2 .\" Copyright (c) 2001 Charles Mott <cm@linktel.net>
3 .\" All rights reserved.
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33 .Nd packet aliasing library for masquerading and network address translation
39 Function prototypes are given in the main body of the text.
43 library is a collection of functions for aliasing and de-aliasing of IP
44 packets, intended for masquerading and network address translation (NAT).
46 This library is a moderately portable set of functions designed to assist
47 in the process of IP masquerading and network address translation.
48 Outgoing packets from a local network with unregistered IP addresses can
49 be aliased to appear as if they came from an accessible IP address.
50 Incoming packets are then de-aliased so that they are sent to the correct
51 machine on the local network.
53 A certain amount of flexibility is built into the packet aliasing engine.
54 In the simplest mode of operation, a many-to-one address mapping takes
55 place between local network and the packet aliasing host.
56 This is known as IP masquerading.
57 In addition, one-to-one mappings between local and public addresses can
58 also be implemented, which is known as static NAT.
59 In between these extremes, different groups of private addresses can be
60 linked to different public addresses, comprising several distinct
62 Also, a given public address and port can be statically redirected to a
65 The packet aliasing engine was designed to operate in user space outside
66 of the kernel, without any access to private kernel data structure, but
67 the source code can also be ported to a kernel environment.
68 .Sh INITIALIZATION AND CONTROL
69 Two special functions,
72 .Fn PacketAliasSetAddress ,
73 must always be called before any packet handling may be performed.
74 In addition, the operating mode of the packet aliasing engine can be
76 .Fn PacketAliasSetMode .
79 .Fn PacketAliasInit void
80 .Bd -ragged -offset indent
81 This function has no arguments or return value and is used to initialize
82 internal data structures.
83 The following mode bits are always set after calling
85 See the description of
86 .Fn PacketAliasSetMode
87 below for the meaning of these mode bits.
89 .Bl -item -offset indent -compact
91 .Dv PKT_ALIAS_SAME_PORTS
93 .Dv PKT_ALIAS_USE_SOCKETS
95 .Dv PKT_ALIAS_RESET_ON_ADDR_CHANGE
98 This function will always return the packet aliasing engine to the same
100 .Fn PacketAliasSetAddress
101 must be called afterwards, and any desired changes from the default mode
102 bits listed above require a call to
103 .Fn PacketAliasSetMode .
105 It is mandatory that this function be called at the beginning of a program
106 prior to any packet handling.
110 .Fn PacketAliasUninit void
111 .Bd -ragged -offset indent
112 This function has no arguments or return value and is used to clear any
113 resources attached to internal data structures.
115 This functions should be called when a program stops using the aliasing
116 engine; it does, amongst other things, clear out any firewall holes.
117 To provide backwards compatibility and extra security, it is added to
121 .Fn PacketAliasInit .
122 Calling it multiple times is harmless.
126 .Fn PacketAliasSetAddress "struct in_addr addr"
127 .Bd -ragged -offset indent
128 This function sets the source address to which outgoing packets from the
129 local area network are aliased.
130 All outgoing packets are re-mapped to this address unless overridden by a
131 static address mapping established by
132 .Fn PacketAliasRedirectAddr .
135 .Dv PKT_ALIAS_RESET_ON_ADDR_CHANGE
136 mode bit is set (the default mode of operation), then the internal aliasing
137 link tables will be reset any time the aliasing address changes.
138 This is useful for interfaces such as
141 address may or may not change on successive dial-up attempts.
144 .Dv PKT_ALIAS_RESET_ON_ADDR_CHANGE
145 mode bit is set to zero, this function can also be used to dynamically change
146 the aliasing address on a packet to packet basis (it is a low overhead call).
148 It is mandatory that this function be called prior to any packet handling.
152 .Fn PacketAliasSetMode "unsigned int flags" "unsigned int mask"
153 .Bd -ragged -offset indent
154 This function sets or clears mode bits
155 according to the value of
160 The following mode bits are defined in
162 .Bl -tag -width indent
165 .Pa /var/log/alias.log .
