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 the 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
64 .Sh INITIALIZATION AND CONTROL
67 must always be called before any packet handling may be performed, and
68 the returned instance pointer must be passed to all the other functions.
70 .Fn LibAliasSetAddress
71 function is called afterwards, to set the default aliasing address.
72 In addition, the operating mode of the packet aliasing engine can be
76 .Ft "struct libalias *"
77 .Fn LibAliasInit "struct libalias *"
78 .Bd -ragged -offset indent
79 This function is used to initialize
80 internal data structures.
81 When called the first time, a
83 pointer should be passed as an argument.
84 The following mode bits are always set after calling
86 See the description of
88 below for the meaning of these mode bits.
90 .Bl -item -offset indent -compact
92 .Dv PKT_ALIAS_SAME_PORTS
94 .Dv PKT_ALIAS_USE_SOCKETS
96 .Dv PKT_ALIAS_RESET_ON_ADDR_CHANGE
99 This function will always return the packet aliasing engine to the same
102 .Fn LibAliasSetAddress
103 function is normally called afterwards, and any desired changes from the
104 default mode bits listed above require a call to
105 .Fn LibAliasSetMode .
107 It is mandatory that this function be called at the beginning of a program
108 prior to any packet handling.
112 .Fn LibAliasUninit "struct libalias *"
113 .Bd -ragged -offset indent
114 This function has no return value and is used to clear any
115 resources attached to internal data structures.
117 This function should be called when a program stops using the aliasing
118 engine; amongst other things, it clears out any firewall holes.
119 To provide backwards compatibility and extra security, it is added to
127 .Fn LibAliasSetAddress "struct libalias *" "struct in_addr addr"
128 .Bd -ragged -offset indent
129 This function sets the source address to which outgoing packets from the
130 local area network are aliased.
131 All outgoing packets are re-mapped to this address unless overridden by a
132 static address mapping established by
133 .Fn LibAliasRedirectAddr .
134 If this function has not been called, and no static rules match, an outgoing
135 packet retains its source address.
138 .Dv PKT_ALIAS_RESET_ON_ADDR_CHANGE
139 mode bit is set (the default mode of operation), then the internal aliasing
140 link tables will be reset any time the aliasing address changes.
141 This is useful for interfaces such as
144 address may or may not change on successive dial-up attempts.
147 .Dv PKT_ALIAS_RESET_ON_ADDR_CHANGE
148 mode bit is set to zero, this function can also be used to dynamically change
149 the aliasing address on a packet-to-packet basis (it is a low overhead call).
151 It is mandatory that this function be called prior to any packet handling.
155 .Fn LibAliasSetMode "struct libalias *" "unsigned int flags" "unsigned int mask"
156 .Bd -ragged -offset indent
157 This function sets or clears mode bits
158 according to the value of
163 The following mode bits are defined in
165 .Bl -tag -width indent
168 .Pa /var/log/alias.log .
169 Each time an aliasing link is created or deleted, the log file is appended to
170 with the current number of ICMP, TCP and UDP links.
171 Mainly useful for debugging when the log file is viewed continuously with
173 .It Dv PKT_ALIAS_DENY_INCOMING
174 If this mode bit is set, all incoming packets associated with new TCP
175 connections or new UDP transactions will be marked for being ignored
179 .Dv PKT_ALIAS_IGNORED
182 by the calling program.
183 Response packets to connections or transactions initiated from the packet
184 aliasing host or local network will be unaffected.
185 This mode bit is useful for implementing a one-way firewall.
186 .It Dv PKT_ALIAS_SAME_PORTS
187 If this mode bit is set, the packet-aliasing engine will attempt to leave
188 the alias port numbers unchanged from the actual local port numbers.
189 This can be done as long as the quintuple (proto, alias addr, alias port,
190 remote addr, remote port) is unique.
191 If a conflict exists, a new aliasing port number is chosen even if this
193 .It Dv PKT_ALIAS_USE_SOCKETS
194 This bit should be set when the packet aliasing host originates network
195 traffic as well as forwards it.
196 When the packet aliasing host is waiting for a connection from an unknown
197 host address or unknown port number (e.g.\& an FTP data connection), this
198 mode bit specifies that a socket be allocated as a place holder to prevent
200 Once a connection is established, usually within a minute or so, the socket
202 .It Dv PKT_ALIAS_UNREGISTERED_ONLY
203 If this mode bit is set, traffic on the local network which does not
204 originate from unregistered address spaces will be ignored.
205 Standard Class A, B and C unregistered addresses are:
207 10.0.0.0 -> 10.255.255.255 (Class A subnet)
208 172.16.0.0 -> 172.31.255.255 (Class B subnets)
209 192.168.0.0 -> 192.168.255.255 (Class C subnets)
211 This option is useful in the case that the packet aliasing host has both
212 registered and unregistered subnets on different interfaces.
