FreeBSD/Linux Kernel Cross Reference
sys/netinet/libalias/libalias.3

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    .Dd May 31, 2021
    .Dt LIBALIAS 3
    .Os
    .Sh NAME
    .Nm libalias
    .Nd packet aliasing library for masquerading and network address translation
    .Sh SYNOPSIS
    .In sys/types.h
    .In netinet/in.h
    .In alias.h
    .Pp
    Function prototypes are given in the main body of the text.
    .Sh DESCRIPTION
    The
    .Nm
    library is a collection of functions for aliasing and de-aliasing of IP
    packets, intended for masquerading and network address translation (NAT).
    .Sh 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.
    .Pp
    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.
    .Sh INITIALIZATION AND CONTROL
    One special function,
    .Fn 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
    .Fn 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
    .Fn LibAliasSetMode .
    .Pp
    .Ft "struct libalias *"
    .Fn LibAliasInit "struct libalias *"
    .Bd -ragged -offset indent
    This function is used to initialize
    internal data structures.
    When called the first time, a
    .Dv NULL
    pointer should be passed as an argument.
    The following mode bits are always set after calling
    .Fn LibAliasInit .
    See the description of
    .Fn LibAliasSetMode
    below for the meaning of these mode bits.
    .Pp
    .Bl -item -offset indent -compact
    .It
    .Dv PKT_ALIAS_SAME_PORTS
    .It
    .Dv PKT_ALIAS_USE_SOCKETS
    .It
    .Dv PKT_ALIAS_RESET_ON_ADDR_CHANGE
    .El
    .Pp
    This function will always return the packet aliasing engine to the same
   initial state.
   The
   .Fn LibAliasSetAddress
   function is normally called afterwards, and any desired changes from the
   default mode bits listed above require a call to
   .Fn LibAliasSetMode .
   .Pp
   It is mandatory that this function be called at the beginning of a program
   prior to any packet handling.
   .Ed
   .Pp
   .Ft void
   .Fn LibAliasUninit "struct libalias *"
   .Bd -ragged -offset indent
   This function has no return value and is used to clear any
   resources attached to internal data structures.
   .Pp
   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
   .Xr atexit 3
   chain by
   .Fn LibAliasInit .
   .Ed
   .Pp
   .Ft void
   .Fn LibAliasSetAddress "struct libalias *" "struct in_addr addr"
   .Bd -ragged -offset indent
   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
   .Fn LibAliasRedirectAddr .
   If this function has not been called, and no static rules match, an outgoing
   packet retains its source address.
   .Pp
   If the
   .Dv 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
   .Xr ppp 8 ,
   where the IP
   address may or may not change on successive dial-up attempts.
   .Pp
   If the
   .Dv 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).
   .Pp
   It is mandatory that this function be called prior to any packet handling.
   .Ed
   .Pp
   .Ft unsigned int
   .Fn LibAliasSetMode "struct libalias *" "unsigned int flags" "unsigned int mask"
   .Bd -ragged -offset indent
   This function sets or clears mode bits
   according to the value of
   .Fa flags .
   Only bits marked in
   .Fa mask
   are affected.
   The following mode bits are defined in
   .In alias.h :
   .Bl -tag -width indent
   .It Dv PKT_ALIAS_LOG
   Enables logging into
   .Pa /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
   .Xr tail 1 .
   .It Dv 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
   .Po
   .Fn LibAliasIn
   returns
   .Dv PKT_ALIAS_IGNORED
   code
   .Pc
   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.
   .It Dv 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.
   .It Dv 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.
   .It Dv 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:
   .Pp
   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)
   .Pp
   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.
   .It Dv PKT_ALIAS_UNREGISTERED_CGN
   Like PKT_ALIAS_UNREGISTERED_ONLY, but includes the RFC 6598 (Carrier Grade
   NAT) subnet as follows:
   .Pp
   100.64.0.0   ->  100.127.255.255  (RFC 6598 subnet)
   .It Dv PKT_ALIAS_RESET_ON_ADDR_CHANGE
   When this mode bit is set and
   .Fn 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
   .Xr 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.
