- Uniformly use .In for header file references.
[dragonfly.git] / lib / libalias / libalias.3
CommitLineData
984263bc
MD
1.\"-
2.\" Copyright (c) 2001 Charles Mott <cm@linktel.net>
3.\" All rights reserved.
4.\"
5.\" Redistribution and use in source and binary forms, with or without
6.\" modification, are permitted provided that the following conditions
7.\" are met:
8.\" 1. Redistributions of source code must retain the above copyright
9.\" notice, this list of conditions and the following disclaimer.
10.\" 2. Redistributions in binary form must reproduce the above copyright
11.\" notice, this list of conditions and the following disclaimer in the
12.\" documentation and/or other materials provided with the distribution.
13.\"
14.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
15.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
16.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
17.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
18.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
19.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
20.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
21.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
22.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
23.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
24.\" SUCH DAMAGE.
25.\"
26.\" $FreeBSD: src/lib/libalias/libalias.3,v 1.23.2.11 2001/12/17 10:08:22 ru Exp $
44cb301e 27.\" $DragonFly: src/lib/libalias/libalias.3,v 1.4 2006/05/26 19:39:36 swildner Exp $
984263bc
MD
28.\"
29.Dd April 13, 2000
30.Dt LIBALIAS 3
31.Os
32.Sh NAME
33.Nm libalias
34.Nd packet aliasing library for masquerading and network address translation
35.Sh SYNOPSIS
36.In sys/types.h
37.In netinet/in.h
38.In alias.h
39.Pp
40Function prototypes are given in the main body of the text.
41.Sh DESCRIPTION
42The
43.Nm
44library is a collection of functions for aliasing and de-aliasing of IP
45packets, intended for masquerading and network address translation (NAT).
46.Sh INTRODUCTION
47This library is a moderately portable set of functions designed to assist
48in the process of IP masquerading and network address translation.
49Outgoing packets from a local network with unregistered IP addresses can
50be aliased to appear as if they came from an accessible IP address.
51Incoming packets are then de-aliased so that they are sent to the correct
52machine on the local network.
53.Pp
54A certain amount of flexibility is built into the packet aliasing engine.
55In the simplest mode of operation, a many-to-one address mapping takes
56place between local network and the packet aliasing host.
57This is known as IP masquerading.
58In addition, one-to-one mappings between local and public addresses can
59also be implemented, which is known as static NAT.
60In between these extremes, different groups of private addresses can be
61linked to different public addresses, comprising several distinct
62many-to-one mappings.
63Also, a given public address and port can be statically redirected to a
64private address/port.
65.Pp
66The packet aliasing engine was designed to operate in user space outside
67of the kernel, without any access to private kernel data structure, but
68the source code can also be ported to a kernel environment.
69.Sh INITIALIZATION AND CONTROL
70Two special functions,
71.Fn PacketAliasInit
72and
73.Fn PacketAliasSetAddress ,
74must always be called before any packet handling may be performed.
75In addition, the operating mode of the packet aliasing engine can be
76customized by calling
77.Fn PacketAliasSetMode .
78.Pp
79.Ft void
80.Fn PacketAliasInit void
81.Bd -ragged -offset indent
82This function has no arguments or return value and is used to initialize
83internal data structures.
84The following mode bits are always set after calling
85.Fn PacketAliasInit .
86See the description of
87.Fn PacketAliasSetMode
88below for the meaning of these mode bits.
89.Pp
90.Bl -item -offset indent -compact
91.It
92.Dv PKT_ALIAS_SAME_PORTS
93.It
94.Dv PKT_ALIAS_USE_SOCKETS
95.It
96.Dv PKT_ALIAS_RESET_ON_ADDR_CHANGE
97.El
98.Pp
99This function will always return the packet aliasing engine to the same
100initial state.
101.Fn PacketAliasSetAddress
102must be called afterwards, and any desired changes from the default mode
103bits listed above require a call to
104.Fn PacketAliasSetMode .
105.Pp
106It is mandatory that this function be called at the beginning of a program
107prior to any packet handling.
108.Ed
109.Pp
110.Ft void
111.Fn PacketAliasUninit void
112.Bd -ragged -offset indent
113This function has no arguments or return value and is used to clear any
114resources attached to internal data structures.