213 The registered subnet is fully accessible to the outside world, so traffic
214 from it does not need to be passed through the packet aliasing engine.
215 .It Dv PKT_ALIAS_UNREGISTERED_CGN
216 Like PKT_ALIAS_UNREGISTERED_ONLY, but includes the RFC 6598 (Carrier Grade
217 NAT) subnet as follows:
219 100.64.0.0 -> 100.127.255.255 (RFC 6598 subnet)
220 .It Dv PKT_ALIAS_RESET_ON_ADDR_CHANGE
221 When this mode bit is set and
222 .Fn LibAliasSetAddress
223 is called to change the aliasing address, the internal link table of the
224 packet aliasing engine will be cleared.
225 This operating mode is useful for
227 links where the interface address can sometimes change or remain the same
228 between dial-up attempts.
229 If this mode bit is not set, the link table will never be reset in the event
230 of an address change.
231 .It Dv PKT_ALIAS_PUNCH_FW
237 based firewall for FTP/IRC DCC connections.
238 The holes punched are bound by from/to IP address and port; it will not be
239 possible to use a hole for another connection.
240 A hole is removed when the connection that uses it dies.
241 To cater to unexpected death of a program using
244 changing the state of the flag will clear the entire firewall range
246 This clearing will also happen on the initial call to
247 .Fn LibAliasSetFWBase ,
248 which must happen prior to setting this flag.
249 .It Dv PKT_ALIAS_REVERSE
252 reverse the way it handles incoming and outgoing packets, allowing it
253 to be fed with data that passes through the internal interface rather
254 than the external one.
255 .It Dv PKT_ALIAS_PROXY_ONLY
258 to obey transparent proxy rules only.
259 Normal packet aliasing is not performed.
261 .Fn LibAliasProxyRule
263 .It Dv PKT_ALIAS_SKIP_GLOBAL
264 This option is used by
267 Specifying it as a flag to
271 .Sx NETWORK ADDRESS TRANSLATION
279 .Fn LibAliasSetFWBase "struct libalias *" "unsigned int base" "unsigned int num"
280 .Bd -ragged -offset indent
281 Set the firewall range allocated for punching firewall holes (with the
282 .Dv PKT_ALIAS_PUNCH_FW
284 The range is cleared for all rules on initialization.
288 .Fn LibAliasSkinnyPort "struct libalias *" "unsigned int port"
289 .Bd -ragged -offset indent
290 Set the TCP port used by the Skinny Station protocol.
291 Skinny is used by Cisco IP phones to communicate with
292 Cisco Call Managers to set up voice over IP calls.
293 If this is not set, Skinny aliasing will not be done.
294 The typical port used by Skinny is 2000.
297 The packet handling functions are used to modify incoming (remote to local)
298 and outgoing (local to remote) packets.
299 The calling program is responsible for receiving and sending packets via
305 .Fn LibAliasSetAddress ,
306 the two packet handling functions,
310 comprise the minimal set of functions needed for a basic IP masquerading
314 .Fn LibAliasIn "struct libalias *" "void *buffer" "int maxpacketsize"
315 .Bd -ragged -offset indent
316 An incoming packet coming from a remote machine to the local network is
317 de-aliased by this function.
318 The IP packet is pointed to by
322 indicates the size of the data structure containing the packet and should
323 be at least as large as the actual packet size.
326 .Bl -tag -width indent
328 The packet aliasing process was successful.
329 .It Dv PKT_ALIAS_IGNORED
330 The packet was ignored and not de-aliased.
331 This can happen if the protocol is unrecognized, as for an ICMP message
332 type that is not handled, or if incoming packets for new connections are being
334 .Dv PKT_ALIAS_DENY_INCOMING
335 mode bit was set using
336 .Fn LibAliasSetMode ) .
337 .It Dv PKT_ALIAS_UNRESOLVED_FRAGMENT
338 This is returned when a fragment cannot be resolved because the header
339 fragment has not been sent yet.
340 In this situation, fragments must be saved with
341 .Fn LibAliasSaveFragment
342 until a header fragment is found.
343 .It Dv PKT_ALIAS_FOUND_HEADER_FRAGMENT
344 The packet aliasing process was successful, and a header fragment was found.
345 This is a signal to retrieve any unresolved fragments with
346 .Fn LibAliasGetFragment
347 and de-alias them with
348 .Fn LibAliasFragmentIn .
349 .It Dv PKT_ALIAS_ERROR
350 An internal error within the packet aliasing engine occurred.
355 .Fn LibAliasOut "struct libalias *" "void *buffer" "int maxpacketsize"
356 .Bd -ragged -offset indent
357 An outgoing packet coming from the local network to a remote machine is
358 aliased by this function.