   .It Dv PKT_ALIAS_PUNCH_FW
   This option makes
   .Nm
   .Dq punch holes
   in an
   .Xr 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
   .Nm
   (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
   .Fn LibAliasSetFWBase ,
   which must happen prior to setting this flag.
   .It Dv PKT_ALIAS_REVERSE
   This option makes
   .Nm
   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.
   .It Dv PKT_ALIAS_PROXY_ONLY
   This option tells
   .Nm
   to obey transparent proxy rules only.
   Normal packet aliasing is not performed.
   See
   .Fn LibAliasProxyRule
   below for details.
   .It Dv PKT_ALIAS_SKIP_GLOBAL
   This option is used by
   .Pa ipfw_nat
   only.
   Specifying it as a flag to
   .Fn LibAliasSetMode
   has no effect.
   See section
   .Sx NETWORK ADDRESS TRANSLATION
   in
   .Xr ipfw 8
   for more details.
   .El
   .Ed
   .Pp
   .Ft void
   .Fn LibAliasSetFWBase "struct libalias *" "unsigned int base" "unsigned int num"
   .Bd -ragged -offset indent
   Set the firewall range allocated for punching firewall holes (with the
   .Dv PKT_ALIAS_PUNCH_FW
   flag).
   The range is cleared for all rules on initialization.
   .Ed
   .Pp
   .Ft void
   .Fn LibAliasSkinnyPort "struct libalias *" "unsigned int port"
   .Bd -ragged -offset indent
   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.
   .Ed
   .Sh 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.
   .Pp
   Along with
   .Fn LibAliasInit
   and
   .Fn LibAliasSetAddress ,
   the two packet handling functions,
   .Fn LibAliasIn
   and
   .Fn LibAliasOut ,
   comprise the minimal set of functions needed for a basic IP masquerading
   implementation.
   .Pp
   .Ft int
   .Fn LibAliasIn "struct libalias *" "void *buffer" "int maxpacketsize"
   .Bd -ragged -offset indent
   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
   .Fa buffer ,
   and
   .Fa maxpacketsize
   indicates the size of the data structure containing the packet and should
   be at least as large as the actual packet size.
   .Pp
   Return codes:
   .Bl -tag -width indent
   .It Dv PKT_ALIAS_OK
   The packet aliasing process was successful.
   .It Dv 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
   .Dv PKT_ALIAS_DENY_INCOMING
   mode bit was set using
   .Fn LibAliasSetMode ) .
   .It Dv 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
   .Fn LibAliasSaveFragment
   until a header fragment is found.
   .It Dv 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
   .Fn LibAliasGetFragment
   and de-alias them with
   .Fn LibAliasFragmentIn .
   .It Dv PKT_ALIAS_ERROR
   An internal error within the packet aliasing engine occurred.
   .El
   .Ed
   .Pp
   .Ft int
   .Fn LibAliasOut "struct libalias *" "void *buffer" "int maxpacketsize"
   .Bd -ragged -offset indent
   An outgoing packet coming from the local network to a remote machine is
   aliased by this function.
   The IP packet is pointed to by
   .Fa buffer ,
   and
   .Fa 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.
   .Pp
   Return codes:
   .Bl -tag -width indent
   .It Dv PKT_ALIAS_OK
   The packet aliasing process was successful.
   .It Dv 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.
   .It Dv PKT_ALIAS_ERROR
   An internal error within the packet aliasing engine occurred.
   .El
   .Ed
   .Sh 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.
