Skip site navigation (1)Skip section navigation (2)

FreeBSD Manual Pages


home | help
libalias(3)	       FreeBSD Library Functions Manual		   libalias(3)

     libalias Packet Aliasing Library.	A collection of	functions for aliasing
     and de-aliasing of	IP packets, intended for masquerading and network
     address translation (NAT).

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

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

     1.	Introduction
     2.	Initialization and Control
	 2.1 PacketAliasInit()
	 2.2 PacketAliasUninit()
	 2.3 PacketAliasSetAddress()
	 2.4 PacketAliasSetMode()
	 2.5 PacketAliasSetFWBase()
     3.	Packet Handling
	 3.1 PacketAliasIn()
	 3.2 PacketAliasOut()
     4.	Port and Address Redirection
	 4.1 PacketAliasRedirectPort()
	 4.2 PacketAliasRedirectAddr()
	 4.3 PacketAliasRedirectDelete()
     5.	Fragment Handling
	 5.1 PacketAliasSaveFragment()
	 5.2 PacketAliasGetFragment()
	 5.3 PacketAliasFragmentIn()
     6.	Miscellaneous Functions
	 6.1 PacketAliasSetTarget()
	 6.2 PacketAliasCheckNewLink()
	 6.3 PacketAliasInternetChecksum()
     7.	Authors
     8.	Acknowledgments

     Appendix A: Conceptual Background
	 A.1 Aliasing Links
	 A.2 Static and	Dynamic	Links
	 A.3 Partially Specified Links
	 A.4 Dynamic Link Creation

1. 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.

2. Initialization and Control
     Two specific functions, PacketAliasInit() and PacketAliasSetAddress(),
     must always be called before any packet handling may be performed.	 In
     addition, the operating mode of the packet	aliasing engine	can be cus-
     tomized by	calling	PacketAliasSetMode().

   2.1 PacketAliasInit()
     void PacketAliasInit(void)

     This function has no argument or return value and is used to initialize
     internal data structures. The following mode bits are always set after
     calling PacketAliasInit().	 See section 2.3 for the meaning of these mode


     This function will	always return the packet aliasing engine to the	same
     initial state.  PacketAliasSetAddress() must be 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	pro-
     gram prior	to any packet handling.

   2.2 PacketAliasUninit()
     void PacketAliasUninit(void)

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

     This functions should be called when a program stop using the aliasing
     engine; it	do, among other	things,	clear out any firewall holes.  To pro-
     vide backwards compatibility and extra security, it is added to the
     atexit() chain by PacketAliasInit().  Calling it multiple times is	harm-

   2.3 PacketAliasSetAddress()
     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 remapped	to
     this address unless overridden by a static	address	mapping	established by

     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,	as if PacketAliasReset() were called.
     This is useful for	interfaces such	as ppp 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 func-
     tion 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 han-

   2.4 PacketAliasSetMode()
     unsigned int PacketAliasSetMode(unsigned int mode,	unsigned int mask)

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

	 PKT_ALIAS_LOG.	Enables	logging	/var/log/alias.log.  The log file
		 shows total numbers of	links (icmp, tcp, udp) each time an
		 aliasing link is created or deleted.  Mainly useful for
		 debugging when	the log	file is	viewed continuously with "tail

	 PKT_ALIAS_DENY_INCOMING. If this mode bit is set, all incoming	pack-
		 ets associated	with new TCP connections or new	UDP transac-
		 tions will be marked for being	ignored	(PacketAliasIn()
		 return	code PKT_ALIAS_IGNORED)	by the calling program.
		 Response packets to connections or transactions initiated
		 from the packet aliasing host or local	network	will be	unaf-
		 fected.  This mode bit	is useful for implementing a one-way

	 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 number.  This can be done as long
		 as the	quintuple (proto, alias	addr, alias port, remote addr,
		 remote	port) is unique.  If a conflict	exists,	an 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	the packet
		 aliasing host originates network traffic as well as forwards
		 it.  When the packet aliasing host is waiting for a connec-
		 tion 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 con-
		 flicts.  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:	->   (Class A subnet)	->   (Class B subnets)	->  (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 doesn't need	to be passed
		 through the packet aliasing engine.

	 PKT_ALIAS_RESET_ON_ADDR_CHANGE. When this mode	bit is set and Packe-
		 tAliasSetAddress() 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 links where
		 the interface address can sometimes change or remain the same
		 between dial-ups.  If this mode bit is	not set, it the	link
		 table will never be reset in the event	of an address change.

	 PKT_ALIAS_PUNCH_FW. This option make libalias `punch holes' in	an
		 ipfw 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 connection.  A hole is
		 removed when the connection that use it die.  To cater	for
		 unexpected death of a program using libalias (e.g kill	-9),
		 changing the state of the flag	will clear the entire ipfw
		 range allocated for holes.  This will also happen on the ini-
		 tial call to PacketAliasSetFWBase().  This call must happen
		 prior to setting this flag.

   2.5 PacketAliasSetFWBase()
     void PacketAliasSetFWBase(unsigned	int base, unsigned int num)

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

3. Packet Handling
     The packet	handling functions are used to modify incoming (remote->local)
     and outgoing (local->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.