115.Pp
116This functions should be called when a program stops using the aliasing
117engine; it does, amongst other things, clear out any firewall holes.
118To provide backwards compatibility and extra security, it is added to
119the
120.Xr atexit 3
121chain by
122.Fn PacketAliasInit .
123Calling it multiple times is harmless.
124.Ed
125.Pp
126.Ft void
127.Fn PacketAliasSetAddress "struct in_addr addr"
128.Bd -ragged -offset indent
129This function sets the source address to which outgoing packets from the
130local area network are aliased.
131All outgoing packets are re-mapped to this address unless overridden by a
132static address mapping established by
133.Fn PacketAliasRedirectAddr .
134.Pp
135If the
136.Dv PKT_ALIAS_RESET_ON_ADDR_CHANGE
137mode bit is set (the default mode of operation), then the internal aliasing
138link tables will be reset any time the aliasing address changes.
139This is useful for interfaces such as
140.Xr ppp 8 ,
141where the IP
142address may or may not change on successive dial-up attempts.
143.Pp
144If the
145.Dv PKT_ALIAS_RESET_ON_ADDR_CHANGE
146mode bit is set to zero, this function can also be used to dynamically change
147the aliasing address on a packet to packet basis (it is a low overhead call).
148.Pp
149It is mandatory that this function be called prior to any packet handling.
150.Ed
151.Pp
152.Ft unsigned int
153.Fn PacketAliasSetMode "unsigned int flags" "unsigned int mask"
154.Bd -ragged -offset indent
155This function sets or clears mode bits
156according to the value of
157.Fa flags .
158Only bits marked in
159.Fa mask
160are affected.
161The following mode bits are defined in
44cb301e 162.In alias.h :
984263bc
MD
163.Bl -tag -width indent
164.It Dv PKT_ALIAS_LOG
165Enables logging into
166.Pa /var/log/alias.log .
167Each time an aliasing link is created or deleted, the log file is appended
168with the current number of ICMP, TCP and UDP links.
169Mainly useful for debugging when the log file is viewed continuously with
170.Xr tail 1 .
171.It Dv PKT_ALIAS_DENY_INCOMING
172If this mode bit is set, all incoming packets associated with new TCP
173connections or new UDP transactions will be marked for being ignored
174.Fn ( PacketAliasIn
175returns
176.Dv PKT_ALIAS_IGNORED
177code)
178by the calling program.
179Response packets to connections or transactions initiated from the packet
180aliasing host or local network will be unaffected.
181This mode bit is useful for implementing a one-way firewall.
182.It Dv PKT_ALIAS_SAME_PORTS
183If this mode bit is set, the packet aliasing engine will attempt to leave
184the alias port numbers unchanged from the actual local port numbers.
185This can be done as long as the quintuple (proto, alias addr, alias port,
186remote addr, remote port) is unique.
187If a conflict exists, a new aliasing port number is chosen even if this
188mode bit is set.
189.It Dv PKT_ALIAS_USE_SOCKETS
190This bit should be set when the packet aliasing host originates network
191traffic as well as forwards it.
192When the packet aliasing host is waiting for a connection from an unknown
193host address or unknown port number (e.g. an FTP data connection), this
194mode bit specifies that a socket be allocated as a place holder to prevent
195port conflicts.
196Once a connection is established, usually within a minute or so, the socket
197is closed.
198.It Dv PKT_ALIAS_UNREGISTERED_ONLY
199If this mode bit is set, traffic on the local network which does not
200originate from unregistered address spaces will be ignored.
201Standard Class A, B and C unregistered addresses are:
202.Bd -literal -offset indent
20310.0.0.0 -> 10.255.255.255 (Class A subnet)
204172.16.0.0 -> 172.31.255.255 (Class B subnets)
205192.168.0.0 -> 192.168.255.255 (Class C subnets)
206.Ed
207.Pp
208This option is useful in the case that packet aliasing host has both
209registered and unregistered subnets on different interfaces.
210The registered subnet is fully accessible to the outside world, so traffic
211from it does not need to be passed through the packet aliasing engine.
212.It Dv PKT_ALIAS_RESET_ON_ADDR_CHANGE
213When this mode bit is set and
214.Fn PacketAliasSetAddress
215is called to change the aliasing address, the internal link table of the
216packet aliasing engine will be cleared.
