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LIBALIAS(3)		 BSD 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, LibAliasInit(), must always be called before any
     packet handling may be performed and the returned instance pointer 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 customized 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.

		 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 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
	   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 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 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 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>:

	   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 (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‐
		   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 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
		   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
		   LibAliasSetFWBase().	 This call must happen prior to set‐
		   ting 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
		   LibAliasProxyRule() below for details.

     void LibAliasSetFWBase(struct libalias *, 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 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.

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

     int LibAliasIn(struct libalias *, 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 LibAliasSetMode()).

	   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 LibAliasSaveFragment()
		   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 LibAliasGetFragment() and de-alias
		   them with LibAliasFragmentIn().

	   PKT_ALIAS_ERROR
		   An internal error within the packet aliasing engine
		   occurred.

     int LibAliasOut(struct libalias *, 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 * 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
	   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 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 for a 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.
	   Almost always, the remote port specification will be zero, but non-
	   zero remote addresses can sometimes be useful for firewalling.  If
	   two calls to LibAliasRedirectPort() 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
	   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 for a load sharing, then
	   local_addr 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("192.168.0.2"),
					 inet_aton("141.221.254.101"));
		 LibAliasRedirectAddr(la, inet_aton("192.168.0.3"),
					 inet_aton("141.221.254.101"));
		 LibAliasRedirectAddr(la, 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 LibAliasRedirectPort() will have precedence over
	   address 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 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 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
	   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 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
	   operation could result, so it is necessary to be careful 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
		   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 * 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 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 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.

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

     Outgoing fragments are handled within LibAliasOut() by changing the
     address 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 *, char *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.

     char * LibAliasGetFragment(struct libalias *, char *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
	   retrieved, 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
	   NULL.

     void LibAliasFragmentIn(struct libalias *, char *header, char *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.

MISCELLANEOUS FUNCTIONS
     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
	   address 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
	   LibAliasSetAddress().

	   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 LibAliasCheckNewLink(struct libalias *)

	   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 LibAliasSetTarget() is called to change the default
	   target address.

     u_short LibAliasInternetChecksum(struct libalias *, 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,
	   LibAliasInternetChecksum() will return zero.

     int LibAliasUnaliasOut(struct libalias *, 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 (e.g. logging).

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.
     Paolo Pisati ⟨piso@FreeBSD.org⟩ made the library modular, moving support
     for all protocols (except for IP, TCP and UDP) to external modules.

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.

MODULAR ARCHITECTURE (AND ipfw(4) SUPPORT)
     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.

   LIBALIAS MODULES IN KERNEL LAND
     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
     example:

	   kldload alias_ftp

     When support for a protocol is no longer needed, its module can be
     unloaded:

	   kldunload alias_ftp

   LIBALIAS MODULES IN USERLAND
     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
     libraries, residing in /usr/lib, and recognizable with the libalias_ pre‐
     fix.

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

	   /usr/lib/libalias_cuseeme.so
	   /usr/lib/libalias_ftp.so
	   /usr/lib/libalias_irc.so
	   /usr/lib/libalias_nbt.so
	   /usr/lib/libalias_pptp.so
	   /usr/lib/libalias_skinny.so
	   /usr/lib/libalias_smedia.so

     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()
     inside the handler, and everytime you want to refresh the loaded modules,
     send it the SIGHUP signal:

	   kill -HUP <process_pid>

   MODULAR ARCHITECURE: HOW IT WORKS
     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 keep tracks of all the protocol handlers loaded, while
     ddl_chain takes care of userland modules 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);
	     LIST_ENTRY(proto_handler) entries;
     };

     where:

     pri is the priority assigned to a protocol handler, lower is better.

     dir is the direction of packets: ingoing or outgoing.

     proto says at 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 double of scope of checking if the
     incoming packet is found and if it belongs to any categories that this
     module can handle.

     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 = {
		   lnk,
		   &original_address,
		   &alias_address,
		   &alias_port,
		   &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
     responsible for walking out 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, cause
     care has to be taken of module handling too (avoiding duplicate load of
     module, avoiding module with same name, etc.) so dll_chain was intro‐
     duced.

     dll_chain contains a list of all userland libalias modules loaded.

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

     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.

   HOW TO WRITE A MODULE FOR LIBALIAS
     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 module.
     From alias_dummy.c:

     struct proto_handler handlers [] = {{666, IN|OUT, UDP|TCP,
					 &fingerprint, &protohandler}};

     The variable handlers is the “most important thing” in a module cause 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 = 0;
		     attach_handlers(handlers);
		     break;
	     case MOD_UNLOAD:
		     error = 0;
		     detach_handlers(handlers;
		     break;
	     default:
		     error = EINVAL;
	     }
	     return (error);
     }
     When running as KLD, mod_handler() register/deregister the module using
     attach_handlers() and detach_handlers(), respectively.

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

     #ifdef _KERNEL
     static
     #endif
     int
     fingerprint(struct libalias *la, struct ip *pip, struct alias_data *ah)
     {

     ...
     }

     #ifdef _KERNEL
     static
     #endif
     int
     protohandler(struct libalias *la, struct ip *pip,
		  struct alias_data *ah)
     {

     ...
     }
     and they must accept exactly these input parameters.

   PATCHING AN APPLICATION FOR USERLAND LIBALIAS MODULES
     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)
		{

			LibAliasRefreshModules();
		}

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

	   LibAliasRefreshModules()

     recompile and you are done.

   LOGGING SUPPORT IN KERNEL LAND
     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
					      */
     #else
	     FILE	    *logDesc;	     /*
					      * ptr to /var/log/alias.log
					      * when libalias runs as a
					      * userland lib
					      */
     #endif

	     ...
     }
     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.

BSD				October 1, 2006				   BSD
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