359 The IP packet is pointed to by
363 indicates the maximum packet size permissible should the packet length be
365 IP encoding protocols place address and port information in the encapsulated
366 data stream which has to be modified and can account for changes in packet
368 Well known examples of such protocols are FTP and IRC DCC.
371 .Bl -tag -width indent
373 The packet aliasing process was successful.
374 .It Dv PKT_ALIAS_IGNORED
375 The packet was ignored and not aliased.
376 This can happen if the protocol is unrecognized, or possibly an ICMP message
378 .It Dv PKT_ALIAS_ERROR
379 An internal error within the packet aliasing engine occurred.
382 .Sh PORT AND ADDRESS REDIRECTION
383 The functions described in this section allow machines on the local network
384 to be accessible in some degree to new incoming connections from the external
386 Individual ports can be re-mapped or static network address translations can
389 .Ft struct alias_link *
390 .Fo LibAliasRedirectPort
391 .Fa "struct libalias *"
392 .Fa "struct in_addr local_addr"
393 .Fa "u_short local_port"
394 .Fa "struct in_addr remote_addr"
395 .Fa "u_short remote_port"
396 .Fa "struct in_addr alias_addr"
397 .Fa "u_short alias_port"
400 .Bd -ragged -offset indent
401 This function specifies that traffic from a given remote address/port to
402 an alias address/port be redirected to a specified local address/port.
416 is zero, this indicates that the packet aliasing address as established
418 .Fn LibAliasSetAddress
421 .Fn LibAliasSetAddress
422 is called to change the address after
423 .Fn LibAliasRedirectPort
424 is called, a zero reference will track this change.
426 If the link is further set up to operate with load sharing, then
430 are ignored, and are selected dynamically from the server pool, as described in
431 .Fn LibAliasAddServer
436 is zero, this indicates to redirect packets from any remote address.
439 is zero, this indicates to redirect packets originating from any remote
441 The remote port specification will almost always be zero, but non-zero
442 remote addresses can sometimes be useful for firewalling.
444 .Fn LibAliasRedirectPort
445 overlap in their address/port specifications, then the most recent call
446 will have precedence.
448 This function returns a pointer which can subsequently be used by
449 .Fn LibAliasRedirectDelete .
452 is returned, then the function call did not complete successfully.
454 All port numbers should be in network address byte order, so it is necessary
457 to convert these parameters from internally readable numbers to network byte
459 Addresses are also in network byte order, which is implicit in the use of the
464 .Ft struct alias_link *
465 .Fo LibAliasRedirectAddr
466 .Fa "struct libalias *"
467 .Fa "struct in_addr local_addr"
468 .Fa "struct in_addr alias_addr"
470 .Bd -ragged -offset indent
471 This function designates that all incoming traffic to
475 Similarly, all outgoing traffic from
484 is zero, this indicates that the packet aliasing address as established by
485 .Fn LibAliasSetAddress
488 .Fn LibAliasSetAddress
489 is called to change the address after
490 .Fn LibAliasRedirectAddr
491 is called, a zero reference will track this change.
493 If the link is further set up to operate with load sharing, then the
495 argument is ignored, and is selected dynamically from the server pool,
497 .Fn LibAliasAddServer
500 If subsequent calls to
501 .Fn LibAliasRedirectAddr
502 use the same aliasing address, all new incoming traffic to this aliasing
503 address will be redirected to the local address made in the last function
505 New traffic generated by any of the local machines, designated in the
506 several function calls, will be aliased to the same address.
507 Consider the following example:
509 LibAliasRedirectAddr(la, inet_aton("192.168.0.2"),
510 inet_aton("141.221.254.101"));
511 LibAliasRedirectAddr(la, inet_aton("192.168.0.3"),
512 inet_aton("141.221.254.101"));
513 LibAliasRedirectAddr(la, inet_aton("192.168.0.4"),
514 inet_aton("141.221.254.101"));
516 Any outgoing connections such as
520 from 192.168.0.2, 192.168.0.3 and 192.168.0.4 will appear to come from
522 Any incoming connections to 141.221.254.101 will be directed to 192.168.0.4.
525 .Fn LibAliasRedirectPort
526 will have precedence over address mappings designated by
527 .Fn LibAliasRedirectAddr .
529 This function returns a pointer which can subsequently be used by
530 .Fn LibAliasRedirectDelete .
533 is returned, then the function call did not complete successfully.
537 .Fo LibAliasAddServer
538 .Fa "struct libalias *"
539 .Fa "struct alias_link *link"
540 .Fa "struct in_addr addr"
543 .Bd -ragged -offset indent
544 This function sets the
546 up for Load Sharing using IP Network Address Translation (RFC 2391, LSNAT).