166 Each time an aliasing link is created or deleted, the log file is appended
167 with the current number of ICMP, TCP and UDP links.
168 Mainly useful for debugging when the log file is viewed continuously with
170 .It Dv PKT_ALIAS_DENY_INCOMING
171 If this mode bit is set, all incoming packets associated with new TCP
172 connections or new UDP transactions will be marked for being ignored
175 .Dv PKT_ALIAS_IGNORED
177 by the calling program.
178 Response packets to connections or transactions initiated from the packet
179 aliasing host or local network will be unaffected.
180 This mode bit is useful for implementing a one-way firewall.
181 .It Dv PKT_ALIAS_SAME_PORTS
182 If this mode bit is set, the packet aliasing engine will attempt to leave
183 the alias port numbers unchanged from the actual local port numbers.
184 This can be done as long as the quintuple (proto, alias addr, alias port,
185 remote addr, remote port) is unique.
186 If a conflict exists, a new aliasing port number is chosen even if this
188 .It Dv PKT_ALIAS_USE_SOCKETS
189 This bit should be set when the packet aliasing host originates network
190 traffic as well as forwards it.
191 When the packet aliasing host is waiting for a connection from an unknown
192 host address or unknown port number (e.g. an FTP data connection), this
193 mode bit specifies that a socket be allocated as a place holder to prevent
195 Once a connection is established, usually within a minute or so, the socket
197 .It Dv PKT_ALIAS_UNREGISTERED_ONLY
198 If this mode bit is set, traffic on the local network which does not
199 originate from unregistered address spaces will be ignored.
200 Standard Class A, B and C unregistered addresses are:
201 .Bd -literal -offset indent
202 10.0.0.0 -> 10.255.255.255 (Class A subnet)
203 172.16.0.0 -> 172.31.255.255 (Class B subnets)
204 192.168.0.0 -> 192.168.255.255 (Class C subnets)
207 This option is useful in the case that packet aliasing host has both
208 registered and unregistered subnets on different interfaces.
209 The registered subnet is fully accessible to the outside world, so traffic
210 from it does not need to be passed through the packet aliasing engine.
211 .It Dv PKT_ALIAS_RESET_ON_ADDR_CHANGE
212 When this mode bit is set and
213 .Fn PacketAliasSetAddress
214 is called to change the aliasing address, the internal link table of the
215 packet aliasing engine will be cleared.
216 This operating mode is useful for
218 links where the interface address can sometimes change or remain the same
219 between dial-up attempts.
220 If this mode bit is not set, the link table will never be reset in the event
221 of an address change.
222 .It Dv PKT_ALIAS_PUNCH_FW
227 based firewall for FTP/IRC DCC connections.
228 The holes punched are bound by from/to IP address and port; it will not be
229 possible to use a hole for another connection.
230 A hole is removed when the connection that uses it dies.
231 To cater to unexpected death of a program using
234 changing the state of the flag will clear the entire firewall range
236 This will also happen on the initial call to
237 .Fn PacketAliasSetFWBase .
238 This call must happen prior to setting this flag.
239 .It Dv PKT_ALIAS_REVERSE
242 reverse the way it handles incoming and outgoing packets, allowing it
243 to be fed with data that passes through the internal interface rather
244 than the external one.
245 .It Dv PKT_ALIAS_PROXY_ONLY
248 to obey transparent proxy rules only.
249 Normal packet aliasing is not performed.
251 .Fn PacketAliasProxyRule
257 .Fn PacketAliasSetFWBase "unsigned int base" "unsigned int num"
258 .Bd -ragged -offset indent
259 Set firewall range allocated for punching firewall holes (with the
260 .Dv PKT_ALIAS_PUNCH_FW
262 The range will be cleared for all rules on initialization.
265 The packet handling functions are used to modify incoming (remote to local)
266 and outgoing (local to remote) packets.