   .Pp
   .Ft struct alias_link *
   .Fo LibAliasRedirectPort
   .Fa "struct libalias *"
   .Fa "struct in_addr local_addr"
   .Fa "u_short local_port"
   .Fa "struct in_addr remote_addr"
   .Fa "u_short remote_port"
   .Fa "struct in_addr alias_addr"
   .Fa "u_short alias_port"
   .Fa "u_char proto"
   .Fc
   .Bd -ragged -offset indent
   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
   .Fa proto
   can be either
   .Dv IPPROTO_TCP
   or
   .Dv IPPROTO_UDP ,
   as defined in
   .In netinet/in.h .
   .Pp
   If
   .Fa local_addr
   or
   .Fa alias_addr
   is zero, this indicates that the packet aliasing address as established
   by
   .Fn LibAliasSetAddress
   is to be used.
   Even if
   .Fn LibAliasSetAddress
   is called to change the address after
   .Fn LibAliasRedirectPort
   is called, a zero reference will track this change.
   .Pp
   If the link is further set up to operate with load sharing, then
   .Fa local_addr
   and
   .Fa local_port
   are ignored, and are selected dynamically from the server pool, as described in
   .Fn LibAliasAddServer
   below.
   .Pp
   If
   .Fa remote_addr
   is zero, this indicates to redirect packets from any remote address.
   Likewise, if
   .Fa 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
   .Fn LibAliasRedirectPort
   overlap in their address/port specifications, then the most recent call
   will have precedence.
   .Pp
   This function returns a pointer which can subsequently be used by
   .Fn LibAliasRedirectDelete .
   If
   .Dv NULL
   is returned, then the function call did not complete successfully.
   .Pp
   All port numbers should be in network address byte order, so it is necessary
   to use
   .Xr 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
   .Fa struct in_addr
   data type.
   .Ed
   .Pp
   .Ft struct alias_link *
   .Fo LibAliasRedirectAddr
   .Fa "struct libalias *"
   .Fa "struct in_addr local_addr"
   .Fa "struct in_addr alias_addr"
   .Fc
   .Bd -ragged -offset indent
   This function designates that all incoming traffic to
   .Fa alias_addr
   be redirected to
   .Fa local_addr .
   Similarly, all outgoing traffic from
   .Fa local_addr
   is aliased to
   .Fa alias_addr .
   .Pp
   If
   .Fa local_addr
   or
   .Fa alias_addr
   is zero, this indicates that the packet aliasing address as established by
   .Fn LibAliasSetAddress
   is to be used.
   Even if
   .Fn LibAliasSetAddress
   is called to change the address after
   .Fn LibAliasRedirectAddr
   is called, a zero reference will track this change.
   .Pp
   If the link is further set up to operate with load sharing, then the
   .Fa local_addr
   argument is ignored, and is selected dynamically from the server pool,
   as described in
   .Fn LibAliasAddServer
   below.
   .Pp
   If subsequent calls to
   .Fn 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:
   .Pp
   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"));
   .Pp
   Any outgoing connections such as
   .Xr telnet 1
   or
   .Xr 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.
   .Pp
   Any calls to
   .Fn LibAliasRedirectPort
   will have precedence over address mappings designated by
   .Fn LibAliasRedirectAddr .
   .Pp
   This function returns a pointer which can subsequently be used by
   .Fn LibAliasRedirectDelete .
   If
   .Dv NULL
   is returned, then the function call did not complete successfully.
   .Ed
   .Pp
   .Ft int
   .Fo LibAliasAddServer
   .Fa "struct libalias *"
   .Fa "struct alias_link *link"
   .Fa "struct in_addr addr"
   .Fa "u_short port"
   .Fc
   .Bd -ragged -offset indent
   This function sets the
   .Fa 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.
   .Pp
   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.
   .Pp
   First, the
   .Fa link
   is created by either
   .Fn LibAliasRedirectPort
   or
   .Fn LibAliasRedirectAddr .
   Then,
   .Fn LibAliasAddServer
   is called multiple times to add entries to the
   .Fa link Ns 's
   server pool.
