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libalias(3)            FreeBSD Library Functions Manual            libalias(3)

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

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

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

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

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

1. Introduction
     This library is a moderately portable set of functions designed to assist
     in the process of IP masquerading and network address translation.
     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 local network and the packet aliasing host.  This is known
     as IP masquerading.  In addition, one-to-one mappings between local and
     public addresses can also be implemented, which is known as static NAT.
     In between these extremes, different groups of private addresses can be
     linked to different public addresses, comprising several distinct many-
     to-one mappings.  Also, a given public address and port can be statically
     redirected to a private address/port.

     The packet aliasing engine was designed to operate in user space outside
     of the kernel, without any access to private kernel data structure, but
     the source code can also be ported to a kernel environment.

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

   2.1 PacketAliasInit()
     void PacketAliasInit(void)

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


     This function will always return the packet aliasing engine to the same
     initial state.  PacketAliasSetAddress() must be called afterwards, and
     any desired changes from the default mode bits listed above require a
     call to PacketAliasSetMode().

     It is mandatory that this function be called at the beginning of a
     program prior to any packet handling.

   2.2 PacketAliasUninit()
     void PacketAliasUninit(void)

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

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

   2.3 PacketAliasSetAddress()
     void PacketAliasSetAddress(struct in_addr addr)

     This function sets the source address to which outgoing packets from the
     local area network are aliased.  All outgoing packets are remapped to
     this address unless overridden by a static address mapping established by

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

     If the PKT_ALIAS_RESET_ON_ADDR_CHANGE mode bit is set to zero, this
     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

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

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

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

         PKT_ALIAS_DENY_INCOMING. If this mode bit is set, all incoming
                 packets associated with new TCP connections or new UDP
                 transactions will be marked for being ignored
                 (PacketAliasIn() return code PKT_ALIAS_IGNORED) by the
                 calling program.  Response packets to connections or
                 transactions initiated from the packet aliasing host or local
                 network will be 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 number.  This can be done as long
                 as the quintuple (proto, alias addr, alias port, remote addr,
                 remote port) is unique.  If a conflict exists, an new
                 aliasing port number is chosen even if this mode bit is set.

         PKT_ALIAS_USE_SOCKETS. This bit should be set when the the packet
                 aliasing host originates network traffic as well as forwards
                 it.  When the packet aliasing host is waiting for a
                 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:

                      ->   (Class A subnet)
                    ->   (Class B subnets)
                   ->  (Class C subnets)

                 This option is useful in the case that packet aliasing host
                 has both registered and unregistered subnets on different
                 interfaces.  The registered subnet is fully accessible to the
                 outside world, so traffic from it doesn't need to be passed
                 through the packet aliasing engine.

         PKT_ALIAS_RESET_ON_ADDR_CHANGE. When this mode bit is set and
                 PacketAliasSetAddress() is called to change the aliasing
                 address, the internal link table of the packet aliasing
                 engine will be cleared.  This operating mode is useful for
                 ppp links where the interface address can sometimes change or
                 remain the same between dial-ups.  If this mode bit is not
                 set, it the link table will never be reset in the event of an
                 address change.

         PKT_ALIAS_PUNCH_FW. This option make libalias `punch holes' in an
                 ipfw based firewall for FTP/IRC DCC connections.  The holes
                 punched are bound by from/to IP address and port; it will not
                 be possible to use a hole for another connection.  A hole is
                 removed when the connection that use it die.  To cater for
                 unexpected death of a program using libalias (e.g kill -9),
                 changing the state of the flag will clear the entire ipfw
                 range allocated for holes.  This will also happen on the
                 initial call to PacketAliasSetFWBase().  This call must
                 happen prior to setting this flag.

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

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

3. Packet Handling
     The packet handling functions are used to modify incoming (remote->local)
     and outgoing (local->remote) packets.  The calling program is responsible
     for receiving and sending packets via network interfaces.

