2 .\" Copyright (c) 2001 Charles Mott <cm@linktel.net>
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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_RESET_ON_ADDR_CHANGE
216 When this mode bit is set and
217 .Fn LibAliasSetAddress
218 is called to change the aliasing address, the internal link table of the
219 packet aliasing engine will be cleared.
220 This operating mode is useful for
222 links where the interface address can sometimes change or remain the same
223 between dial-up attempts.
224 If this mode bit is not set, the link table will never be reset in the event
225 of an address change.
226 .It Dv PKT_ALIAS_PUNCH_FW
232 based firewall for FTP/IRC DCC connections.
233 The holes punched are bound by from/to IP address and port; it will not be
234 possible to use a hole for another connection.
235 A hole is removed when the connection that uses it dies.
236 To cater to unexpected death of a program using
239 changing the state of the flag will clear the entire firewall range
241 This clearing will also happen on the initial call to
242 .Fn LibAliasSetFWBase ,
243 which must happen prior to setting this flag.
244 .It Dv PKT_ALIAS_REVERSE
247 reverse the way it handles incoming and outgoing packets, allowing it
248 to be fed with data that passes through the internal interface rather
249 than the external one.
250 .It Dv PKT_ALIAS_PROXY_ONLY
253 to obey transparent proxy rules only.
254 Normal packet aliasing is not performed.
256 .Fn LibAliasProxyRule
258 .It Dv PKT_ALIAS_SKIP_GLOBAL
259 This option is used by
262 Specifying it as a flag to
266 .Sx NETWORK ADDRESS TRANSLATION
274 .Fn LibAliasSetFWBase "struct libalias *" "unsigned int base" "unsigned int num"
275 .Bd -ragged -offset indent
276 Set the firewall range allocated for punching firewall holes (with the
277 .Dv PKT_ALIAS_PUNCH_FW
279 The range is cleared for all rules on initialization.
283 .Fn LibAliasSkinnyPort "struct libalias *" "unsigned int port"
284 .Bd -ragged -offset indent
285 Set the TCP port used by the Skinny Station protocol.
286 Skinny is used by Cisco IP phones to communicate with
287 Cisco Call Managers to set up voice over IP calls.
288 If this is not set, Skinny aliasing will not be done.
289 The typical port used by Skinny is 2000.
292 The packet handling functions are used to modify incoming (remote to local)
293 and outgoing (local to remote) packets.
294 The calling program is responsible for receiving and sending packets via
300 .Fn LibAliasSetAddress ,
301 the two packet handling functions,
305 comprise the minimal set of functions needed for a basic IP masquerading
309 .Fn LibAliasIn "struct libalias *" "char *buffer" "int maxpacketsize"
310 .Bd -ragged -offset indent
311 An incoming packet coming from a remote machine to the local network is
312 de-aliased by this function.
313 The IP packet is pointed to by
317 indicates the size of the data structure containing the packet and should
318 be at least as large as the actual packet size.
321 .Bl -tag -width indent
323 The packet aliasing process was successful.
324 .It Dv PKT_ALIAS_IGNORED
325 The packet was ignored and not de-aliased.
326 This can happen if the protocol is unrecognized, as for an ICMP message
327 type that is not handled, or if incoming packets for new connections are being
329 .Dv PKT_ALIAS_DENY_INCOMING
330 mode bit was set using
331 .Fn LibAliasSetMode ) .
332 .It Dv PKT_ALIAS_UNRESOLVED_FRAGMENT
333 This is returned when a fragment cannot be resolved because the header
334 fragment has not been sent yet.
335 In this situation, fragments must be saved with
336 .Fn LibAliasSaveFragment
337 until a header fragment is found.
338 .It Dv PKT_ALIAS_FOUND_HEADER_FRAGMENT
339 The packet aliasing process was successful, and a header fragment was found.
340 This is a signal to retrieve any unresolved fragments with
341 .Fn LibAliasGetFragment
342 and de-alias them with
343 .Fn LibAliasFragmentIn .