547 LSNAT operates as follows.
548 A client attempts to access a server by using the server virtual address.
549 The LSNAT router transparently redirects the request to one of the hosts
550 in the server pool, using a real-time load sharing algorithm.
551 Multiple sessions may be initiated from the same client, and each session
552 could be directed to a different host based on the load balance across server
553 pool hosts when the sessions are initiated.
554 If load sharing is desired for just a few specific services, the configuration
555 on LSNAT could be defined to restrict load sharing to just the services
558 Currently, only the simplest selection algorithm is implemented, where a
559 host is selected on a round-robin basis only, without regard to load on
565 .Fn LibAliasRedirectPort
567 .Fn LibAliasRedirectAddr .
569 .Fn LibAliasAddServer
570 is called multiple times to add entries to the
574 For links created with
575 .Fn LibAliasRedirectAddr ,
578 argument is ignored and could have any value, e.g.\& htons(~0).
580 This function returns 0 on success, \-1 otherwise.
584 .Fn LibAliasRedirectDynamic "struct libalias *" "struct alias_link *link"
585 .Bd -ragged -offset indent
586 This function marks the specified static redirect rule entered by
587 .Fn LibAliasRedirectPort
589 This can be used to e.g.\& dynamically redirect a single TCP connection,
590 after which the rule is removed.
591 Only fully specified links can be made dynamic.
593 .Sx STATIC AND DYNAMIC LINKS
595 .Sx PARTIALLY SPECIFIED ALIASING LINKS
596 sections below for a definition of static vs.\& dynamic,
597 and partially vs.\& fully specified links.)
599 This function returns 0 on success, \-1 otherwise.
603 .Fn LibAliasRedirectDelete "struct libalias *" "struct alias_link *link"
604 .Bd -ragged -offset indent
605 This function will delete a specific static redirect rule entered by
606 .Fn LibAliasRedirectPort
608 .Fn LibAliasRedirectAddr .
611 is the pointer returned by either of the redirection functions.
612 If an invalid pointer is passed to
613 .Fn LibAliasRedirectDelete ,
614 then a program crash or unpredictable operation could result, so
615 care is needed when using this function.
619 .Fn LibAliasProxyRule "struct libalias *" "const char *cmd"
620 .Bd -ragged -offset indent
623 string consists of one or more pairs of words.
624 The first word in each pair is a token and the second is the value that
625 should be applied for that token.
626 Tokens and their argument types are as follows:
627 .Bl -tag -width indent
628 .It Cm type encode_ip_hdr | encode_tcp_stream | no_encode
629 In order to support transparent proxying, it is necessary to somehow
630 pass the original address and port information into the new destination
634 is specified, the original destination address and port are passed
635 as an extra IP option.
637 .Cm encode_tcp_stream
638 is specified, the original destination address and port are passed
639 as the first piece of data in the TCP stream in the format
640 .Dq Li DEST Ar IP port .
641 .It Cm port Ar portnum
642 Only packets with the destination port
645 .It Cm server Ar host Ns Op : Ns Ar portnum
650 that the data is to be redirected to.
652 must be an IP address rather than a DNS host name.
655 is not specified, the destination port number is not changed.
659 specification is mandatory unless the
661 command is being used.
663 Normally, each call to
664 .Fn LibAliasProxyRule
665 inserts the next rule at the start of a linear list of rules.
668 is specified, the new rule will be checked after all rules with lower
671 .Fn LibAliasProxyRule
672 that do not specify a rule are assigned rule 0.
673 .It Cm delete Ar index
674 This token and its argument MUST NOT be used with any other tokens.
675 When used, all existing rules with the given
678 .It Cm proto tcp | udp
679 If specified, only packets of the given protocol type are matched.
680 .It Cm src Ar IP Ns Op / Ns Ar bits
681 If specified, only packets with a source address matching the given
686 is also specified, then the first
690 are taken as a network specification, and all IP addresses from that
691 network will be matched.
692 .It Cm dst Ar IP Ns Op / Ns Ar bits
693 If specified, only packets with a destination address matching the given
698 is also specified, then the first
702 are taken as a network specification, and all IP addresses from that
703 network will be matched.
706 This function is usually used to redirect outgoing connections for
707 internal machines that are not permitted certain types of internet
708 access, or to restrict access to certain external machines.
711 .Ft struct alias_link *
712 .Fo LibAliasRedirectProto
713 .Fa "struct libalias *"
714 .Fa "struct in_addr local_addr"
715 .Fa "struct in_addr remote_addr"
716 .Fa "struct in_addr alias_addr"
719 .Bd -ragged -offset indent
720 This function specifies that any IP packet with protocol number of
722 from a given remote address to an alias address will be
723 redirected to a specified local address.