217This operating mode is useful for
218.Xr ppp 8
219links where the interface address can sometimes change or remain the same
220between dial-up attempts.
221If this mode bit is not set, the link table will never be reset in the event
222of an address change.
223.It Dv PKT_ALIAS_PUNCH_FW
224This option makes
225.Nm
226`punch holes' in an
227.Xr ipfirewall 4
228based firewall for FTP/IRC DCC connections.
229The holes punched are bound by from/to IP address and port; it will not be
230possible to use a hole for another connection.
231A hole is removed when the connection that uses it dies.
232To cater to unexpected death of a program using
233.Nm
234(e.g. kill -9),
235changing the state of the flag will clear the entire firewall range
236allocated for holes.
237This will also happen on the initial call to
238.Fn PacketAliasSetFWBase .
239This call must happen prior to setting this flag.
240.It Dv PKT_ALIAS_REVERSE
241This option makes
242.Nm
243reverse the way it handles incoming and outgoing packets, allowing it
244to be fed with data that passes through the internal interface rather
245than the external one.
246.It Dv PKT_ALIAS_PROXY_ONLY
247This option tells
248.Nm
249to obey transparent proxy rules only.
250Normal packet aliasing is not performed.
251See
252.Fn PacketAliasProxyRule
253below for details.
254.El
255.Ed
256.Pp
257.Ft void
258.Fn PacketAliasSetFWBase "unsigned int base" "unsigned int num"
259.Bd -ragged -offset indent
260Set firewall range allocated for punching firewall holes (with the
261.Dv PKT_ALIAS_PUNCH_FW
262flag).
263The range will be cleared for all rules on initialization.
264.Ed
265.Sh PACKET HANDLING
266The packet handling functions are used to modify incoming (remote to local)
267and outgoing (local to remote) packets.
268The calling program is responsible for receiving and sending packets via
269network interfaces.
270.Pp
271Along with
272.Fn PacketAliasInit
273and
274.Fn PacketAliasSetAddress ,
275the two packet handling functions,
276.Fn PacketAliasIn
277and
278.Fn PacketAliasOut ,
279comprise minimal set of functions needed for a basic IP masquerading
280implementation.
281.Pp
282.Ft int
283.Fn PacketAliasIn "char *buffer" "int maxpacketsize"
284.Bd -ragged -offset indent
285An incoming packet coming from a remote machine to the local network is
286de-aliased by this function.
287The IP packet is pointed to by
288.Fa buffer ,
289and
290.Fa maxpacketsize
291indicates the size of the data structure containing the packet and should
292be at least as large as the actual packet size.
293.Pp
294Return codes:
295.Bl -tag -width indent
296.It Dv PKT_ALIAS_OK
297The packet aliasing process was successful.
298.It Dv PKT_ALIAS_IGNORED
299The packet was ignored and not de-aliased.
300This can happen if the protocol is unrecognized, possibly an ICMP message
301type is not handled or if incoming packets for new connections are being
302ignored (if
303.Dv PKT_ALIAS_DENY_INCOMING
304mode bit was set by
305.Fn PacketAliasSetMode ) .
306.It Dv PKT_ALIAS_UNRESOLVED_FRAGMENT
307This is returned when a fragment cannot be resolved because the header
308fragment has not been sent yet.
309In this situation, fragments must be saved with
310.Fn PacketAliasSaveFragment
311until a header fragment is found.
312.It Dv PKT_ALIAS_FOUND_HEADER_FRAGMENT
313The packet aliasing process was successful, and a header fragment was found.
314This is a signal to retrieve any unresolved fragments with
315.Fn PacketAliasGetFragment
316and de-alias them with
317.Fn PacketAliasFragmentIn .
318.It Dv PKT_ALIAS_ERROR
319An internal error within the packet aliasing engine occurred.
320.El
321.Ed
322.Pp
323.Ft int
324.Fn PacketAliasOut "char *buffer" "int maxpacketsize"
325.Bd -ragged -offset indent
326An outgoing packet coming from the local network to a remote machine is
327aliased by this function.
328The IP packet is pointed to by
329.Fa buffer ,
330and
331.Fa maxpacketsize
332indicates the maximum packet size permissible should the packet length be
333changed.
334IP encoding protocols place address and port information in the encapsulated
335data stream which has to be modified and can account for changes in packet
336length.