267 The calling program is responsible for receiving and sending packets via
273 .Fn PacketAliasSetAddress ,
274 the two packet handling functions,
278 comprise minimal set of functions needed for a basic IP masquerading
282 .Fn PacketAliasIn "char *buffer" "int maxpacketsize"
283 .Bd -ragged -offset indent
284 An incoming packet coming from a remote machine to the local network is
285 de-aliased by this function.
286 The IP packet is pointed to by
290 indicates the size of the data structure containing the packet and should
291 be at least as large as the actual packet size.
294 .Bl -tag -width indent
296 The packet aliasing process was successful.
297 .It Dv PKT_ALIAS_IGNORED
298 The packet was ignored and not de-aliased.
299 This can happen if the protocol is unrecognized, possibly an ICMP message
300 type is not handled or if incoming packets for new connections are being
302 .Dv PKT_ALIAS_DENY_INCOMING
304 .Fn PacketAliasSetMode ) .
305 .It Dv PKT_ALIAS_UNRESOLVED_FRAGMENT
306 This is returned when a fragment cannot be resolved because the header
307 fragment has not been sent yet.
308 In this situation, fragments must be saved with
309 .Fn PacketAliasSaveFragment
310 until a header fragment is found.
311 .It Dv PKT_ALIAS_FOUND_HEADER_FRAGMENT
312 The packet aliasing process was successful, and a header fragment was found.
313 This is a signal to retrieve any unresolved fragments with
314 .Fn PacketAliasGetFragment
315 and de-alias them with
316 .Fn PacketAliasFragmentIn .
317 .It Dv PKT_ALIAS_ERROR
318 An internal error within the packet aliasing engine occurred.
323 .Fn PacketAliasOut "char *buffer" "int maxpacketsize"
324 .Bd -ragged -offset indent
325 An outgoing packet coming from the local network to a remote machine is
326 aliased by this function.
327 The IP packet is pointed to by
331 indicates the maximum packet size permissible should the packet length be
333 IP encoding protocols place address and port information in the encapsulated
334 data stream which has to be modified and can account for changes in packet
336 Well known examples of such protocols are FTP and IRC DCC.
339 .Bl -tag -width indent
341 The packet aliasing process was successful.
342 .It Dv PKT_ALIAS_IGNORED
343 The packet was ignored and not aliased.
344 This can happen if the protocol is unrecognized, or possibly an ICMP message
346 .It Dv PKT_ALIAS_ERROR
347 An internal error within the packet aliasing engine occurred.
350 .Sh PORT AND ADDRESS REDIRECTION
351 The functions described in this section allow machines on the local network
352 to be accessible in some degree to new incoming connections from the external
354 Individual ports can be re-mapped or static network address translations can
357 .Ft struct alias_link *
358 .Fo PacketAliasRedirectPort
359 .Fa "struct in_addr local_addr"
360 .Fa "u_short local_port"
361 .Fa "struct in_addr remote_addr"
362 .Fa "u_short remote_port"
363 .Fa "struct in_addr alias_addr"
364 .Fa "u_short alias_port"
367 .Bd -ragged -offset indent
368 This function specifies that traffic from a given remote address/port to
369 an alias address/port be redirected to a specified local address/port.
377 .Aq Pa netinet/in.h .
383 is zero, this indicates that the packet aliasing address as established
385 .Fn PacketAliasSetAddress
388 .Fn PacketAliasSetAddress
389 is called to change the address after
390 .Fn PacketAliasRedirectPort
391 is called, a zero reference will track this change.
393 If the link is further set up to operate for a load sharing, then
397 are ignored, and are selected dynamically from the server pool, as described in
398 .Fn PacketAliasAddServer
403 is zero, this indicates to redirect packets from any remote address.
406 is zero, this indicates to redirect packets originating from any remote
408 Almost always, the remote port specification will be zero, but non-zero
409 remote addresses can sometimes be useful for firewalling.
411 .Fn PacketAliasRedirectPort
412 overlap in their address/port specifications, then the most recent call
413 will have precedence.
415 This function returns a pointer which can subsequently be used by
416 .Fn PacketAliasRedirectDelete .
419 is returned, then the function call did not complete successfully.