729 is zero, this indicates that the packet aliasing address as established
731 .Fn LibAliasSetAddress
734 .Fn LibAliasSetAddress
735 is called to change the address after
736 .Fn LibAliasRedirectProto
737 is called, a zero reference will track this change.
741 is zero, this indicates to redirect packets from any remote address.
742 Non-zero remote addresses can sometimes be useful for firewalling.
745 .Fn LibAliasRedirectProto
746 overlap in their address specifications, then the most recent call
747 will have precedence.
749 This function returns a pointer which can subsequently be used by
750 .Fn LibAliasRedirectDelete .
753 is returned, then the function call did not complete successfully.
755 .Sh FRAGMENT HANDLING
756 The functions in this section are used to deal with incoming fragments.
758 Outgoing fragments are handled within
760 by changing the address according to any applicable mapping set by
761 .Fn LibAliasRedirectAddr ,
762 or the default aliasing address set by
763 .Fn LibAliasSetAddress .
765 Incoming fragments are handled in one of two ways.
766 If the header of a fragmented IP packet has already been seen, then all
767 subsequent fragments will be re-mapped in the same manner the header
769 Fragments which arrive before the header are saved and then retrieved
770 once the header fragment has been resolved.
773 .Fn LibAliasSaveFragment "struct libalias *" "void *ptr"
774 .Bd -ragged -offset indent
778 .Dv PKT_ALIAS_UNRESOLVED_FRAGMENT ,
779 this function can be used to save the pointer to the unresolved fragment.
781 It is implicitly assumed that
783 points to a block of memory allocated by
785 If the fragment is never resolved, the packet aliasing engine will
786 automatically free the memory after a timeout period.
787 [Eventually this function should be modified so that a callback function
788 for freeing memory is passed as an argument.]
790 This function returns
792 if it was successful and
794 if there was an error.
798 .Fn LibAliasGetFragment "struct libalias *" "void *buffer"
799 .Bd -ragged -offset indent
800 This function can be used to retrieve fragment pointers saved by
801 .Fn LibAliasSaveFragment .
802 The IP header fragment pointed to by
804 is the header fragment indicated when
807 .Dv PKT_ALIAS_FOUND_HEADER_FRAGMENT .
808 Once a fragment pointer is retrieved, it becomes the calling program's
809 responsibility to free the dynamically allocated memory for the fragment.
812 .Fn LibAliasGetFragment
813 function can be called sequentially until there are no more fragments
814 available, at which time it returns
819 .Fn LibAliasFragmentIn "struct libalias *" "void *header" "void *fragment"
820 .Bd -ragged -offset indent
821 When a fragment is retrieved with
822 .Fn LibAliasGetFragment ,
823 it can then be de-aliased with a call to
824 .Fn LibAliasFragmentIn .
827 argument is the pointer to a header fragment used as a template, and
829 is the pointer to the packet to be de-aliased.
831 .Sh MISCELLANEOUS FUNCTIONS
832 .Ft struct alias_link *
833 .Fn AddLink "struct libalias *" "struct in_addr src_addr" "struct in_addr dst_addr" \
834 "struct in_addr alias_addr" "u_short src_port" "u_short dst_port" \
835 "int alias_param" "int link_type"
836 .Bd -ragged -offset indent
837 This function adds new state to the instance hash table.
838 The dst_address and/or dst_port may be given as zero, which
839 introduces some dynamic character into the link, since
840 LibAliasSetAddress can change the address that is used.
841 However, in the current implementation, such links can only be used
842 for inbound (ext -> int) traffic.
846 .Fn LibAliasSetTarget "struct libalias *" "struct in_addr addr"
847 .Bd -ragged -offset indent
848 When an incoming packet not associated with any pre-existing aliasing link
849 arrives at the host machine, it will be sent to the address indicated by a
851 .Fn LibAliasSetTarget .
853 If this function is called with an
855 address argument, then all new incoming packets go to the address set by
856 .Fn LibAliasSetAddress .
858 If this function is not called, or is called with an
860 address argument, then all new incoming packets go to the address specified
862 This allows external machines to talk directly to internal machines if they
863 can route packets to the machine in question.
867 .Fn LibAliasInternetChecksum "struct libalias *" "u_short *buffer" "int nbytes"
868 .Bd -ragged -offset indent
869 This is a utility function that does not seem to be available elsewhere and
870 is included as a convenience.
871 It computes the internet checksum, which is used in both IP and
872 protocol-specific headers (TCP, UDP, ICMP).
876 argument points to the data block to be checksummed, and
878 is the number of bytes.