344 .It Dv PKT_ALIAS_ERROR
345 An internal error within the packet aliasing engine occurred.
350 .Fn LibAliasOut "struct libalias *" "char *buffer" "int maxpacketsize"
351 .Bd -ragged -offset indent
352 An outgoing packet coming from the local network to a remote machine is
353 aliased by this function.
354 The IP packet is pointed to by
358 indicates the maximum packet size permissible should the packet length be
360 IP encoding protocols place address and port information in the encapsulated
361 data stream which has to be modified and can account for changes in packet
363 Well known examples of such protocols are FTP and IRC DCC.
366 .Bl -tag -width indent
368 The packet aliasing process was successful.
369 .It Dv PKT_ALIAS_IGNORED
370 The packet was ignored and not aliased.
371 This can happen if the protocol is unrecognized, or possibly an ICMP message
373 .It Dv PKT_ALIAS_ERROR
374 An internal error within the packet aliasing engine occurred.
377 .Sh PORT AND ADDRESS REDIRECTION
378 The functions described in this section allow machines on the local network
379 to be accessible in some degree to new incoming connections from the external
381 Individual ports can be re-mapped or static network address translations can
384 .Ft struct alias_link *
385 .Fo LibAliasRedirectPort
386 .Fa "struct libalias *"
387 .Fa "struct in_addr local_addr"
388 .Fa "u_short local_port"
389 .Fa "struct in_addr remote_addr"
390 .Fa "u_short remote_port"
391 .Fa "struct in_addr alias_addr"
392 .Fa "u_short alias_port"
395 .Bd -ragged -offset indent
396 This function specifies that traffic from a given remote address/port to
397 an alias address/port be redirected to a specified local address/port.
411 is zero, this indicates that the packet aliasing address as established
413 .Fn LibAliasSetAddress
416 .Fn LibAliasSetAddress
417 is called to change the address after
418 .Fn LibAliasRedirectPort
419 is called, a zero reference will track this change.
421 If the link is further set up to operate with load sharing, then
425 are ignored, and are selected dynamically from the server pool, as described in
426 .Fn LibAliasAddServer
431 is zero, this indicates to redirect packets from any remote address.
434 is zero, this indicates to redirect packets originating from any remote
436 The remote port specification will almost always be zero, but non-zero
437 remote addresses can sometimes be useful for firewalling.
439 .Fn LibAliasRedirectPort
440 overlap in their address/port specifications, then the most recent call
441 will have precedence.
443 This function returns a pointer which can subsequently be used by
444 .Fn LibAliasRedirectDelete .
447 is returned, then the function call did not complete successfully.
449 All port numbers should be in network address byte order, so it is necessary
452 to convert these parameters from internally readable numbers to network byte
454 Addresses are also in network byte order, which is implicit in the use of the
459 .Ft struct alias_link *
460 .Fo LibAliasRedirectAddr
461 .Fa "struct libalias *"
462 .Fa "struct in_addr local_addr"
463 .Fa "struct in_addr alias_addr"
465 .Bd -ragged -offset indent
466 This function designates that all incoming traffic to
470 Similarly, all outgoing traffic from
479 is zero, this indicates that the packet aliasing address as established by
480 .Fn LibAliasSetAddress
483 .Fn LibAliasSetAddress
484 is called to change the address after
485 .Fn LibAliasRedirectAddr
486 is called, a zero reference will track this change.
488 If the link is further set up to operate with load sharing, then the
490 argument is ignored, and is selected dynamically from the server pool,
492 .Fn LibAliasAddServer
495 If subsequent calls to
496 .Fn LibAliasRedirectAddr
497 use the same aliasing address, all new incoming traffic to this aliasing
498 address will be redirected to the local address made in the last function
500 New traffic generated by any of the local machines, designated in the
501 several function calls, will be aliased to the same address.
502 Consider the following example:
504 LibAliasRedirectAddr(la, inet_aton("192.168.0.2"),
505 inet_aton("141.221.254.101"));
506 LibAliasRedirectAddr(la, inet_aton("192.168.0.3"),
507 inet_aton("141.221.254.101"));
508 LibAliasRedirectAddr(la, inet_aton("192.168.0.4"),
509 inet_aton("141.221.254.101"));
511 Any outgoing connections such as
515 from 192.168.0.2, 192.168.0.3 and 192.168.0.4 will appear to come from
517 Any incoming connections to 141.221.254.101 will be directed to 192.168.0.4.
520 .Fn LibAliasRedirectPort
521 will have precedence over address mappings designated by
522 .Fn LibAliasRedirectAddr .