421 All port numbers should be in network address byte order, so it is necessary
424 to convert these parameters from internally readable numbers to network byte
426 Addresses are also in network byte order, which is implicit in the use of the
431 .Ft struct alias_link *
432 .Fo PacketAliasRedirectAddr
433 .Fa "struct in_addr local_addr"
434 .Fa "struct in_addr alias_addr"
436 .Bd -ragged -offset indent
437 This function designates that all incoming traffic to
441 Similarly, all outgoing traffic from
450 is zero, this indicates that the packet aliasing address as established by
451 .Fn PacketAliasSetAddress
454 .Fn PacketAliasSetAddress
455 is called to change the address after
456 .Fn PacketAliasRedirectAddr
457 is called, a zero reference will track this change.
459 If the link is further set up to operate for a load sharing, then
461 is ignored, and is selected dynamically from the server pool, as described in
462 .Fn PacketAliasAddServer
465 If subsequent calls to
466 .Fn PacketAliasRedirectAddr
467 use the same aliasing address, all new incoming traffic to this aliasing
468 address will be redirected to the local address made in the last function
470 New traffic generated by any of the local machines, designated in the
471 several function calls, will be aliased to the same address.
472 Consider the following example:
473 .Bd -literal -offset indent
474 PacketAliasRedirectAddr(inet_aton("192.168.0.2"),
475 inet_aton("141.221.254.101"));
476 PacketAliasRedirectAddr(inet_aton("192.168.0.3"),
477 inet_aton("141.221.254.101"));
478 PacketAliasRedirectAddr(inet_aton("192.168.0.4"),
479 inet_aton("141.221.254.101"));
482 Any outgoing connections such as
486 from 192.168.0.2, 192.168.0.3 and 192.168.0.4 will appear to come from
488 Any incoming connections to 141.221.254.101 will be directed to 192.168.0.4.
491 .Fn PacketAliasRedirectPort
492 will have precedence over address mappings designated by
493 .Fn PacketAliasRedirectAddr .
495 This function returns a pointer which can subsequently be used by
496 .Fn PacketAliasRedirectDelete .
499 is returned, then the function call did not complete successfully.
503 .Fo PacketAliasAddServer
504 .Fa "struct alias_link *link"
505 .Fa "struct in_addr addr"
508 .Bd -ragged -offset indent
509 This function sets the
511 up for Load Sharing using IP Network Address Translation (RFC 2391, LSNAT).
512 LSNAT operates as follows.
513 A client attempts to access a server by using the server virtual address.
514 The LSNAT router transparently redirects the request to one of the hosts
515 in server pool, selected using a real-time load sharing algorithm.
516 Multiple sessions may be initiated from the same client, and each session
517 could be directed to a different host based on load balance across server
518 pool hosts at the time.
519 If load share is desired for just a few specific services, the configuration
520 on LSNAT could be defined to restrict load share for just the services
523 Currently, only the simplest selection algorithm is implemented, where a
524 host is selected on a round-robin basis only, without regard to load on
530 .Fn PacketAliasRedirectPort
532 .Fn PacketAliasRedirectAddr .
534 .Fn PacketAliasAddServer
535 is called multiple times to add entries to the
539 For links created with
540 .Fn PacketAliasRedirectAddr ,
543 argument is ignored and could have any value, e.g. htons(~0).
545 This function returns 0 on success, -1 otherwise.
549 .Fn PacketAliasRedirectDelete "struct alias_link *link"
550 .Bd -ragged -offset indent
551 This function will delete a specific static redirect rule entered by
552 .Fn PacketAliasRedirectPort
554 .Fn PacketAliasRedirectAddr .
557 is the pointer returned by either of the redirection functions.
558 If an invalid pointer is passed to
559 .Fn PacketAliasRedirectDelete ,
560 then a program crash or unpredictable operation could result, so it is
561 necessary to be careful using this function.
565 .Fn PacketAliasProxyRule "const char *cmd"
566 .Bd -ragged -offset indent
569 string consists of one or more pairs of words.