   .Pp
   For links created with
   .Fn LibAliasRedirectAddr ,
   the
   .Fa port
   argument is ignored and could have any value, e.g.\& htons(~0).
   .Pp
   This function returns 0 on success, \-1 otherwise.
   .Ed
   .Pp
   .Ft int
   .Fn LibAliasRedirectDynamic "struct libalias *" "struct alias_link *link"
   .Bd -ragged -offset indent
   This function marks the specified static redirect rule entered by
   .Fn 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
   .Sx STATIC AND DYNAMIC LINKS
   and
   .Sx PARTIALLY SPECIFIED ALIASING LINKS
   sections below for a definition of static vs.\& dynamic,
   and partially vs.\& fully specified links.)
   .Pp
   This function returns 0 on success, \-1 otherwise.
   .Ed
   .Pp
   .Ft void
   .Fn LibAliasRedirectDelete "struct libalias *" "struct alias_link *link"
   .Bd -ragged -offset indent
   This function will delete a specific static redirect rule entered by
   .Fn LibAliasRedirectPort
   or
   .Fn LibAliasRedirectAddr .
   The parameter
   .Fa link
   is the pointer returned by either of the redirection functions.
   If an invalid pointer is passed to
   .Fn LibAliasRedirectDelete ,
   then a program crash or unpredictable operation could result, so
   care is needed when using this function.
   .Ed
   .Pp
   .Ft int
   .Fn LibAliasProxyRule "struct libalias *" "const char *cmd"
   .Bd -ragged -offset indent
   The passed
   .Fa 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:
   .Bl -tag -width indent
   .It Cm 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
   .Cm encode_ip_hdr
   is specified, the original destination address and port are passed
   as an extra IP option.
   If
   .Cm 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
   .Dq Li DEST Ar IP port .
   .It Cm port Ar portnum
   Only packets with the destination port
   .Ar portnum
   are proxied.
   .It Cm server Ar host Ns Op : Ns Ar portnum
   This specifies the
   .Ar host
   and
   .Ar portnum
   that the data is to be redirected to.
   .Ar host
   must be an IP address rather than a DNS host name.
   If
   .Ar portnum
   is not specified, the destination port number is not changed.
   .Pp
   The
   .Ar server
   specification is mandatory unless the
   .Cm delete
   command is being used.
   .It Cm rule Ar index
   Normally, each call to
   .Fn LibAliasProxyRule
   inserts the next rule at the start of a linear list of rules.
   If an
   .Ar index
   is specified, the new rule will be checked after all rules with lower
   indices.
   Calls to
   .Fn LibAliasProxyRule
   that do not specify a rule are assigned rule 0.
   .It Cm delete Ar index
   This token and its argument MUST NOT be used with any other tokens.
   When used, all existing rules with the given
   .Ar index
   are deleted.
   .It Cm proto tcp | udp
   If specified, only packets of the given protocol type are matched.
   .It Cm src Ar IP Ns Op / Ns Ar bits
   If specified, only packets with a source address matching the given
   .Ar IP
   are matched.
   If
   .Ar bits
   is also specified, then the first
   .Ar bits
   bits of
   .Ar IP
   are taken as a network specification, and all IP addresses from that
   network will be matched.
   .It Cm dst Ar IP Ns Op / Ns Ar bits
   If specified, only packets with a destination address matching the given
   .Ar IP
   are matched.
   If
   .Ar bits
   is also specified, then the first
   .Ar bits
   bits of
   .Ar IP
   are taken as a network specification, and all IP addresses from that
   network will be matched.
   .El
   .Pp
   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.
   .Ed
   .Pp
   .Ft struct alias_link *
   .Fo LibAliasRedirectProto
   .Fa "struct libalias *"
   .Fa "struct in_addr local_addr"
   .Fa "struct in_addr remote_addr"
   .Fa "struct in_addr alias_addr"
   .Fa "u_char proto"
   .Fc
   .Bd -ragged -offset indent
   This function specifies that any IP packet with protocol number of
   .Fa proto
   from a given remote address to an alias address will be
   redirected to a specified local address.