337Well known examples of such protocols are FTP and IRC DCC.
338.Pp
339Return codes:
340.Bl -tag -width indent
341.It Dv PKT_ALIAS_OK
342The packet aliasing process was successful.
343.It Dv PKT_ALIAS_IGNORED
344The packet was ignored and not aliased.
345This can happen if the protocol is unrecognized, or possibly an ICMP message
346type is not handled.
347.It Dv PKT_ALIAS_ERROR
348An internal error within the packet aliasing engine occurred.
349.El
350.Ed
351.Sh PORT AND ADDRESS REDIRECTION
352The functions described in this section allow machines on the local network
353to be accessible in some degree to new incoming connections from the external
354network.
355Individual ports can be re-mapped or static network address translations can
356be designated.
357.Pp
358.Ft struct alias_link *
359.Fo PacketAliasRedirectPort
360.Fa "struct in_addr local_addr"
361.Fa "u_short local_port"
362.Fa "struct in_addr remote_addr"
363.Fa "u_short remote_port"
364.Fa "struct in_addr alias_addr"
365.Fa "u_short alias_port"
366.Fa "u_char proto"
367.Fc
368.Bd -ragged -offset indent
369This function specifies that traffic from a given remote address/port to
370an alias address/port be redirected to a specified local address/port.
371The parameter
372.Fa proto
373can be either
374.Dv IPPROTO_TCP
375or
376.Dv IPPROTO_UDP ,
377as defined in
44cb301e 378.In netinet/in.h .
984263bc
MD
379.Pp
380If
381.Fa local_addr
382or
383.Fa alias_addr
384is zero, this indicates that the packet aliasing address as established
385by
386.Fn PacketAliasSetAddress
387is to be used.
388Even if
389.Fn PacketAliasSetAddress
390is called to change the address after
391.Fn PacketAliasRedirectPort
392is called, a zero reference will track this change.
393.Pp
394If the link is further set up to operate for a load sharing, then
395.Fa local_addr
396and
397.Fa local_port
398are ignored, and are selected dynamically from the server pool, as described in
399.Fn PacketAliasAddServer
400below.
401.Pp
402If
403.Fa remote_addr
404is zero, this indicates to redirect packets from any remote address.
405Likewise, if
406.Fa remote_port
407is zero, this indicates to redirect packets originating from any remote
408port number.
409Almost always, the remote port specification will be zero, but non-zero
410remote addresses can sometimes be useful for firewalling.
411If two calls to
412.Fn PacketAliasRedirectPort
413overlap in their address/port specifications, then the most recent call
414will have precedence.
415.Pp
416This function returns a pointer which can subsequently be used by
417.Fn PacketAliasRedirectDelete .
418If
419.Dv NULL
420is returned, then the function call did not complete successfully.
421.Pp
422All port numbers should be in network address byte order, so it is necessary
423to use
424.Xr htons 3
425to convert these parameters from internally readable numbers to network byte
426order.
427Addresses are also in network byte order, which is implicit in the use of the
428.Fa struct in_addr
429data type.
430.Ed
431.Pp
432.Ft struct alias_link *
433.Fo PacketAliasRedirectAddr
434.Fa "struct in_addr local_addr"
435.Fa "struct in_addr alias_addr"
436.Fc
437.Bd -ragged -offset indent
438This function designates that all incoming traffic to
439.Fa alias_addr
440be redirected to
441.Fa local_addr .
442Similarly, all outgoing traffic from
443.Fa local_addr
444is aliased to
445.Fa alias_addr .
446.Pp
447If
448.Fa local_addr
449or
450.Fa alias_addr
451is zero, this indicates that the packet aliasing address as established by
452.Fn PacketAliasSetAddress
453is to be used.
454Even if
455.Fn PacketAliasSetAddress
456is called to change the address after
457.Fn PacketAliasRedirectAddr
458is called, a zero reference will track this change.
459.Pp
460If the link is further set up to operate for a load sharing, then
461.Fa local_addr
462is ignored, and is selected dynamically from the server pool, as described in
463.Fn PacketAliasAddServer
464below.
465.Pp
466If subsequent calls to
467.Fn PacketAliasRedirectAddr
468use the same aliasing address, all new incoming traffic to this aliasing
469address will be redirected to the local address made in the last function
470call.