524 This function returns a pointer which can subsequently be used by
525 .Fn LibAliasRedirectDelete .
528 is returned, then the function call did not complete successfully.
532 .Fo LibAliasAddServer
533 .Fa "struct libalias *"
534 .Fa "struct alias_link *link"
535 .Fa "struct in_addr addr"
538 .Bd -ragged -offset indent
539 This function sets the
541 up for Load Sharing using IP Network Address Translation (RFC 2391, LSNAT).
542 LSNAT operates as follows.
543 A client attempts to access a server by using the server virtual address.
544 The LSNAT router transparently redirects the request to one of the hosts
545 in the server pool, using a real-time load sharing algorithm.
546 Multiple sessions may be initiated from the same client, and each session
547 could be directed to a different host based on the load balance across server
548 pool hosts when the sessions are initiated.
549 If load sharing is desired for just a few specific services, the configuration
550 on LSNAT could be defined to restrict load sharing to just the services
553 Currently, only the simplest selection algorithm is implemented, where a
554 host is selected on a round-robin basis only, without regard to load on
560 .Fn LibAliasRedirectPort
562 .Fn LibAliasRedirectAddr .
564 .Fn LibAliasAddServer
565 is called multiple times to add entries to the
569 For links created with
570 .Fn LibAliasRedirectAddr ,
573 argument is ignored and could have any value, e.g.\& htons(~0).
575 This function returns 0 on success, \-1 otherwise.
579 .Fn LibAliasRedirectDynamic "struct libalias *" "struct alias_link *link"
580 .Bd -ragged -offset indent
581 This function marks the specified static redirect rule entered by
582 .Fn LibAliasRedirectPort
584 This can be used to e.g.\& dynamically redirect a single TCP connection,
585 after which the rule is removed.
586 Only fully specified links can be made dynamic.
588 .Sx STATIC AND DYNAMIC LINKS
590 .Sx PARTIALLY SPECIFIED ALIASING LINKS
591 sections below for a definition of static vs.\& dynamic,
592 and partially vs.\& fully specified links.)
594 This function returns 0 on success, \-1 otherwise.
598 .Fn LibAliasRedirectDelete "struct libalias *" "struct alias_link *link"
599 .Bd -ragged -offset indent
600 This function will delete a specific static redirect rule entered by
601 .Fn LibAliasRedirectPort
603 .Fn LibAliasRedirectAddr .
606 is the pointer returned by either of the redirection functions.
607 If an invalid pointer is passed to
608 .Fn LibAliasRedirectDelete ,
609 then a program crash or unpredictable operation could result, so
610 care is needed when using this function.
614 .Fn LibAliasProxyRule "struct libalias *" "const char *cmd"
615 .Bd -ragged -offset indent
618 string consists of one or more pairs of words.
619 The first word in each pair is a token and the second is the value that
620 should be applied for that token.
621 Tokens and their argument types are as follows:
622 .Bl -tag -width indent
623 .It Cm type encode_ip_hdr | encode_tcp_stream | no_encode
624 In order to support transparent proxying, it is necessary to somehow
625 pass the original address and port information into the new destination
629 is specified, the original destination address and port are passed
630 as an extra IP option.
632 .Cm encode_tcp_stream
633 is specified, the original destination address and port are passed
634 as the first piece of data in the TCP stream in the format
635 .Dq Li DEST Ar IP port .
636 .It Cm port Ar portnum
637 Only packets with the destination port
640 .It Cm server Ar host Ns Op : Ns Ar portnum
645 that the data is to be redirected to.
647 must be an IP address rather than a DNS host name.
650 is not specified, the destination port number is not changed.
654 specification is mandatory unless the
656 command is being used.
658 Normally, each call to
659 .Fn LibAliasProxyRule
660 inserts the next rule at the start of a linear list of rules.