570 The first word in each pair is a token and the second is the value that
571 should be applied for that token.
572 Tokens and their argument types are as follows:
573 .Bl -tag -width indent
574 .It Cm type encode_ip_hdr | encode_tcp_stream | no_encode
575 In order to support transparent proxying, it is necessary to somehow
576 pass the original address and port information into the new destination
580 is specified, the original address and port is passed as an extra IP
583 .Cm encode_tcp_stream
584 is specified, the original address and port is passed as the first
585 piece of data in the TCP stream in the format
586 .Dq DEST Ar IP port .
587 .It Cm port Ar portnum
588 Only packets with the destination port
591 .It Cm server Ar host Ns Xo
598 that the data is to be redirected to.
600 must be an IP address rather than a DNS host name.
603 is not specified, the destination port number is not changed.
607 specification is mandatory unless the
609 command is being used.
611 Normally, each call to
612 .Fn PacketAliasProxyRule
613 inserts the next rule at the start of a linear list of rules.
616 is specified, the new rule will be checked after all rules with lower
619 .Fn PacketAliasProxyRule
620 that do not specify a rule are assigned rule 0.
621 .It Cm delete Ar index
622 This token and its argument MUST NOT be used with any other tokens.
623 When used, all existing rules with the given
626 .It Cm proto tcp | udp
627 If specified, only packets of the given protocol type are matched.
628 .It Cm src Ar IP Ns Xo
631 If specified, only packets with a source address matching the given
636 is also specified, then the first
640 are taken as a network specification, and all IP addresses from that
641 network will be matched.
642 .It Cm dst Ar IP Ns Xo
645 If specified, only packets with a destination address matching the given
650 is also specified, then the first
654 are taken as a network specification, and all IP addresses from that
655 network will be matched.
658 This function is usually used to redirect outgoing connections for
659 internal machines that are not permitted certain types of internet
660 access, or to restrict access to certain external machines.
663 .Ft struct alias_link *
664 .Fo PacketAliasRedirectProto
665 .Fa "struct in_addr local_addr"
666 .Fa "struct in_addr remote_addr"
667 .Fa "struct in_addr alias_addr"
670 .Bd -ragged -offset indent
671 This function specifies that any IP packet with protocol number of
673 from a given remote address to an alias address be
674 redirected to a specified local address.
680 is zero, this indicates that the packet aliasing address as established
682 .Fn PacketAliasSetAddress
685 .Fn PacketAliasSetAddress
686 is called to change the address after
687 .Fn PacketAliasRedirectProto
688 is called, a zero reference will track this change.
692 is zero, this indicates to redirect packets from any remote address.
693 Non-zero remote addresses can sometimes be useful for firewalling.
696 .Fn PacketAliasRedirectProto
697 overlap in their address specifications, then the most recent call
698 will have precedence.
700 This function returns a pointer which can subsequently be used by
701 .Fn PacketAliasRedirectDelete .
704 is returned, then the function call did not complete successfully.
706 .Sh FRAGMENT HANDLING
707 The functions in this section are used to deal with incoming fragments.
709 Outgoing fragments are handled within
711 by changing the address according to any applicable mapping set by
712 .Fn PacketAliasRedirectAddr ,
713 or the default aliasing address set by
714 .Fn PacketAliasSetAddress .
716 Incoming fragments are handled in one of two ways.
717 If the header of a fragmented IP packet has already been seen, then all
718 subsequent fragments will be re-mapped in the same manner the header
720 Fragments which arrive before the header are saved and then retrieved
721 once the header fragment has been resolved.
724 .Fn PacketAliasSaveFragment "char *ptr"
725 .Bd -ragged -offset indent
729 .Dv PKT_ALIAS_UNRESOLVED_FRAGMENT ,
730 this function can be used to save the pointer to the unresolved fragment.
732 It is implicitly assumed that
734 points to a block of memory allocated by
736 If the fragment is never resolved, the packet aliasing engine will
737 automatically free the memory after a timeout period.
738 [Eventually this function should be modified so that a callback function
739 for freeing memory is passed as an argument.]