     Along with PacketAliasInit() and PacketAliasSetAddress(), the two packet
     handling functions, PacketAliasIn() and PacketAliasOut(), comprise
     minimal set of functions needed for a basic IP masquerading

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

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

     Return codes:

         PKT_ALIAS_ERROR. An internal error within the packet aliasing engine

         PKT_ALIAS_OK. The packet aliasing process was successful.

         PKT_ALIAS_IGNORED. The packet was ignored and not de-aliased.  This
                 can happen if the protocal is unrecognized, possibly an ICMP
                 message type is not handled or if incoming packets for new
                 connections are being ignored (see PKT_ALIAS_DENY_INCOMING in
                 section 2.2).

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

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

   3.2 PacketAliasOut()
     int PacketAliasOut(char *buffer, int maxpacketsize)

     An outgoing packet coming from the local network to a remote machine is
     aliased by this function.  The IP packet is pointed to by buffer r, and
     maxpacketsize indicates the maximum packet size permissible should the
     packet length be changed.  IP encoding protocols place address and port
     information in the encapsulated data stream which have to be modified and
     can account for changes in packet length.  Well known examples of such
     protocols are FTP and IRC DCC.

     Return codes:

         PKT_ALIAS_ERROR. An internal error within the packet aliasing engine

         PKT_ALIAS_OK. The packet aliasing process was successful.

         PKT_ALIAS_IGNORED. The packet was ignored and not de-aliased.  This
                 can happen if the protocal is unrecognized, or possibly an
                 ICMP message type is not handled.

4. Port and Address Redirection
     The functions described in this section allow machines on the local
     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.

   4.1 PacketAliasRedirectPort()
     struct alias_link * PacketAliasRedirectPort(struct in_addr local_addr,
     u_short local_port, struct in_addr remote_addr, u_short remote_port,
     struct in_addr alias_addr, u_short alias_port, u_char proto)

     This function specifies that traffic from a given remote address/port to
     an alias address/port be redirected to a specified local address/port.
     The parameter proto can be either IPPROTO_TCP or IPPROTO_UDP, as defined
     in <netinet/in.h>.

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

     If remote_addr is zero, this indicates to redirect packets from any
     remote address.  Likewise, if remote_port is zero, this indicates to
     redirect packets originating from any remote port number.  Almost always,
     the remote port specification will be zero, but non-zero remote addresses
     can be sometimes be useful for firewalling.  If two calls to
     PacketAliasRedirectPort() overlap in their address/port specifications,
     then the most recent call will have precedence.

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

     All port numbers are in network address byte order, so it is necessary to
     use htons() to convert these parameters from internally readable numbers
     to network byte order.  Addresses are also in network byte order, which
     is implicit in the use of the struct in_addr data type.

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

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

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

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


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

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

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

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

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

5. Fragment Handling
     The functions in this section are used to deal with incoming fragments.

     Outgoing fragments are handled within PacketAliasOut() by changing the
     address according to any applicable mapping set by
     PacketAliasRedirectAddress(), or the default aliasing address set by

     Incoming fragments are handled in one of two ways.  If the header of a
     fragmented IP packet has already been seen, then all subsequent fragments
     will be re-mapped in the same manner the header fragment was.  Fragments
     which arrive before the header are saved and then retrieved once the
     header fragment has been resolved.

   5.1 PacketAliasSaveFragment()
     int PacketAliasSaveFragment(char *ptr)

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

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

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

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

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

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

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

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

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

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

   6.2 PacketAliasCheckNewLink()
     int PacketAliasCheckNewLink(void)

     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 PacketAliasSetTarget() is called to change the default target

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

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

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

     Checksums can also be verified by operating on a block of data including
     its checksum.  If the checksum is valid, PacketAliasInternetChecksum()
     will return zero.

7. Authors
     Charles Mott (, versions 1.0 - 1.8, 2.0 - 2.4.

     Eivind Eklund (, versions 1.8b, 1.9 and 2.5.  Added
     IRC DCC support as well as contributing a number of architectural
     improvements; added the firewall bypass for FTP/IRC DCC.