663 is specified, the new rule will be checked after all rules with lower
666 .Fn LibAliasProxyRule
667 that do not specify a rule are assigned rule 0.
668 .It Cm delete Ar index
669 This token and its argument MUST NOT be used with any other tokens.
670 When used, all existing rules with the given
673 .It Cm proto tcp | udp
674 If specified, only packets of the given protocol type are matched.
675 .It Cm src Ar IP Ns Op / Ns Ar bits
676 If specified, only packets with a source address matching the given
681 is also specified, then the first
685 are taken as a network specification, and all IP addresses from that
686 network will be matched.
687 .It Cm dst Ar IP Ns Op / Ns Ar bits
688 If specified, only packets with a destination address matching the given
693 is also specified, then the first
697 are taken as a network specification, and all IP addresses from that
698 network will be matched.
701 This function is usually used to redirect outgoing connections for
702 internal machines that are not permitted certain types of internet
703 access, or to restrict access to certain external machines.
706 .Ft struct alias_link *
707 .Fo LibAliasRedirectProto
708 .Fa "struct libalias *"
709 .Fa "struct in_addr local_addr"
710 .Fa "struct in_addr remote_addr"
711 .Fa "struct in_addr alias_addr"
714 .Bd -ragged -offset indent
715 This function specifies that any IP packet with protocol number of
717 from a given remote address to an alias address will be
718 redirected to a specified local address.
724 is zero, this indicates that the packet aliasing address as established
726 .Fn LibAliasSetAddress
729 .Fn LibAliasSetAddress
730 is called to change the address after
731 .Fn LibAliasRedirectProto
732 is called, a zero reference will track this change.
736 is zero, this indicates to redirect packets from any remote address.
737 Non-zero remote addresses can sometimes be useful for firewalling.
740 .Fn LibAliasRedirectProto
741 overlap in their address specifications, then the most recent call
742 will have precedence.
744 This function returns a pointer which can subsequently be used by
745 .Fn LibAliasRedirectDelete .
748 is returned, then the function call did not complete successfully.
750 .Sh FRAGMENT HANDLING
751 The functions in this section are used to deal with incoming fragments.
753 Outgoing fragments are handled within
755 by changing the address according to any applicable mapping set by
756 .Fn LibAliasRedirectAddr ,
757 or the default aliasing address set by
758 .Fn LibAliasSetAddress .
760 Incoming fragments are handled in one of two ways.
761 If the header of a fragmented IP packet has already been seen, then all
762 subsequent fragments will be re-mapped in the same manner the header
764 Fragments which arrive before the header are saved and then retrieved
765 once the header fragment has been resolved.
768 .Fn LibAliasSaveFragment "struct libalias *" "char *ptr"
769 .Bd -ragged -offset indent
773 .Dv PKT_ALIAS_UNRESOLVED_FRAGMENT ,
774 this function can be used to save the pointer to the unresolved fragment.
776 It is implicitly assumed that
778 points to a block of memory allocated by
780 If the fragment is never resolved, the packet aliasing engine will
781 automatically free the memory after a timeout period.
782 [Eventually this function should be modified so that a callback function
783 for freeing memory is passed as an argument.]
785 This function returns
787 if it was successful and
789 if there was an error.
793 .Fn LibAliasGetFragment "struct libalias *" "char *buffer"
794 .Bd -ragged -offset indent
795 This function can be used to retrieve fragment pointers saved by
796 .Fn LibAliasSaveFragment .
797 The IP header fragment pointed to by
799 is the header fragment indicated when
802 .Dv PKT_ALIAS_FOUND_HEADER_FRAGMENT .
803 Once a fragment pointer is retrieved, it becomes the calling program's
804 responsibility to free the dynamically allocated memory for the fragment.
807 .Fn LibAliasGetFragment
808 function can be called sequentially until there are no more fragments
809 available, at which time it returns
814 .Fn LibAliasFragmentIn "struct libalias *" "char *header" "char *fragment"
815 .Bd -ragged -offset indent
816 When a fragment is retrieved with
817 .Fn LibAliasGetFragment ,
818 it can then be de-aliased with a call to
819 .Fn LibAliasFragmentIn .