   .Pp
   If
   .Fa local_addr
   or
   .Fa alias_addr
   is zero, this indicates that the packet aliasing address as established
   by
   .Fn LibAliasSetAddress
   is to be used.
   Even if
   .Fn LibAliasSetAddress
   is called to change the address after
   .Fn LibAliasRedirectProto
   is called, a zero reference will track this change.
   .Pp
   If
   .Fa remote_addr
   is zero, this indicates to redirect packets from any remote address.
   Non-zero remote addresses can sometimes be useful for firewalling.
   .Pp
   If two calls to
   .Fn LibAliasRedirectProto
   overlap in their address specifications, then the most recent call
   will have precedence.
   .Pp
   This function returns a pointer which can subsequently be used by
   .Fn LibAliasRedirectDelete .
   If
   .Dv NULL
   is returned, then the function call did not complete successfully.
   .Ed
   .Sh FRAGMENT HANDLING
   The functions in this section are used to deal with incoming fragments.
   .Pp
   Outgoing fragments are handled within
   .Fn LibAliasOut
   by changing the address according to any applicable mapping set by
   .Fn LibAliasRedirectAddr ,
   or the default aliasing address set by
   .Fn LibAliasSetAddress .
   .Pp
   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.
   .Pp
   .Ft int
   .Fn LibAliasSaveFragment "struct libalias *" "void *ptr"
   .Bd -ragged -offset indent
   When
   .Fn LibAliasIn
   returns
   .Dv PKT_ALIAS_UNRESOLVED_FRAGMENT ,
   this function can be used to save the pointer to the unresolved fragment.
   .Pp
   It is implicitly assumed that
   .Fa ptr
   points to a block of memory allocated by
   .Xr 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.]
   .Pp
   This function returns
   .Dv PKT_ALIAS_OK
   if it was successful and
   .Dv PKT_ALIAS_ERROR
   if there was an error.
   .Ed
   .Pp
   .Ft void *
   .Fn LibAliasGetFragment "struct libalias *" "void *buffer"
   .Bd -ragged -offset indent
   This function can be used to retrieve fragment pointers saved by
   .Fn LibAliasSaveFragment .
   The IP header fragment pointed to by
   .Fa buffer
   is the header fragment indicated when
   .Fn LibAliasIn
   returns
   .Dv 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.
   .Pp
   The
   .Fn LibAliasGetFragment
   function can be called sequentially until there are no more fragments
   available, at which time it returns
   .Dv NULL .
   .Ed
   .Pp
   .Ft void
   .Fn LibAliasFragmentIn "struct libalias *" "void *header" "void *fragment"
   .Bd -ragged -offset indent
   When a fragment is retrieved with
   .Fn LibAliasGetFragment ,
   it can then be de-aliased with a call to
   .Fn LibAliasFragmentIn .
   The
   .Fa header
   argument is the pointer to a header fragment used as a template, and
   .Fa fragment
   is the pointer to the packet to be de-aliased.
   .Ed
   .Sh MISCELLANEOUS FUNCTIONS
   .Ft struct alias_link *
   .Fn 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"
   .Bd -ragged -offset indent
   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.
   .Ed
   .Pp
   .Ft void
   .Fn LibAliasSetTarget "struct libalias *" "struct in_addr addr"
   .Bd -ragged -offset indent
   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
   .Fn LibAliasSetTarget .
   .Pp
   If this function is called with an
   .Dv INADDR_NONE
   address argument, then all new incoming packets go to the address set by
   .Fn LibAliasSetAddress .
   .Pp
   If this function is not called, or is called with an
   .Dv 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.