   3.1 PacketAliasIn()
     int PacketAliasIn(char *buffer, int maxpacketsize)

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

     Return codes:

	 PKT_ALIAS_ERROR. An internal error within the packet aliasing engine

	 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 protocal is unrecognized, possibly an ICMP
		 message type is not handled or	if incoming packets for	new
		 connections are being ignored (see PKT_ALIAS_DENY_INCOMING in
		 section 2.2).

	 PKT_ALIAS_UNRESOLVED_FRAGMENT.	This is	returned when a	fragment can-
		 not be	resolved because the header fragment has not been sent
		 yet.  In this situation, fragments must be saved with Packe-
		 tAliasSaveFragment() until a header fragment is found.

	 PKT_ALIAS_FOUND_HEADER_FRAGMENT. The packet aliasing process was suc-
		 cessful, and a	header fragment	was found.  This is a signal
		 to retrieve any unresolved fragments with PacketAliasGetFrag-
		 ment()	and de-alias them with PacketAliasFragmentIn().

   3.2 PacketAliasOut()
     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 r, and
     maxpacketsize indicates the maximum packet	size permissible should	the
     packet length be changed.	IP encoding protocols place address and	port
     information in the	encapsulated data stream which have 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_ERROR. An internal error within the packet aliasing engine

	 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 protocal is unrecognized, or	possibly an
		 ICMP message type is not handled.

4. 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.

   4.1 PacketAliasRedirectPort()
     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 PacketAliasAddress() is called to change the address after	Packe-
     tAliasRedirectPort() is called, a zero reference will track this change.

     If	remote_addr is zero, this indicates to redirect	packets	from any
     remote address.  Likewise,	if remote_port is zero,	this indicates to re-
     direct packets originating	from any remote	port number.  Almost always,
     the remote	port specification will	be zero, but non-zero remote addresses
     can be sometimes be useful	for firewalling.  If two calls to PacketAl-
     iasRedirectPort() 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 Packe-
     tAliasRedirectDelete().  If NULL is returned, then	the function call did
     not complete successfully.

     All port numbers are in network address byte order, so it is necessary to
     use htons() to convert these parameters from internally 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.

   4.2 PacketAliasRedirectAddr()
     struct alias_link * PacketAliasRedirectAddr(struct	in_addr	local_addr,
     struct in_addr alias_addr)

     This function desgnates that all incoming traffic to alias_addr be	redi-
     rected 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 PacketAliasAddress() is called to change the address after	Packe-
     tAliasRedirectAddr() is called, a zero reference will track this change.

     If	subsequent calls to PacketAliasRedirectAddr() use the same aliasing
     address, all new incoming traffic to this aliasing	address	will be	redi-
     rected to the local address made in the last function call, but new traf-
     fic all of	the local machines designated in the several function calls
     will be aliased to	the same address.  Consider the	following example:


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

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

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

   4.3 PacketAliasRedirectDelete()
     void PacketAliasRedirectDelete(struct alias_link *ptr)

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

5. 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	PacketAliasRedirectAd-
     dress(), 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.

   5.1 PacketAliasSaveFragment()
     int PacketAliasSaveFragment(char *ptr)

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

     It	is implicitly assumed that ptr points to a block of memory allocated
     by	malloc().  If the fragment is never resolved, the packet aliasing
     engine will automatically free the	memory after a timeout period. [Even-
     tually 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.

   5.2 PacketAliasGetNextFragment()
     char * PacketAliasGetFragment(char	*buffer)

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

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

   5.3 PacketAliasFragmentIn()
     void PacketAliasFragmentIn(char *header, char *fragment)

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

6. Miscellaneous Functions
   6.1 PacketAliasSetTarget()
     void PacketAliasSetTarget(struct in_addr addr)

     When an incoming packet not associated with any pre-existing aliasing
     link arrives at the host machine, it will be sent to the address indi-
     cated by a	call to	PacketAliasSetTarget().

     If	this function is not called, or	is called with a zero address argu-
     ment, then	all new	incoming packets go to the address set by PacketAlias-

   6.2 PacketAliasCheckNewLink()
     int PacketAliasCheckNewLink(void)

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

   6.3 PacketAliasInternetChecksum()
     u_short PacketAliasInternetChecksum(u_short *buffer, int nbytes)

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

     buffer points to the data block to	be checksummed,	and nbytes is the num-
     ber of bytes.  The	16-bit checksum	field should be	zeroed before comput-
     ing 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.

7. Authors
     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
     improvements; added the firewall bypass for FTP/IRC DCC.

8. 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 Bialeki
	 Gordon	Burditt

Appendix: Conceptual Background
     This appendix 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.

   A.1 Aliasing	Links
     There is a	notion of an "aliasing link", which is 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
     aliased IP	address.  In cases where conflicts might arise,	the aliasing
     port is chosen so that uniqueness is maintained.

   A.2 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.

   A.3 Partially Specified Aliasing Links
     Aliasing links can	be partially specified,	meaning	that the remote
     address and/or remote ports 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

	   (, 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
     incoming 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.

   A.4 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-

	   (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 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 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	11.1		      September	24, 2017		  FreeBSD 11.1

NAME | SYNOPSIS | CONTENTS | 1. Introduction | 2. Initialization and Control | 3. Packet Handling | 4. Port and Address Redirection | 5. Fragment Handling | 6. Miscellaneous Functions | 7. Authors | 8. Acknowledgments | Appendix: Conceptual Background

Want to link to this manual page? Use this URL:

home | help