822 argument is the pointer to a header fragment used as a template, and
824 is the pointer to the packet to be de-aliased.
826 .Sh MISCELLANEOUS FUNCTIONS
827 .Ft struct alias_link *
828 .Fn AddLink "struct libalias *" "struct in_addr src_addr" "struct in_addr dst_addr" \
829 "struct in_addr alias_addr" "u_short src_port" "u_short dst_port" \
830 "int alias_param" "int link_type"
831 .Bd -ragged -offset indent
832 This function adds new state to the instance hash table.
833 The dst_address and/or dst_port may be given as zero, which
834 introduces some dynamic character into the link, since
835 LibAliasSetAddress can change the address that is used.
836 However, in the current implementation, such links can only be used
837 for inbound (ext -> int) traffic.
841 .Fn LibAliasSetTarget "struct libalias *" "struct in_addr addr"
842 .Bd -ragged -offset indent
843 When an incoming packet not associated with any pre-existing aliasing link
844 arrives at the host machine, it will be sent to the address indicated by a
846 .Fn LibAliasSetTarget .
848 If this function is called with an
850 address argument, then all new incoming packets go to the address set by
851 .Fn LibAliasSetAddress .
853 If this function is not called, or is called with an
855 address argument, then all new incoming packets go to the address specified
857 This allows external machines to talk directly to internal machines if they
858 can route packets to the machine in question.
862 .Fn LibAliasCheckNewLink "struct libalias *"
863 .Bd -ragged -offset indent
864 This function returns a non-zero value when a new aliasing link is created.
865 In circumstances where incoming traffic is being sequentially sent to
866 different local servers, this function can be used to trigger when
867 .Fn LibAliasSetTarget
868 is called to change the default target address.
872 .Fn LibAliasInternetChecksum "struct libalias *" "u_short *buffer" "int nbytes"
873 .Bd -ragged -offset indent
874 This is a utility function that does not seem to be available elsewhere and
875 is included as a convenience.
876 It computes the internet checksum, which is used in both IP and
877 protocol-specific headers (TCP, UDP, ICMP).
881 argument points to the data block to be checksummed, and
883 is the number of bytes.
884 The 16-bit checksum field should be zeroed before computing the checksum.
886 Checksums can also be verified by operating on a block of data including
888 If the checksum is valid,
889 .Fn LibAliasInternetChecksum
894 .Fn LibAliasUnaliasOut "struct libalias *" "char *buffer" "int maxpacketsize"
895 .Bd -ragged -offset indent
896 An outgoing packet, which has already been aliased,
897 has its private address/port information restored by this function.
898 The IP packet is pointed to by
902 is provided for error checking purposes.
903 This function can be used if an already-aliased packet needs to have its
904 original IP header restored for further processing (e.g.\& logging).
906 .Sh CONCEPTUAL BACKGROUND
907 This section is intended for those who are planning to modify the source
908 code or want to create somewhat esoteric applications using the packet
911 The conceptual framework under which the packet aliasing engine operates
913 Central to the discussion is the idea of an
915 which describes the relationship for a given packet transaction between
916 the local machine, aliased identity and remote machine.
917 It is discussed how such links come into existence and are destroyed.
919 There is a notion of an
921 which is a 7-tuple describing a specific translation:
922 .Bd -literal -offset indent
923 (local addr, local port, alias addr, alias port,
924 remote addr, remote port, protocol)
927 Outgoing packets have the local address and port number replaced with the
928 alias address and port number.
929 Incoming packets undergo the reverse process.
930 The packet aliasing engine attempts to match packets against an internal
931 table of aliasing links to determine how to modify a given IP packet.
932 Both the IP header and protocol dependent headers are modified as necessary.
933 Aliasing links are created and deleted as necessary according to network
936 Protocols can be TCP, UDP or even ICMP in certain circumstances.
937 (Some types of ICMP packets can be aliased according to sequence or ID
938 number which acts as an equivalent port number for identifying how
939 individual packets should be handled.)
941 Each aliasing link must have a unique combination of the following five
942 quantities: alias address/port, remote address/port and protocol.
943 This ensures that several machines on a local network can share the
944 same aliasing IP address.
