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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 4.10                    April 13, 2000                    FreeBSD 4.10

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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