879 The 16-bit checksum field should be zeroed before computing the checksum.
881 Checksums can also be verified by operating on a block of data including
883 If the checksum is valid,
884 .Fn LibAliasInternetChecksum
889 .Fn LibAliasUnaliasOut "struct libalias *" "void *buffer" "int maxpacketsize"
890 .Bd -ragged -offset indent
891 An outgoing packet, which has already been aliased,
892 has its private address/port information restored by this function.
893 The IP packet is pointed to by
897 is provided for error checking purposes.
898 This function can be used if an already-aliased packet needs to have its
899 original IP header restored for further processing (e.g.\& logging).
901 .Sh CONCEPTUAL BACKGROUND
902 This section is intended for those who are planning to modify the source
903 code or want to create somewhat esoteric applications using the packet
906 The conceptual framework under which the packet aliasing engine operates
908 Central to the discussion is the idea of an
910 which describes the relationship for a given packet transaction between
911 the local machine, aliased identity and remote machine.
912 It is discussed how such links come into existence and are destroyed.
914 There is a notion of an
916 which is a 7-tuple describing a specific translation:
917 .Bd -literal -offset indent
918 (local addr, local port, alias addr, alias port,
919 remote addr, remote port, protocol)
922 Outgoing packets have the local address and port number replaced with the
923 alias address and port number.
924 Incoming packets undergo the reverse process.
925 The packet aliasing engine attempts to match packets against an internal
926 table of aliasing links to determine how to modify a given IP packet.
927 Both the IP header and protocol dependent headers are modified as necessary.
928 Aliasing links are created and deleted as necessary according to network
931 Protocols can be TCP, UDP or even ICMP in certain circumstances.
932 (Some types of ICMP packets can be aliased according to sequence or ID
933 number which acts as an equivalent port number for identifying how
934 individual packets should be handled.)
936 Each aliasing link must have a unique combination of the following five
937 quantities: alias address/port, remote address/port and protocol.
938 This ensures that several machines on a local network can share the
939 same aliasing IP address.
940 In cases where conflicts might arise, the aliasing port is chosen so that
941 uniqueness is maintained.
942 .Ss STATIC AND DYNAMIC LINKS
943 Aliasing links can either be static or dynamic.
944 Static links persist indefinitely and represent fixed rules for translating
946 Dynamic links come into existence for a specific TCP connection or UDP
947 transaction or ICMP ECHO sequence.
948 For the case of TCP, the connection can be monitored to see when the
949 associated aliasing link should be deleted.
950 Aliasing links for UDP transactions (and ICMP ECHO and TIMESTAMP requests)
951 work on a simple timeout rule.
952 When no activity is observed on a dynamic link for a certain amount of time
953 it is automatically deleted.
954 Timeout rules also apply to TCP connections which do not open or close
956 .Ss PARTIALLY SPECIFIED ALIASING LINKS
957 Aliasing links can be partially specified, meaning that the remote address
958 and/or remote port are unknown.
959 In this case, when a packet matching the incomplete specification is found,
960 a fully specified dynamic link is created.
961 If the original partially specified link is dynamic, it will be deleted
962 after the fully specified link is created, otherwise it will persist.
964 For instance, a partially specified link might be
965 .Bd -literal -offset indent
966 (192.168.0.4, 23, 204.228.203.215, 8066, 0, 0, tcp)
969 The zeros denote unspecified components for the remote address and port.
970 If this link were static it would have the effect of redirecting all
971 incoming traffic from port 8066 of 204.228.203.215 to port 23 (telnet)
972 of machine 192.168.0.4 on the local network.
973 Each individual telnet connection would initiate the creation of a distinct
975 .Ss DYNAMIC LINK CREATION
976 In addition to aliasing links, there are also address mappings that can be
977 stored within the internal data table of the packet aliasing mechanism.
978 .Bd -literal -offset indent
979 (local addr, alias addr)
982 Address mappings are searched when creating new dynamic links.
984 All outgoing packets from the local network automatically create a dynamic
985 link if they do not match an already existing fully specified link.
986 If an address mapping exists for the outgoing packet, this determines
987 the alias address to be used.
988 If no mapping exists, then a default address, usually the address of the
989 packet aliasing host, is used.
990 If necessary, this default address can be changed as often as each individual
993 The aliasing port number is determined such that the new dynamic link does
994 not conflict with any existing links.
995 In the default operating mode, the packet aliasing engine attempts to set
996 the aliasing port equal to the local port number.
997 If this results in a conflict, then port numbers are randomly chosen until
998 a unique aliasing link can be established.
999 In an alternate operating mode, the first choice of an aliasing port is also
1000 random and unrelated to the local port number.
1001 .Sh MODULAR ARCHITECTURE Po AND Xr ipfw 4 SUPPORT Pc
1002 One of the latest improvements to
1004 was to make its support
1005 for new protocols independent from the rest of the library, giving it
1006 the ability to load/unload support for new protocols at run-time.