741 This function returns
743 if it was successful and
745 if there was an error.
749 .Fn PacketAliasGetFragment "char *buffer"
750 .Bd -ragged -offset indent
751 This function can be used to retrieve fragment pointers saved by
752 .Fn PacketAliasSaveFragment .
753 The IP header fragment pointed to by
755 is the header fragment indicated when
758 .Dv PKT_ALIAS_FOUND_HEADER_FRAGMENT .
759 Once a fragment pointer is retrieved, it becomes the calling program's
760 responsibility to free the dynamically allocated memory for the fragment.
762 .Fn PacketAliasGetFragment
763 can be called sequentially until there are no more fragments available,
764 at which time it returns
769 .Fn PacketAliasFragmentIn "char *header" "char *fragment"
770 .Bd -ragged -offset indent
771 When a fragment is retrieved with
772 .Fn PacketAliasGetFragment ,
773 it can then be de-aliased with a call to
774 .Fn PacketAliasFragmentIn .
777 argument is the pointer to a header fragment used as a template, and
779 is the pointer to the packet to be de-aliased.
781 .Sh MISCELLANEOUS FUNCTIONS
783 .Fn PacketAliasSetTarget "struct in_addr addr"
784 .Bd -ragged -offset indent
785 When an incoming packet not associated with any pre-existing aliasing link
786 arrives at the host machine, it will be sent to the address indicated by a
788 .Fn PacketAliasSetTarget .
790 If this function is called with an
792 address argument, then all new incoming packets go to the address set by
793 .Fn PacketAliasSetAddress .
795 If this function is not called, or is called with an
797 address argument, then all new incoming packets go to the address specified
799 This allows external machines to talk directly to internal machines if they
800 can route packets to the machine in question.
804 .Fn PacketAliasCheckNewLink void
805 .Bd -ragged -offset indent
806 This function returns a non-zero value when a new aliasing link is created.
807 In circumstances where incoming traffic is being sequentially sent to
808 different local servers, this function can be used to trigger when
809 .Fn PacketAliasSetTarget
810 is called to change the default target address.
814 .Fn PacketAliasInternetChecksum "u_short *buffer" "int nbytes"
815 .Bd -ragged -offset indent
816 This is a utility function that does not seem to be available elsewhere and
817 is included as a convenience.
818 It computes the internet checksum, which is used in both IP and
819 protocol-specific headers (TCP, UDP, ICMP).
823 argument points to the data block to be checksummed, and
825 is the number of bytes.
826 The 16-bit checksum field should be zeroed before computing the checksum.
828 Checksums can also be verified by operating on a block of data including
830 If the checksum is valid,
831 .Fn PacketAliasInternetChecksum
836 .Fn PacketUnaliasOut "char *buffer" "int maxpacketsize"
837 .Bd -ragged -offset indent
838 An outgoing packet, which has already been aliased,
839 has its private address/port information restored by this function.
840 The IP packet is pointed to by
844 is provided for error checking purposes.
845 This function can be used if an already-aliased packet needs to have its
846 original IP header restored for further processing (eg. logging).
849 PPTP aliasing does not work when more than one internal client
850 connects to the same external server at the same time, because
851 PPTP requires a single TCP control connection to be established
852 between any two IP addresses.
854 .An Charles Mott Aq cm@linktel.net ,
855 versions 1.0 - 1.8, 2.0 - 2.4.
856 .An Eivind Eklund Aq eivind@FreeBSD.org ,
857 versions 1.8b, 1.9 and 2.5.
858 Added IRC DCC support as well as contributing a number of architectural
859 improvements; added the firewall bypass for FTP/IRC DCC.
860 .An Erik Salander Aq erik@whistle.com
861 added support for PPTP and RTSP.
862 .An Junichi Satoh Aq junichi@junichi.org
863 added support for RTSP/PNA.
865 Listed below, in approximate chronological order, are individuals who
866 have provided valuable comments and/or debugging assistance.