   .Ed
   .Pp
   .Ft u_short
   .Fn LibAliasInternetChecksum "struct libalias *" "u_short *buffer" "int nbytes"
   .Bd -ragged -offset indent
   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).
   .Pp
   The
   .Fa buffer
   argument points to the data block to be checksummed, and
   .Fa nbytes
   is the number of bytes.
   The 16-bit checksum field should be zeroed before computing the checksum.
   .Pp
   Checksums can also be verified by operating on a block of data including
   its checksum.
   If the checksum is valid,
   .Fn LibAliasInternetChecksum
   will return zero.
   .Ed
   .Pp
   .Ft int
   .Fn LibAliasUnaliasOut "struct libalias *" "void *buffer" "int maxpacketsize"
   .Bd -ragged -offset indent
   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
   .Fa buffer ,
   and
   .Fa 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).
   .Ed
   .Sh 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.
   .Pp
   The conceptual framework under which the packet aliasing engine operates
   is described here.
   Central to the discussion is the idea of an
   .Em 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.
   .Ss ALIASING LINKS
   There is a notion of an
   .Em aliasing link ,
   which is a 7-tuple describing a specific translation:
   .Bd -literal -offset indent
   (local addr, local port, alias addr, alias port,
    remote addr, remote port, protocol)
   .Ed
   .Pp
   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.
   .Pp
   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.)
   .Pp
   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.
   .Ss 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.
   .Ss 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.
   .Pp
   For instance, a partially specified link might be
   .Bd -literal -offset indent
   (192.168.0.4, 23, 204.228.203.215, 8066, 0, 0, tcp)
   .Ed
   .Pp
   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.
   .Ss 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.
   .Bd -literal -offset indent
   (local addr, alias addr)
   .Ed
   .Pp
   Address mappings are searched when creating new dynamic links.
   .Pp
   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.
   .Pp
   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.
  .Sh MODULAR ARCHITECTURE Po AND Xr ipfw 4 SUPPORT Pc
  One of the latest improvements to
  .Nm
  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.
  .Ss LIBALIAS MODULES IN KERNEL LAND
  When compiled for the kernel,
  .Nm
  modules are plain KLDs recognizable with the
  .Pa alias_
  prefix.
  .Pp
  To add support for a new protocol, load the corresponding module.
  For example:
  .Pp
  .Dl "kldload alias_ftp"
  .Pp
  When support for a protocol is no longer needed, its module can be unloaded:
  .Pp
  .Dl "kldunload alias_ftp"
  .Ss 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.
  .Pp
  While compiled for a userland
  .Nm ,
  all the modules are plain libraries, residing in
  .Pa /usr/lib ,
  and recognizable with the
  .Pa libalias_
  prefix.
  .Pp
  There is a configuration file,
  .Pa /etc/libalias.conf ,
  with the following contents (by default):
  .Bd -literal -offset indent
  /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
  .Ed
  .Pp
  This file contains the paths to the modules that
  .Nm
  will load.
  To load/unload a new module, just add its path to
  .Pa libalias.conf
  and call
  .Fn LibAliasRefreshModules
  from the program.
  In case the application provides a
  .Dv SIGHUP
  signal handler, add a call to
  .Fn LibAliasRefreshModules
  inside the handler, and every time you want to refresh the loaded modules,
  send it the
  .Dv SIGHUP
  signal:
  .Pp
  .Dl "kill -HUP <process_pid>"
  .Ss MODULAR ARCHITECURE: HOW IT WORKS
  The modular architecture of
  .Nm
  works similar whether it is running inside the
  kernel or in userland.
  From
  .Pa alias_mod.c :
  .Bd -literal
  /* Protocol and userland module handlers chains. */
  LIST_HEAD(handler_chain, proto_handler) handler_chain ...
  \&...
  SLIST_HEAD(dll_chain, dll) dll_chain ...