471New traffic generated by any of the local machines, designated in the
472several function calls, will be aliased to the same address.
473Consider the following example:
474.Bd -literal -offset indent
475PacketAliasRedirectAddr(inet_aton("192.168.0.2"),
476 inet_aton("141.221.254.101"));
477PacketAliasRedirectAddr(inet_aton("192.168.0.3"),
478 inet_aton("141.221.254.101"));
479PacketAliasRedirectAddr(inet_aton("192.168.0.4"),
480 inet_aton("141.221.254.101"));
481.Ed
482.Pp
483Any outgoing connections such as
484.Xr telnet 1
485or
486.Xr ftp 1
487from 192.168.0.2, 192.168.0.3 and 192.168.0.4 will appear to come from
488141.221.254.101.
489Any incoming connections to 141.221.254.101 will be directed to 192.168.0.4.
490.Pp
491Any calls to
492.Fn PacketAliasRedirectPort
493will have precedence over address mappings designated by
494.Fn PacketAliasRedirectAddr .
495.Pp
496This function returns a pointer which can subsequently be used by
497.Fn PacketAliasRedirectDelete .
498If
499.Dv NULL
500is returned, then the function call did not complete successfully.
501.Ed
502.Pp
503.Ft int
504.Fo PacketAliasAddServer
505.Fa "struct alias_link *link"
506.Fa "struct in_addr addr"
507.Fa "u_short port"
508.Fc
509.Bd -ragged -offset indent
510This function sets the
511.Fa link
512up for Load Sharing using IP Network Address Translation (RFC 2391, LSNAT).
513LSNAT operates as follows.
514A client attempts to access a server by using the server virtual address.
515The LSNAT router transparently redirects the request to one of the hosts
516in server pool, selected using a real-time load sharing algorithm.
517Multiple sessions may be initiated from the same client, and each session
518could be directed to a different host based on load balance across server
519pool hosts at the time.
520If load share is desired for just a few specific services, the configuration
521on LSNAT could be defined to restrict load share for just the services
522desired.
523.Pp
524Currently, only the simplest selection algorithm is implemented, where a
525host is selected on a round-robin basis only, without regard to load on
526the host.
527.Pp
528First, the
529.Fa link
530is created by either
531.Fn PacketAliasRedirectPort
532or
533.Fn PacketAliasRedirectAddr .
534Then,
535.Fn PacketAliasAddServer
536is called multiple times to add entries to the
537.Fa link Ns 's
538server pool.
539.Pp
540For links created with
541.Fn PacketAliasRedirectAddr ,
542the
543.Fa port
544argument is ignored and could have any value, e.g. htons(~0).
545.Pp
546This function returns 0 on success, -1 otherwise.
547.Ed
548.Pp
549.Ft void
550.Fn PacketAliasRedirectDelete "struct alias_link *link"
551.Bd -ragged -offset indent
552This function will delete a specific static redirect rule entered by
553.Fn PacketAliasRedirectPort
554or
555.Fn PacketAliasRedirectAddr .
556The parameter
557.Fa link
558is the pointer returned by either of the redirection functions.
559If an invalid pointer is passed to
560.Fn PacketAliasRedirectDelete ,
561then a program crash or unpredictable operation could result, so it is
562necessary to be careful using this function.
563.Ed
564.Pp
565.Ft int
566.Fn PacketAliasProxyRule "const char *cmd"
567.Bd -ragged -offset indent
568The passed
569.Fa cmd
570string consists of one or more pairs of words.
571The first word in each pair is a token and the second is the value that
572should be applied for that token.
573Tokens and their argument types are as follows:
574.Bl -tag -width indent
575.It Cm type encode_ip_hdr | encode_tcp_stream | no_encode
576In order to support transparent proxying, it is necessary to somehow
577pass the original address and port information into the new destination
578server.
579If
580.Cm encode_ip_hdr
581is specified, the original address and port is passed as an extra IP
582option.
583If
584.Cm encode_tcp_stream
585is specified, the original address and port is passed as the first
586piece of data in the TCP stream in the format
587.Dq DEST Ar IP port .
588.It Cm port Ar portnum
589Only packets with the destination port
590.Ar portnum
591are proxied.