1007 To achieve this feature, all the code for protocol handling was moved
1008 to a series of modules outside of the main library.
1009 These modules are compiled from the same sources but work in
1010 different ways, depending on whether they are compiled to work inside a kernel
1011 or as part of the userland library.
1012 .Ss LIBALIAS MODULES IN KERNEL LAND
1013 When compiled for the kernel,
1015 modules are plain KLDs recognizable with the
1019 To add support for a new protocol, load the corresponding module.
1022 .Dl "kldload alias_ftp"
1024 When support for a protocol is no longer needed, its module can be unloaded:
1026 .Dl "kldunload alias_ftp"
1027 .Ss LIBALIAS MODULES IN USERLAND
1028 Due to the differences between kernel and userland (no KLD mechanism,
1029 many different address spaces, etc.), we had to change a bit how to
1030 handle module loading/tracking/unloading in userland.
1032 While compiled for a userland
1034 all the modules are plain libraries, residing in
1036 and recognizable with the
1040 There is a configuration file,
1041 .Pa /etc/libalias.conf ,
1042 with the following contents (by default):
1043 .Bd -literal -offset indent
1044 /usr/lib/libalias_ftp.so
1045 /usr/lib/libalias_irc.so
1046 /usr/lib/libalias_nbt.so
1047 /usr/lib/libalias_pptp.so
1048 /usr/lib/libalias_skinny.so
1049 /usr/lib/libalias_smedia.so
1052 This file contains the paths to the modules that
1055 To load/unload a new module, just add its path to
1058 .Fn LibAliasRefreshModules
1060 In case the application provides a
1062 signal handler, add a call to
1063 .Fn LibAliasRefreshModules
1064 inside the handler, and every time you want to refresh the loaded modules,
1069 .Dl "kill -HUP <process_pid>"
1070 .Ss MODULAR ARCHITECURE: HOW IT WORKS
1071 The modular architecture of
1073 works similar whether it is running inside the
1074 kernel or in userland.
1078 /* Protocol and userland module handlers chains. */
1079 LIST_HEAD(handler_chain, proto_handler) handler_chain ...
1081 SLIST_HEAD(dll_chain, dll) dll_chain ...
1085 keeps track of all the protocol handlers loaded, while
1087 tracks which userland modules are loaded.
1091 .Vt "struct proto_handler"
1094 struct proto_handler {
1098 int (*fingerprint)(struct libalias *la,
1099 struct ip *pip, struct alias_data *ah);
1100 int (*protohandler)(struct libalias *la,
1101 struct ip *pip, struct alias_data *ah);
1102 TAILQ_ENTRY(proto_handler) link;
1109 is the priority assigned to a protocol handler; lower priority
1112 is the direction of packets: ingoing or outgoing.
1114 indicates to which protocol this packet belongs: IP, TCP or UDP.
1116 points to the fingerprint function while protohandler points
1117 to the protocol handler function.
1122 function has the dual role of checking if the
1123 incoming packet is found, and if it belongs to any categories that this
1128 function actually manipulates
1133 When a packet enters
1135 if it meets a module hook,
1137 is searched to see if there is an handler that matches
1138 this type of a packet (it checks protocol and direction of packet).
1139 Then, if more than one handler is found, it starts with the module with
1140 the lowest priority number: it calls the
1142 function and interprets the result.
1144 If the result value is equal to 0 then it calls the protocol handler
1145 of this handler and returns.
1146 Otherwise, it proceeds to the next eligible module until the
1152 the module hook looks like this:
1153 .Bd -literal -offset indent
1154 struct alias_data ad = {
1159 &ud->uh_sport, /* original source port */
1160 &ud->uh_dport, /* original dest port */
1161 256 /* maxpacketsize */
1166 /* walk out chain */
1167 err = find_handler(IN, UDP, la, pip, &ad);
1170 All data useful to a module are gathered together in an
1177 function is responsible for walking the handler
1178 chain; it receives as input parameters:
1179 .Bl -tag -width indent
1185 pointer to this instance of libalias
1191 .Vt "struct alias_data"
1197 will search only for modules registered for
1198 supporting INcoming UDP packets.
1200 As was mentioned earlier,
1202 in userland is a bit different, as
1203 care must be taken in module handling as well (avoiding duplicate load of
1204 modules, avoiding modules with same name, etc.) so
1209 contains a list of all userland
1213 When an application calls
1214 .Fn LibAliasRefreshModules ,
1216 first unloads all the loaded modules, then reloads all the modules listed in
1217 .Pa /etc/libalias.conf :
1218 for every module loaded, a new entry is added to
1227 /* name of module */
1230 * ptr to shared obj obtained through
1231 * dlopen() - use this ptr to get access
1232 * to any symbols from a loaded module
1241 is the name of the module.