945 In cases where conflicts might arise, the aliasing port is chosen so that
946 uniqueness is maintained.
947 .Ss STATIC AND DYNAMIC LINKS
948 Aliasing links can either be static or dynamic.
949 Static links persist indefinitely and represent fixed rules for translating
951 Dynamic links come into existence for a specific TCP connection or UDP
952 transaction or ICMP ECHO sequence.
953 For the case of TCP, the connection can be monitored to see when the
954 associated aliasing link should be deleted.
955 Aliasing links for UDP transactions (and ICMP ECHO and TIMESTAMP requests)
956 work on a simple timeout rule.
957 When no activity is observed on a dynamic link for a certain amount of time
958 it is automatically deleted.
959 Timeout rules also apply to TCP connections which do not open or close
961 .Ss PARTIALLY SPECIFIED ALIASING LINKS
962 Aliasing links can be partially specified, meaning that the remote address
963 and/or remote port are unknown.
964 In this case, when a packet matching the incomplete specification is found,
965 a fully specified dynamic link is created.
966 If the original partially specified link is dynamic, it will be deleted
967 after the fully specified link is created, otherwise it will persist.
969 For instance, a partially specified link might be
970 .Bd -literal -offset indent
971 (192.168.0.4, 23, 204.228.203.215, 8066, 0, 0, tcp)
974 The zeros denote unspecified components for the remote address and port.
975 If this link were static it would have the effect of redirecting all
976 incoming traffic from port 8066 of 204.228.203.215 to port 23 (telnet)
977 of machine 192.168.0.4 on the local network.
978 Each individual telnet connection would initiate the creation of a distinct
980 .Ss DYNAMIC LINK CREATION
981 In addition to aliasing links, there are also address mappings that can be
982 stored within the internal data table of the packet aliasing mechanism.
983 .Bd -literal -offset indent
984 (local addr, alias addr)
987 Address mappings are searched when creating new dynamic links.
989 All outgoing packets from the local network automatically create a dynamic
990 link if they do not match an already existing fully specified link.
991 If an address mapping exists for the outgoing packet, this determines
992 the alias address to be used.
993 If no mapping exists, then a default address, usually the address of the
994 packet aliasing host, is used.
995 If necessary, this default address can be changed as often as each individual
998 The aliasing port number is determined such that the new dynamic link does
999 not conflict with any existing links.
1000 In the default operating mode, the packet aliasing engine attempts to set
1001 the aliasing port equal to the local port number.
1002 If this results in a conflict, then port numbers are randomly chosen until
1003 a unique aliasing link can be established.
1004 In an alternate operating mode, the first choice of an aliasing port is also
1005 random and unrelated to the local port number.
1006 .Sh MODULAR ARCHITECTURE Po AND Xr ipfw 4 SUPPORT Pc
1007 One of the latest improvements to
1009 was to make its support
1010 for new protocols independent from the rest of the library, giving it
1011 the ability to load/unload support for new protocols at run-time.
1012 To achieve this feature, all the code for protocol handling was moved
1013 to a series of modules outside of the main library.
1014 These modules are compiled from the same sources but work in
1015 different ways, depending on whether they are compiled to work inside a kernel
1016 or as part of the userland library.
1017 .Ss LIBALIAS MODULES IN KERNEL LAND
1018 When compiled for the kernel,
1020 modules are plain KLDs recognizable with the
1024 To add support for a new protocol, load the corresponding module.
1027 .Dl "kldload alias_ftp"
1029 When support for a protocol is no longer needed, its module can be unloaded:
1031 .Dl "kldunload alias_ftp"
1032 .Ss LIBALIAS MODULES IN USERLAND
1033 Due to the differences between kernel and userland (no KLD mechanism,
1034 many different address spaces, etc.), we had to change a bit how to
1035 handle module loading/tracking/unloading in userland.