592.It Cm server Ar host Ns Xo
593.Op : Ns Ar portnum
594.Xc
595This specifies the
596.Ar host
597and
598.Ar portnum
599that the data is to be redirected to.
600.Ar host
601must be an IP address rather than a DNS host name.
602If
603.Ar portnum
604is not specified, the destination port number is not changed.
605.Pp
606The
607.Ar server
608specification is mandatory unless the
609.Cm delete
610command is being used.
611.It Cm rule Ar index
612Normally, each call to
613.Fn PacketAliasProxyRule
614inserts the next rule at the start of a linear list of rules.
615If an
616.Ar index
617is specified, the new rule will be checked after all rules with lower
618indices.
619Calls to
620.Fn PacketAliasProxyRule
621that do not specify a rule are assigned rule 0.
622.It Cm delete Ar index
623This token and its argument MUST NOT be used with any other tokens.
624When used, all existing rules with the given
625.Ar index
626are deleted.
627.It Cm proto tcp | udp
628If specified, only packets of the given protocol type are matched.
629.It Cm src Ar IP Ns Xo
630.Op / Ns Ar bits
631.Xc
632If specified, only packets with a source address matching the given
633.Ar IP
634are matched.
635If
636.Ar bits
637is also specified, then the first
638.Ar bits
639bits of
640.Ar IP
641are taken as a network specification, and all IP addresses from that
642network will be matched.
643.It Cm dst Ar IP Ns Xo
644.Op / Ns Ar bits
645.Xc
646If specified, only packets with a destination address matching the given
647.Ar IP
648are matched.
649If
650.Ar bits
651is also specified, then the first
652.Ar bits
653bits of
654.Ar IP
655are taken as a network specification, and all IP addresses from that
656network will be matched.
657.El
658.Pp
659This function is usually used to redirect outgoing connections for
660internal machines that are not permitted certain types of internet
661access, or to restrict access to certain external machines.
662.Ed
663.Pp
664.Ft struct alias_link *
665.Fo PacketAliasRedirectProto
666.Fa "struct in_addr local_addr"
667.Fa "struct in_addr remote_addr"
668.Fa "struct in_addr alias_addr"
669.Fa "u_char proto"
670.Fc
671.Bd -ragged -offset indent
672This function specifies that any IP packet with protocol number of
673.Fa proto
674from a given remote address to an alias address be
675redirected to a specified local address.
676.Pp
677If
678.Fa local_addr
679or
680.Fa alias_addr
681is zero, this indicates that the packet aliasing address as established
682by
683.Fn PacketAliasSetAddress
684is to be used.
685Even if
686.Fn PacketAliasSetAddress
687is called to change the address after
688.Fn PacketAliasRedirectProto
689is called, a zero reference will track this change.
690.Pp
691If
692.Fa remote_addr
693is zero, this indicates to redirect packets from any remote address.
694Non-zero remote addresses can sometimes be useful for firewalling.
695.Pp
696If two calls to
697.Fn PacketAliasRedirectProto
698overlap in their address specifications, then the most recent call
699will have precedence.
700.Pp
701This function returns a pointer which can subsequently be used by
702.Fn PacketAliasRedirectDelete .
703If
704.Dv NULL
705is returned, then the function call did not complete successfully.
706.Ed
707.Sh FRAGMENT HANDLING
708The functions in this section are used to deal with incoming fragments.
709.Pp
710Outgoing fragments are handled within
711.Fn PacketAliasOut
712by changing the address according to any applicable mapping set by
713.Fn PacketAliasRedirectAddr ,
714or the default aliasing address set by
715.Fn PacketAliasSetAddress .
716.Pp
717Incoming fragments are handled in one of two ways.
718If the header of a fragmented IP packet has already been seen, then all
719subsequent fragments will be re-mapped in the same manner the header
720fragment was.
721Fragments which arrive before the header are saved and then retrieved
722once the header fragment has been resolved.
723.Pp
724.Ft int
725.Fn PacketAliasSaveFragment "char *ptr"
726.Bd -ragged -offset indent
727When
728.Fn PacketAliasIn
729returns
730.Dv PKT_ALIAS_UNRESOLVED_FRAGMENT ,
731this function can be used to save the pointer to the unresolved fragment.
732.Pp
733It is implicitly assumed that
734.Fa ptr
735points to a block of memory allocated by
736.Xr malloc 3 .