1243 is a pointer to the module obtained through
1246 Whenever a module is loaded in userland, an entry is added to
1248 then every protocol handler present in that module
1249 is resolved and registered in
1251 .Ss HOW TO WRITE A MODULE FOR LIBALIAS
1252 There is a module (called
1253 .Pa alias_dummy.[ch] )
1256 that can be used as a skeleton for future work.
1257 Here we analyse some parts of that module.
1261 struct proto_handler handlers[] = {
1266 .fingerprint = fingerprint,
1267 .protohandler= protohandler,
1276 .Dq "most important thing"
1278 since it describes the handlers present and lets the outside world use
1279 it in an opaque way.
1281 It must ALWAYS be present in every module, and it MUST retain
1284 otherwise attempting to load a module in userland will fail and
1285 complain about missing symbols: for more information about module
1286 load/unload, please refer to
1287 .Fn LibAliasRefreshModules ,
1288 .Fn LibAliasLoadModule
1290 .Fn LibAliasUnloadModule
1297 structures present in a module.
1300 mod_handler(module_t mod, int type, void *data)
1306 error = LibAliasAttachHandlers(handlers);
1309 error = LibAliasDetachHandlers(handlers);
1317 When running as KLD,
1319 registers/deregisters the module using
1320 .Fn LibAliasAttachHandlers
1322 .Fn LibAliasDetachHandlers ,
1325 Every module must contain at least 2 functions: one fingerprint
1326 function and a protocol handler function.
1332 fingerprint(struct libalias *la, struct ip *pip, struct alias_data *ah)
1342 protohandler(struct libalias *la, struct ip *pip,
1343 struct alias_data *ah)
1349 and they must accept exactly these input parameters.
1350 .Ss PATCHING AN APPLICATION FOR USERLAND LIBALIAS MODULES
1351 To add module support into an application that uses
1353 the following simple steps can be followed.
1356 Find the main file of an application
1360 Add this to the header section of
1362 if not already present:
1364 .Dl "#include <signal.h>"
1366 and this just after the header section:
1368 .Dl "static void signal_handler(int);"
1370 Add the following line to the init function of an application or,
1371 if it does not have any init function, put it in
1374 .Dl "signal(SIGHUP, signal_handler);"
1378 function somewhere in
1380 .Bd -literal -offset indent
1382 signal_handler(int sig)
1385 LibAliasRefreshModules();
1389 Otherwise, if an application already traps the
1391 signal, just add a call to
1392 .Fn LibAliasRefreshModules
1393 in the signal handler function.
1395 For example, to patch
1399 modules, just add the following line to
1400 .Fn RefreshAddr "int sig __unused" :
1402 .Dl "LibAliasRefreshModules()"
1404 recompile and you are done.
1405 .Ss LOGGING SUPPORT IN KERNEL LAND
1406 When working as KLD,
1408 now has log support that
1409 happens on a buffer allocated inside
1410 .Vt "struct libalias"
1412 .Pa alias_local.h ) :
1417 /* log descriptor */
1420 * ptr to an auto-malloced
1421 * memory buffer when libalias
1426 * ptr to /var/log/alias.log
1427 * when libalias runs as a
1435 so all applications using
1437 will be able to handle their
1438 own logs, if they want, accessing
1440 Moreover, every change to a log buffer is automatically added to
1448 .An Charles Mott Aq cm@linktel.net ,
1449 versions 1.0 - 1.8, 2.0 - 2.4.
1450 .An Eivind Eklund Aq eivind@FreeBSD.org ,
1451 versions 1.8b, 1.9 and 2.5.
1452 Added IRC DCC support as well as contributing a number of architectural
1453 improvements; added the firewall bypass for FTP/IRC DCC.
1454 .An Erik Salander Aq erik@whistle.com
1455 added support for PPTP and RTSP.
1456 .An Junichi Satoh Aq junichi@junichi.org
1457 added support for RTSP/PNA.
1458 .An Ruslan Ermilov Aq ru@FreeBSD.org
1459 added support for PPTP and LSNAT as well as general hacking.
1460 .An Gleb Smirnoff Aq glebius@FreeBSD.org
1461 ported the library to kernel space.
1462 .An Paolo Pisati Aq piso@FreeBSD.org
1463 made the library modular, moving support for all
1464 protocols (except for IP, TCP and UDP) to external modules.
1465 .Sh ACKNOWLEDGEMENTS
1466 Listed below, in approximate chronological order, are individuals who
1467 have provided valuable comments and/or debugging assistance.
1468 .Bd -ragged -offset indent
1479 .An Andrzej Bialecki