  .Ed
  .Pp
  .Va handler_chain
  keeps track of all the protocol handlers loaded, while
  .Va ddl_chain
  tracks which userland modules are loaded.
  .Pp
  .Va handler_chain
  is composed of
  .Vt "struct proto_handler"
  entries:
  .Bd -literal
  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;
  };
  .Ed
  .Pp
  where:
  .Bl -inset
  .It Va pri
  is the priority assigned to a protocol handler; lower priority
  is better.
  .It Va dir
  is the direction of packets: ingoing or outgoing.
  .It Va proto
  indicates to which protocol this packet belongs: IP, TCP or UDP.
  .It Va fingerprint
  points to the fingerprint function while protohandler points
  to the protocol handler function.
  .El
  .Pp
  The
  .Va 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.
  .Pp
  The
  .Va protohandler
  function actually manipulates
  the packet to make
  .Nm
  correctly NAT it.
  .Pp
  When a packet enters
  .Nm ,
  if it meets a module hook,
  .Va 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
  .Va fingerprint
  function and interprets the result.
  .Pp
  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
  .Va handler_chain
  is exhausted.
  .Pp
  Inside
  .Nm ,
  the module hook looks like this:
  .Bd -literal -offset indent
  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);
  .Ed
  .Pp
  All data useful to a module are gathered together in an
  .Vt alias_data
  structure, then
  .Fn find_handler
  is called.
  The
  .Fn find_handler
  function is responsible for walking the handler
  chain; it receives as input parameters:
  .Bl -tag -width indent
  .It Fa IN
  direction
  .It Fa UDP
  working protocol
  .It Fa la
  pointer to this instance of libalias
  .It Fa pip
  pointer to a
  .Vt "struct ip"
  .It Fa ad
  pointer to
  .Vt "struct alias_data"
  (see above)
  .El
  .Pp
  In this case,
  .Fn find_handler
  will search only for modules registered for
  supporting INcoming UDP packets.
  .Pp
  As was mentioned earlier,
  .Nm
  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
  .Va dll_chain
  was introduced.
  .Pp
  .Va dll_chain
  contains a list of all userland
  .Nm
  modules loaded.
  .Pp
  When an application calls
  .Fn LibAliasRefreshModules ,
  .Nm
  first unloads all the loaded modules, then reloads all the modules listed in
  .Pa /etc/libalias.conf :
  for every module loaded, a new entry is added to
  .Va dll_chain .
  .Pp
  .Va dll_chain
  is composed of
  .Vt "struct dll"
  entries:
  .Bd -literal
  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;
  };
  .Ed
  .Bl -inset
  .It Va name
  is the name of the module.
  .It Va handle
  is a pointer to the module obtained through
  .Xr dlopen 3 .
  .El
  Whenever a module is loaded in userland, an entry is added to
  .Va dll_chain ,
  then every protocol handler present in that module
  is resolved and registered in
  .Va handler_chain .
  .Ss HOW TO WRITE A MODULE FOR LIBALIAS
  There is a module (called
  .Pa alias_dummy.[ch] )
  in
  .Nm
  that can be used as a skeleton for future work.
  Here we analyse some parts of that module.
  From
  .Pa alias_dummy.c :
  .Bd -literal
  struct proto_handler handlers[] = {
      {
          .pri = 666,
          .dir = IN|OUT,
          .proto = UDP|TCP,
          .fingerprint = fingerprint,
          .protohandler= protohandler,
      },
      { EOH }
  };
  .Ed
  .Pp
  The variable
  .Va handlers
  is the
  .Dq "most important thing"
  in a module
  since it describes the handlers present and lets the outside world use
  it in an opaque way.
  .Pp
  It must ALWAYS be present in every module, and it MUST retain
  the name
  .Va 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
  .Fn LibAliasRefreshModules ,
  .Fn LibAliasLoadModule
  and
  .Fn LibAliasUnloadModule
  in
  .Pa alias.c .
