libalias

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.
     Outgoing packets from a local network with unregistered IP addresses can be
     aliased to appear as if they came from an accessible IP address.  Incoming
     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 the local network and the packet aliasing host.  This is
     known as IP masquerading.  In addition, one-to-one mappings between local
     and public addresses can also be implemented, which is known as static NAT.
     In between these extremes, different groups of private addresses can be
     linked to different public addresses, comprising several distinct many-to-
     one mappings.  Also, a given public address and port can be statically
     redirected to a private address/port.

INITIALIZATION AND CONTROL
     One special function, LibAliasInit(), must always be called before any
     packet handling may be performed, and the returned instance pointer must be
     passed to all the other functions.  Normally, the LibAliasSetAddress()
     function is called afterwards, to set the default aliasing address.  In
     addition, the operating mode of the packet aliasing engine can be
     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
           argument.  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 function should be called when a program stops using the
           aliasing engine; amongst other things, it clears out any firewall
           holes.  To provide backwards compatibility and extra security, it is
           added to the atexit(3) chain by LibAliasInit().

     void LibAliasSetAddress(struct libalias *, struct in_addr addr)

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

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

           If the PKT_ALIAS_RESET_ON_ADDR_CHANGE mode bit is set to zero, this
           function can also be used to dynamically change the aliasing 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
                   to with the current number of ICMP, TCP and UDP links.
                   Mainly useful for debugging when the log file is viewed
                   continuously with tail(1).

           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 firewall.

           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 aliasing 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
                   originates 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 the 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 interface 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 connection.  A hole is
                   removed when the connection that uses it dies.  To cater to
                   unexpected death of a program using libalias (e.g. kill -9),
                   changing the state of the flag will clear the entire firewall
                   range allocated for holes.  This clearing will also happen on
                   the initial call to LibAliasSetFWBase(), which must happen
                   prior to setting this flag.

           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.

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

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

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

     void LibAliasSkinnyPort(struct libalias *, unsigned int port)

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

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 handling
     functions, LibAliasIn() and LibAliasOut(), comprise the 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 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_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, as for an ICMP message type
                   that is not handled, or if incoming packets for new
                   connections are being ignored (if the PKT_ALIAS_DENY_INCOMING
                   mode bit was set using 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
                   situation, 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
                   unresolved 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 permissible
           should the packet length be changed.  IP encoding protocols place
           address and port information in the encapsulated data stream which
           has to be modified and can account for changes in packet length.
           Well known examples of such protocols are FTP and IRC DCC.

           Return codes:

           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 network
     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 with load sharing, then
           local_addr and local_port are ignored, and are selected dynamically
           from the server pool, as described in LibAliasAddServer() below.

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

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

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

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

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

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

           LibAliasRedirectAddr(la, inet_aton("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 the server pool, using a real-time load sharing
           algorithm.  Multiple sessions may be initiated from the same client,
           and each session could be directed to a different host based on the
           load balance across server pool hosts when the sessions are
           initiated.  If load sharing is desired for just a few specific
           services, the configuration on LSNAT could be defined to restrict
           load sharing to just the services desired.

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

           First, the link is created by either LibAliasRedirectPort() or
           LibAliasRedirectAddr().  Then, LibAliasAddServer() is called multiple
           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.
           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 partially
           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 functions.  If
           an invalid pointer is passed to LibAliasRedirectDelete(), then a
           program crash or unpredictable operation could result, so care is
           needed when using this function.

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

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

           type encode_ip_hdr | encode_tcp_stream | no_encode
                   In order to support transparent proxying, it is necessary to
                   somehow pass the original address and port information into
                   the new destination server.  If encode_ip_hdr is specified,
                   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 destination 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 specification,
                   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
                   specified, then the first bits bits of IP are taken as a
                   network specification, and all IP addresses from that network
                   will be matched.

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

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

           This function specifies that any IP packet with protocol number of
           proto from a given remote address to an alias address will be
           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
           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(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
     struct alias_link * AddLink(struct libalias *, struct in_addr src_addr,
     struct in_addr dst_addr, struct in_addr alias_addr, u_short src_port,
     u_short dst_port, int alias_param, int link_type)

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

     void LibAliasSetTarget(struct libalias *, 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
           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
           sequentially 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
           elsewhere 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).

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 TIMESTAMP requests)
     work on a simple timeout rule.  When no activity is observed on a dynamic
     link for a certain amount of time it is automatically deleted.  Timeout
     rules also apply to TCP connections which do not open or close 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
     connection 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 mechanism.

           (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 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 numbers 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 recognizable
     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 handle
     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_
     prefix.

     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
     provides a SIGHUP signal handler, add a call to LibAliasRefreshModules()
     inside the handler, and every time 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 keeps track of all the protocol handlers loaded, while
     ddl_chain tracks which userland modules are loaded.

     handler_chain is composed of struct proto_handler entries:

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

     where:

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

     dir is the direction of packets: ingoing or outgoing.

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

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

     The fingerprint function has the dual role of checking if the incoming
     packet is found, and if it belongs to any categories that this module can
     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
     structure, then find_handler() is called.  The find_handler() function is
     responsible for walking the handler chain; it receives as input parameters:

     IN      direction

     UDP     working protocol

     la      pointer to this instance of libalias

     pip     pointer to a struct ip

     ad      pointer to struct alias_data (see above)

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

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

     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 is added to
     dll_chain.

     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
     registered 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[] = {
         {
             .pri = 666,
             .dir = IN|OUT,
             .proto = UDP|TCP,
             .fingerprint = fingerprint,
             .protohandler= protohandler,
         },
         { EOH }
     };

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

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

     handlers contains all the proto_handler structures present in a module.

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

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

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

     #ifdef _KERNEL
     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 following
     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 function.
     For example, to patch natd(8) to use libalias modules, just add the
     following 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 buffer
     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.

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.
     Gleb Smirnoff <glebius@FreeBSD.org> ported the library to kernel space.
     Paolo Pisati <piso@FreeBSD.org> made the library modular, moving support
     for all protocols (except for IP, TCP and UDP) to external modules.

ACKNOWLEDGEMENTS
     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

BSD                             December 25, 2013                            BSD