1037 While compiled for a userland
1039 all the modules are plain libraries, residing in
1041 and recognizable with the
1045 There is a configuration file,
1046 .Pa /etc/libalias.conf ,
1047 with the following contents (by default):
1048 .Bd -literal -offset indent
1049 /usr/lib/libalias_cuseeme.so
1050 /usr/lib/libalias_ftp.so
1051 /usr/lib/libalias_irc.so
1052 /usr/lib/libalias_nbt.so
1053 /usr/lib/libalias_pptp.so
1054 /usr/lib/libalias_skinny.so
1055 /usr/lib/libalias_smedia.so
1058 This file contains the paths to the modules that
1061 To load/unload a new module, just add its path to
1064 .Fn LibAliasRefreshModules
1066 In case the application provides a
1068 signal handler, add a call to
1069 .Fn LibAliasRefreshModules
1070 inside the handler, and every time you want to refresh the loaded modules,
1075 .Dl "kill -HUP <process_pid>"
1076 .Ss MODULAR ARCHITECURE: HOW IT WORKS
1077 The modular architecture of
1079 works similar whether it is running inside the
1080 kernel or in userland.
1084 /* Protocol and userland module handlers chains. */
1085 LIST_HEAD(handler_chain, proto_handler) handler_chain ...
1087 SLIST_HEAD(dll_chain, dll) dll_chain ...
1091 keeps track of all the protocol handlers loaded, while
1093 tracks which userland modules are loaded.
1097 .Vt "struct proto_handler"
1100 struct proto_handler {
1104 int (*fingerprint)(struct libalias *la,
1105 struct ip *pip, struct alias_data *ah);
1106 int (*protohandler)(struct libalias *la,
1107 struct ip *pip, struct alias_data *ah);
1108 TAILQ_ENTRY(proto_handler) link;
1115 is the priority assigned to a protocol handler; lower priority
1118 is the direction of packets: ingoing or outgoing.
1120 indicates to which protocol this packet belongs: IP, TCP or UDP.
1122 points to the fingerprint function while protohandler points
1123 to the protocol handler function.
1128 function has the dual role of checking if the
1129 incoming packet is found, and if it belongs to any categories that this
1134 function actually manipulates
1139 When a packet enters
1141 if it meets a module hook,
1143 is searched to see if there is an handler that matches
1144 this type of a packet (it checks protocol and direction of packet).
1145 Then, if more than one handler is found, it starts with the module with
1146 the lowest priority number: it calls the
1148 function and interprets the result.
1150 If the result value is equal to 0 then it calls the protocol handler
1151 of this handler and returns.
1152 Otherwise, it proceeds to the next eligible module until the
1158 the module hook looks like this:
1159 .Bd -literal -offset indent
1160 struct alias_data ad = {
1165 &ud->uh_sport, /* original source port */
1166 &ud->uh_dport, /* original dest port */
1167 256 /* maxpacketsize */
1172 /* walk out chain */
1173 err = find_handler(IN, UDP, la, pip, &ad);
1176 All data useful to a module are gathered together in an
1183 function is responsible for walking the handler
1184 chain; it receives as input parameters:
1185 .Bl -tag -width indent
1191 pointer to this instance of libalias
1197 .Vt "struct alias_data"
1203 will search only for modules registered for
1204 supporting INcoming UDP packets.
1206 As was mentioned earlier,
1208 in userland is a bit different, as
1209 care must be taken in module handling as well (avoiding duplicate load of
1210 modules, avoiding modules with same name, etc.) so
1215 contains a list of all userland
1219 When an application calls
1220 .Fn LibAliasRefreshModules ,
1222 first unloads all the loaded modules, then reloads all the modules listed in
1223 .Pa /etc/libalias.conf :
1224 for every module loaded, a new entry is added to
1233 /* name of module */
1236 * ptr to shared obj obtained through
1237 * dlopen() - use this ptr to get access
1238 * to any symbols from a loaded module
1247 is the name of the module.
1249 is a pointer to the module obtained through
1252 Whenever a module is loaded in userland, an entry is added to
1254 then every protocol handler present in that module
1255 is resolved and registered in
1257 .Ss HOW TO WRITE A MODULE FOR LIBALIAS
1258 There is a module (called
1259 .Pa alias_dummy.[ch] )
1262 that can be used as a skeleton for future work.
