing process was successful.

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

	   PKT_ALIAS_ERROR
		   An internal error within the	packet aliasing	engine
		   occurred.

PORT AND ADDRESS REDIRECTION
     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 * PacketAliasRedirectPort(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
	   address/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 PacketAliasSetAddress() is to be
	   used.  Even if PacketAliasSetAddress() is called to change the
	   address after PacketAliasRedirectPort() is called, a	zero reference
	   will	track this change.

	   If the link is further set up to operate for	a load sharing,	then
	   local_addr and local_port are ignored, and are selected dynamically
	   from	the server pool, as described in PacketAliasAddServer()	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.
	   Almost always, the remote port specification	will be	zero, but non-
	   zero	remote addresses can sometimes be useful for firewalling.  If
	   two calls to	PacketAliasRedirectPort() overlap in their
	   address/port	specifications,	then the most recent call will have
	   precedence.

	   This	function returns a pointer which can subsequently be used by
	   PacketAliasRedirectDelete().	 If NULL is returned, then the func-
	   tion	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 * PacketAliasRedirectAddr(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 PacketAliasSetAddress() is to be
	   used.  Even if PacketAliasSetAddress() is called to change the
	   address after PacketAliasRedirectAddr() is called, a	zero reference
	   will	track this change.

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

	   If subsequent calls to PacketAliasRedirectAddr() 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, desig-
	   nated in the	several	function calls,	will be	aliased	to the same
	   address.  Consider the following example:

		 PacketAliasRedirectAddr(inet_aton("192.168.0.2"),
					 inet_aton("141.221.254.101"));
		 PacketAliasRedirectAddr(inet_aton("192.168.0.3"),
					 inet_aton("141.221.254.101"));
		 PacketAliasRedirectAddr(inet_aton("192.168.0.4"),
					 inet_aton("141.221.254.101"));

	   Any outgoing	connections such as telnet(1) or ftp(1)	from
	   192.168.0.2,	192.168.0.3 and	192.168.0.4 will appear	to come	from
	   141.221.254.101.  Any incoming connections to 141.221.254.101 will
	   be directed to 192.168.0.4.

	   Any calls to	PacketAliasRedirectPort() will have precedence over
	   address mappings designated by PacketAliasRedirectAddr().

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

     int PacketAliasAddServer(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 server pool, selected 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 load balance across	server pool hosts at the time.	If
	   load	share is desired for just a few	specific services, the config-
	   uration on LSNAT could be defined to	restrict load share for	just
	   the services	desired.

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

	   First, the link is created by either	PacketAliasRedirectPort() or
	   PacketAliasRedirectAddr().  Then, PacketAliasAddServer() is called
	   multiple times to add entries to the	link's server pool.

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

	   This	function returns 0 on success, -1 otherwise.

     int PacketAliasRedirectDynamic(struct alias_link *link)

	   This	function marks the specified static redirect rule entered by
	   PacketAliasRedirectPort() as	dynamic.  This can be used to e.g.
	   dynamically redirect	a single TCP connection, after which the rule
	   is removed.	Only fully specified links can be made dynamic.	 (See
	   the STATIC AND DYNAMIC LINKS	and PARTIALLY SPECIFIED	ALIASING LINKS
	   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 PacketAliasRedirectDelete(struct alias_link *link)

	   This	function will delete a specific	static redirect	rule entered
	   by PacketAliasRedirectPort()	or PacketAliasRedirectAddr().  The
	   parameter link is the pointer returned by either of the redirection
	   functions.  If an invalid pointer is	passed to
	   PacketAliasRedirectDelete(),	then a program crash or	unpredictable
	   operation could result, so it is necessary to be careful using this
	   function.

     int PacketAliasProxyRule(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 PacketAliasProxyRule() 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 PacketAliasProxyRule()
		   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
		   matched.

	   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
		   matched.

	   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 * PacketAliasRedirectProto(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 be redirected
	   to a	specified local	address.

	   If local_addr or alias_addr is zero,	this indicates that the	packet
	   aliasing address as established by PacketAliasSetAddress() is to be
	   used.  Even if PacketAliasSetAddress() is called to change the
	   address after PacketAliasRedirectProto() is called, a zero refer-
	   ence	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 PacketAliasRedirectProto() overlap in their address
	   specifications, then	the most recent	call will have precedence.