8. Acknowledgments
     Listed below, in approximate chronological order, are individuals who
     have provided valuable comments and/or debugging assistance.

         Gary Roberts
         Tom Torrance
         Reto Burkhalter
         Martin Renters
         Brian Somers
         Paul Traina
         Ari Suutari
         Dave Remien
         J. Fortes
         Andrzej Bialeki
         Gordon Burditt

Appendix: Conceptual Background
     This appendix is intended for those who are planning to modify the source
     code or want to create somewhat esoteric applications using the packet
     aliasing functions.

     The conceptual framework under which the packet aliasing engine operates
     is described here.  Central to the discussion is the idea of an "aliasing
     link" which  describes the relationship for a given packet transaction
     between the local machine, aliased identity and remote machine.  It is
     discussed how such links come into existence and are destroyed.

   A.1 Aliasing Links
     There is a notion of an "aliasing link", which is 7-tuple describing a
     specific translation:

           (local addr, local port, alias addr, alias port,
            remote addr, remote port, protocol)

     Outgoing packets have the local address and port number replaced with the
     alias address and port number.  Incoming packets undergo the reverse
     process.  The packet aliasing engine attempts to match packets against an
     internal table of aliasing links to determine how to modify a given IP
     packet.  Both the IP header and protocol dependent headers are modified
     as necessary.  Aliasing links are created and deleted as necessary
     according to network traffic.

     Protocols can be TCP, UDP or even ICMP in certain circumstances.  (Some
     types of ICMP packets can be aliased according to sequence or id number
     which acts as an equivalent port number for identifying how individual
     packets should be handled.)

     Each aliasing link must have a unique combination of the following five
     quantities: alias address/port, remote address/port and protocol.  This
     ensures that several machines on a local network can share the same
     aliased IP address.  In cases where conflicts might arise, the aliasing
     port is chosen so that uniqueness is maintained.

   A.2 Static and Dynamic Links
     Aliasing links can either be static or dynamic.  Static links persist
     indefinitely and represent fixed rules for translating IP packets.
     Dynamic links come into existence for a specific TCP connection or UDP
     transaction or ICMP echo sequence.  For the case of TCP, the connection
     can be monitored to see when the associated aliasing link should be
     deleted.  Aliasing links for UDP transactions (and ICMP echo and
     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.

   A.3 Partially Specified Aliasing Links
     Aliasing links can be partially specified, meaning that the remote
     address and/or remote ports are unknown.  In this case, when a packet
     matching the incomplete specification is found, a fully specified dynamic
     link is created.  If the original partially specified link is dynamic, it
     will be deleted after the fully specified link is created, otherwise it
     will persist.

     For instance, a partially specified link might be

           (, 23,, 8066, 0, 0, tcp)

     The zeros denote unspecified components for the remote address and port.
     If this link were static it would have the effect of redirecting all
     incoming traffic from port 8066 of to port 23 (telnet) of
     machine on the local network.  Each individual telnet
     connection would initiate the creation of a distinct dynamic link.

   A.4 Dynamic Link Creation
     In addition to aliasing links, there are also address mappings that can
     be stored within the internal data table of the packet aliasing

           (local addr, alias addr)

     Address mappings are searched when creating new dynamic links.

     All outgoing packets from the local network automatically create a
     dynamic link if they do not match an already existing fully specified
     link.  If an address mapping exists for the the outgoing packet, this
     determines the alias address to be used.  If no mapping exists, then a
     default address, usually the address of the packet aliasing host, is
     used.  If necessary, this default address can be changed as often as each
     individual packet arrives.

     The aliasing port number is determined such that the new dynamic link
     does not conflict with any existing links.  In the default operating
     mode, the packet aliasing engine attempts to set the aliasing port equal
     to the local port number.  If this results in a conflict, then port
     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.

FreeBSD 11.0-PRERELEASE           July, 1997           FreeBSD 11.0-PRERELEASE

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

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