1263 Here we analyse some parts of that module.
1267 struct proto_handler handlers[] = {
1272 .fingerprint = fingerprint,
1273 .protohandler= protohandler,
1282 .Dq "most important thing"
1284 since it describes the handlers present and lets the outside world use
1285 it in an opaque way.
1287 It must ALWAYS be present in every module, and it MUST retain
1290 otherwise attempting to load a module in userland will fail and
1291 complain about missing symbols: for more information about module
1292 load/unload, please refer to
1293 .Fn LibAliasRefreshModules ,
1294 .Fn LibAliasLoadModule
1296 .Fn LibAliasUnloadModule
1303 structures present in a module.
1306 mod_handler(module_t mod, int type, void *data)
1312 error = LibAliasAttachHandlers(handlers);
1315 error = LibAliasDetachHandlers(handlers);
1323 When running as KLD,
1325 registers/deregisters the module using
1326 .Fn LibAliasAttachHandlers
1328 .Fn LibAliasDetachHandlers ,
1331 Every module must contain at least 2 functions: one fingerprint
1332 function and a protocol handler function.
1338 fingerprint(struct libalias *la, struct ip *pip, struct alias_data *ah)
1348 protohandler(struct libalias *la, struct ip *pip,
1349 struct alias_data *ah)
1355 and they must accept exactly these input parameters.
1356 .Ss PATCHING AN APPLICATION FOR USERLAND LIBALIAS MODULES
1357 To add module support into an application that uses
1359 the following simple steps can be followed.
1362 Find the main file of an application
1366 Add this to the header section of
1368 if not already present:
1370 .Dl "#include <signal.h>"
1372 and this just after the header section:
1374 .Dl "static void signal_handler(int);"
1376 Add the following line to the init function of an application or,
1377 if it does not have any init function, put it in
1380 .Dl "signal(SIGHUP, signal_handler);"
1384 function somewhere in
1386 .Bd -literal -offset indent
1388 signal_handler(int sig)
1391 LibAliasRefreshModules();
1395 Otherwise, if an application already traps the
1397 signal, just add a call to
1398 .Fn LibAliasRefreshModules
1399 in the signal handler function.
1401 For example, to patch
1405 modules, just add the following line to
1406 .Fn RefreshAddr "int sig __unused" :
1408 .Dl "LibAliasRefreshModules()"
1410 recompile and you are done.
1411 .Ss LOGGING SUPPORT IN KERNEL LAND
1412 When working as KLD,
1414 now has log support that
1415 happens on a buffer allocated inside
1416 .Vt "struct libalias"
1418 .Pa alias_local.h ) :
1423 /* log descriptor */
1426 * ptr to an auto-malloced
1427 * memory buffer when libalias
1432 * ptr to /var/log/alias.log
1433 * when libalias runs as a
1441 so all applications using
1443 will be able to handle their
1444 own logs, if they want, accessing
1446 Moreover, every change to a log buffer is automatically added to
1454 .An Charles Mott Aq cm@linktel.net ,
1455 versions 1.0 - 1.8, 2.0 - 2.4.
1456 .An Eivind Eklund Aq eivind@FreeBSD.org ,
1457 versions 1.8b, 1.9 and 2.5.
1458 Added IRC DCC support as well as contributing a number of architectural
1459 improvements; added the firewall bypass for FTP/IRC DCC.
1460 .An Erik Salander Aq erik@whistle.com
1461 added support for PPTP and RTSP.
1462 .An Junichi Satoh Aq junichi@junichi.org
1463 added support for RTSP/PNA.
1464 .An Ruslan Ermilov Aq ru@FreeBSD.org
1465 added support for PPTP and LSNAT as well as general hacking.
1466 .An Gleb Smirnoff Aq glebius@FreeBSD.org
1467 ported the library to kernel space.
1468 .An Paolo Pisati Aq piso@FreeBSD.org
1469 made the library modular, moving support for all
1470 protocols (except for IP, TCP and UDP) to external modules.
1471 .Sh ACKNOWLEDGEMENTS
1472 Listed below, in approximate chronological order, are individuals who
1473 have provided valuable comments and/or debugging assistance.
1474 .Bd -ragged -offset indent
1485 .An Andrzej Bialecki
projects / FreeBSD / FreeBSD.git / blob