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

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

     Outgoing fragments	are handled within PacketAliasOut() by changing	the
     address according to any applicable mapping set by
     PacketAliasRedirectAddr(),	or the default aliasing	address	set by
     PacketAliasSetAddress().

     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 PacketAliasSaveFragment(char *ptr)

	   When	PacketAliasIn()	returns	PKT_ALIAS_UNRESOLVED_FRAGMENT, this
	   function can	be used	to save	the pointer to the unresolved frag-
	   ment.

	   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.

     char * PacketAliasGetFragment(char	*buffer)

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

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

     void PacketAliasFragmentIn(char *header, char *fragment)

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

MISCELLANEOUS FUNCTIONS
     void PacketAliasSetTarget(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
	   address indicated by	a call to PacketAliasSetTarget().

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

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

     int PacketAliasCheckNewLink(void)

	   This	function returns a non-zero value when a new aliasing link is
	   created.  In	circumstances where incoming traffic is	being sequen-
	   tially sent to different local servers, this	function can be	used
	   to trigger when PacketAliasSetTarget() is called to change the
	   default target address.

     u_short PacketAliasInternetChecksum(u_short *buffer, int nbytes)

	   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
	   including its checksum.  If the checksum is valid,
	   PacketAliasInternetChecksum() will return zero.

     int PacketUnaliasOut(char *buffer,	int maxpacketsize)

	   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 (eg. logging).

BUGS
     PPTP aliasing does	not work when more than	one internal client connects
     to	the same external server at the	same time, because PPTP	requires a
     single TCP	control	connection to be established between any two IP
     addresses.

AUTHORS
     Charles Mott <cm@linktel.net>, versions 1.0 - 1.8,	2.0 - 2.4.
     Eivind Eklund <eivind@FreeBSD.org>, versions 1.8b,	1.9 and	2.5.  Added
     IRC DCC support as	well as	contributing a number of architectural
     improvements; added the firewall bypass for FTP/IRC DCC.
     Erik Salander <erik@whistle.com> added support for	PPTP and RTSP.
     Junichi Satoh <junichi@junichi.org> added support for RTSP/PNA.
     Ruslan Ermilov <ru@FreeBSD.org> added support for PPTP and	LSNAT as well
     as	general	hacking.

ACKNOWLEDGMENTS
     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

CONCEPTUAL BACKGROUND
     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
     between the local machine,	aliased	identity and remote machine.  It is
     discussed how such	links come into	existence and are destroyed.

   ALIASING LINKS
     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
     according 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.

   STATIC AND DYNAMIC LINKS
     Aliasing links can	either be static or dynamic.  Static links persist
     indefinitely and represent	fixed rules for	translating IP packets.
     Dynamic links come	into existence for a specific TCP connection or	UDP
     transaction 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 TIME-
     STAMP requests) work on a simple timeout rule.  When no activity is
     observed on a dynamic link	for a certain amount of	time it	is automati-
     cally deleted.  Timeout rules also	apply to TCP connections which do not
     open or close properly.

   PARTIALLY SPECIFIED ALIASING	LINKS
     Aliasing links can	be partially specified,	meaning	that the remote
     address and/or remote port	are unknown.  In this case, when a packet
     matching 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 persist.

     For instance, a partially specified link might be

	   (192.168.0.4, 23, 204.228.203.215, 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
     incoming traffic from port	8066 of	204.228.203.215	to port	23 (telnet) of
     machine 192.168.0.4 on the	local network.	Each individual	telnet connec-
     tion would	initiate the creation of a distinct dynamic link.

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

	   (local addr,	alias addr)

     Address mappings are searched when	creating new dynamic links.

     All outgoing packets from the local network automatically create a
     dynamic link if they do not match an already existing fully specified
     link.  If an address mapping exists for the outgoing packet, this deter-
     mines 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 individ-
     ual packet	arrives.

     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.

FreeBSD	9.2			April 13, 2000			   FreeBSD 9.2

NAME | SYNOPSIS | DESCRIPTION | INTRODUCTION | INITIALIZATION AND CONTROL | PACKET HANDLING | PORT AND ADDRESS REDIRECTION | FRAGMENT HANDLING | MISCELLANEOUS FUNCTIONS | BUGS | AUTHORS | ACKNOWLEDGMENTS | CONCEPTUAL BACKGROUND

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