  .Pp
  .Va handlers
  contains all the
  .Vt proto_handler
  structures present in a module.
  .Bd -literal
  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);
  }
  .Ed
  When running as KLD,
  .Fn mod_handler
  registers/deregisters the module using
  .Fn LibAliasAttachHandlers
  and
  .Fn LibAliasDetachHandlers ,
  respectively.
  .Pp
  Every module must contain at least 2 functions: one fingerprint
  function and a protocol handler function.
  .Bd -literal
  #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)
  {
  
  \&...
  }
  .Ed
  and they must accept exactly these input parameters.
  .Ss PATCHING AN APPLICATION FOR USERLAND LIBALIAS MODULES
  To add module support into an application that uses
  .Nm ,
  the following simple steps can be followed.
  .Bl -enum
  .It
  Find the main file of an application
  (let us call it
  .Pa main.c ) .
  .It
  Add this to the header section of
  .Pa main.c ,
  if not already present:
  .Pp
  .Dl "#include <signal.h>"
  .Pp
  and this just after the header section:
  .Pp
  .Dl "static void signal_handler(int);"
  .It
  Add the following line to the init function of an application or,
  if it does not have any init function, put it in
  .Fn main :
  .Pp
  .Dl "signal(SIGHUP, signal_handler);"
  .Pp
  and place the
  .Fn signal_handler
  function somewhere in
  .Pa main.c :
  .Bd -literal -offset indent
  static void
  signal_handler(int sig)
  {
  
          LibAliasRefreshModules();
  }
  .Ed
  .Pp
  Otherwise, if an application already traps the
  .Dv SIGHUP
  signal, just add a call to
  .Fn LibAliasRefreshModules
  in the signal handler function.
  .El
  For example, to patch
  .Xr natd 8
  to use
  .Nm
  modules, just add the following line to
  .Fn RefreshAddr "int sig __unused" :
  .Pp
  .Dl "LibAliasRefreshModules()"
  .Pp
  recompile and you are done.
  .Ss LOGGING SUPPORT IN KERNEL LAND
  When working as KLD,
  .Nm
  now has log support that
  happens on a buffer allocated inside
  .Vt "struct libalias"
  (from
  .Pa alias_local.h ) :
  .Bd -literal
  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
  
          ...
  }
  .Ed
  so all applications using
  .Nm
  will be able to handle their
  own logs, if they want, accessing
  .Va logDesc .
  Moreover, every change to a log buffer is automatically added to
  .Xr syslog 3
  with the
  .Dv LOG_SECURITY
  facility and the
  .Dv LOG_INFO
  level.
  .Sh AUTHORS
  .An Charles Mott Aq cm@linktel.net ,
  versions 1.0 - 1.8, 2.0 - 2.4.
  .An Eivind Eklund Aq 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.
  .An Erik Salander Aq erik@whistle.com
  added support for PPTP and RTSP.
  .An Junichi Satoh Aq junichi@junichi.org
  added support for RTSP/PNA.
  .An Ruslan Ermilov Aq ru@FreeBSD.org
  added support for PPTP and LSNAT as well as general hacking.
  .An Gleb Smirnoff Aq glebius@FreeBSD.org
  ported the library to kernel space.
  .An Paolo Pisati Aq piso@FreeBSD.org
  made the library modular, moving support for all
  protocols (except for IP, TCP and UDP) to external modules.
  .Sh ACKNOWLEDGEMENTS
  Listed below, in approximate chronological order, are individuals who
  have provided valuable comments and/or debugging assistance.
  .Bd -ragged -offset indent
  .An -split
  .An Gary Roberts
  .An Tom Torrance
  .An Reto Burkhalter
  .An Martin Renters
  .An Brian Somers
  .An Paul Traina
  .An Ari Suutari
  .An Dave Remien
  .An J. Fortes
  .An Andrzej Bialecki
  .An Gordon Burditt
  .Ed

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