868 .Bd -ragged -offset indent
882 .Sh CONCEPTUAL BACKGROUND
883 This section is intended for those who are planning to modify the source
884 code or want to create somewhat esoteric applications using the packet
887 The conceptual framework under which the packet aliasing engine operates
889 Central to the discussion is the idea of an
891 which describes the relationship for a given packet transaction between
892 the local machine, aliased identity and remote machine.
893 It is discussed how such links come into existence and are destroyed.
895 There is a notion of an
897 which is a 7-tuple describing a specific translation:
898 .Bd -literal -offset indent
899 (local addr, local port, alias addr, alias port,
900 remote addr, remote port, protocol)
903 Outgoing packets have the local address and port number replaced with the
904 alias address and port number.
905 Incoming packets undergo the reverse process.
906 The packet aliasing engine attempts to match packets against an internal
907 table of aliasing links to determine how to modify a given IP packet.
908 Both the IP header and protocol dependent headers are modified as necessary.
909 Aliasing links are created and deleted as necessary according to network
912 Protocols can be TCP, UDP or even ICMP in certain circumstances.
913 (Some types of ICMP packets can be aliased according to sequence or ID
914 number which acts as an equivalent port number for identifying how
915 individual packets should be handled.)
917 Each aliasing link must have a unique combination of the following five
918 quantities: alias address/port, remote address/port and protocol.
919 This ensures that several machines on a local network can share the
920 same aliasing IP address.
921 In cases where conflicts might arise, the aliasing port is chosen so that
922 uniqueness is maintained.
923 .Ss STATIC AND DYNAMIC LINKS
924 Aliasing links can either be static or dynamic.
925 Static links persist indefinitely and represent fixed rules for translating
927 Dynamic links come into existence for a specific TCP connection or UDP
928 transaction or ICMP ECHO sequence.
929 For the case of TCP, the connection can be monitored to see when the
930 associated aliasing link should be deleted.
931 Aliasing links for UDP transactions (and ICMP ECHO and TIMESTAMP requests)
932 work on a simple timeout rule.
933 When no activity is observed on a dynamic link for a certain amount of time
934 it is automatically deleted.
935 Timeout rules also apply to TCP connections which do not open or close
937 .Ss PARTIALLY SPECIFIED ALIASING LINKS
938 Aliasing links can be partially specified, meaning that the remote address
939 and/or remote port are unknown.
940 In this case, when a packet matching the incomplete specification is found,
941 a fully specified dynamic link is created.
942 If the original partially specified link is dynamic, it will be deleted
943 after the fully specified link is created, otherwise it will persist.
945 For instance, a partially specified link might be
946 .Bd -literal -offset indent
947 (192.168.0.4, 23, 204.228.203.215, 8066, 0, 0, tcp)
950 The zeros denote unspecified components for the remote address and port.
951 If this link were static it would have the effect of redirecting all
952 incoming traffic from port 8066 of 204.228.203.215 to port 23 (telnet)
953 of machine 192.168.0.4 on the local network.
954 Each individual telnet connection would initiate the creation of a distinct
956 .Ss DYNAMIC LINK CREATION
957 In addition to aliasing links, there are also address mappings that can be
958 stored within the internal data table of the packet aliasing mechanism.
959 .Bd -literal -offset indent
960 (local addr, alias addr)
963 Address mappings are searched when creating new dynamic links.
965 All outgoing packets from the local network automatically create a dynamic
966 link if they do not match an already existing fully specified link.
967 If an address mapping exists for the outgoing packet, this determines
968 the alias address to be used.
969 If no mapping exists, then a default address, usually the address of the
970 packet aliasing host, is used.
971 If necessary, this default address can be changed as often as each individual
974 The aliasing port number is determined such that the new dynamic link does
975 not conflict with any existing links.
976 In the default operating mode, the packet aliasing engine attempts to set
977 the aliasing port equal to the local port number.
978 If this results in a conflict, then port numbers are randomly chosen until
979 a unique aliasing link can be established.
980 In an alternate operating mode, the first choice of an aliasing port is also
981 random and unrelated to the local port number.