737If the fragment is never resolved, the packet aliasing engine will
738automatically free the memory after a timeout period.
739[Eventually this function should be modified so that a callback function
740for freeing memory is passed as an argument.]
741.Pp
742This function returns
743.Dv PKT_ALIAS_OK
744if it was successful and
745.Dv PKT_ALIAS_ERROR
746if there was an error.
747.Ed
748.Pp
749.Ft char *
750.Fn PacketAliasGetFragment "char *buffer"
751.Bd -ragged -offset indent
752This function can be used to retrieve fragment pointers saved by
753.Fn PacketAliasSaveFragment .
754The IP header fragment pointed to by
755.Fa buffer
756is the header fragment indicated when
757.Fn PacketAliasIn
758returns
759.Dv PKT_ALIAS_FOUND_HEADER_FRAGMENT .
760Once a fragment pointer is retrieved, it becomes the calling program's
761responsibility to free the dynamically allocated memory for the fragment.
762.Pp
763.Fn PacketAliasGetFragment
764can be called sequentially until there are no more fragments available,
765at which time it returns
766.Dv NULL .
767.Ed
768.Pp
769.Ft void
770.Fn PacketAliasFragmentIn "char *header" "char *fragment"
771.Bd -ragged -offset indent
772When a fragment is retrieved with
773.Fn PacketAliasGetFragment ,
774it can then be de-aliased with a call to
775.Fn PacketAliasFragmentIn .
776The
777.Fa header
778argument is the pointer to a header fragment used as a template, and
779.Fa fragment
780is the pointer to the packet to be de-aliased.
781.Ed
782.Sh MISCELLANEOUS FUNCTIONS
783.Ft void
784.Fn PacketAliasSetTarget "struct in_addr addr"
785.Bd -ragged -offset indent
786When an incoming packet not associated with any pre-existing aliasing link
787arrives at the host machine, it will be sent to the address indicated by a
788call to
789.Fn PacketAliasSetTarget .
790.Pp
791If this function is called with an
792.Dv INADDR_NONE
793address argument, then all new incoming packets go to the address set by
794.Fn PacketAliasSetAddress .
795.Pp
796If this function is not called, or is called with an
797.Dv INADDR_ANY
798address argument, then all new incoming packets go to the address specified
799in the packet.
800This allows external machines to talk directly to internal machines if they
801can route packets to the machine in question.
802.Ed
803.Pp
804.Ft int
805.Fn PacketAliasCheckNewLink void
806.Bd -ragged -offset indent
807This function returns a non-zero value when a new aliasing link is created.
808In circumstances where incoming traffic is being sequentially sent to
809different local servers, this function can be used to trigger when
810.Fn PacketAliasSetTarget
811is called to change the default target address.
812.Ed
813.Pp
814.Ft u_short
815.Fn PacketAliasInternetChecksum "u_short *buffer" "int nbytes"
816.Bd -ragged -offset indent
817This is a utility function that does not seem to be available elsewhere and
818is included as a convenience.
819It computes the internet checksum, which is used in both IP and
820protocol-specific headers (TCP, UDP, ICMP).
821.Pp
822The
823.Fa buffer
824argument points to the data block to be checksummed, and
825.Fa nbytes
826is the number of bytes.
827The 16-bit checksum field should be zeroed before computing the checksum.
828.Pp
829Checksums can also be verified by operating on a block of data including
830its checksum.
831If the checksum is valid,
832.Fn PacketAliasInternetChecksum
833will return zero.
834.Ed
835.Pp
836.Ft int
837.Fn PacketUnaliasOut "char *buffer" "int maxpacketsize"
838.Bd -ragged -offset indent
839An outgoing packet, which has already been aliased,
840has its private address/port information restored by this function.
841The IP packet is pointed to by
842.Fa buffer ,
843and
844.Fa maxpacketsize
845is provided for error checking purposes.
846This function can be used if an already-aliased packet needs to have its
847original IP header restored for further processing (eg. logging).
848.Ed
984263bc
MD
849.Sh AUTHORS
850.An Charles Mott Aq cm@linktel.net ,
851versions 1.0 - 1.8, 2.0 - 2.4.
852.An Eivind Eklund Aq eivind@FreeBSD.org ,
853versions 1.8b, 1.9 and 2.5.
854Added IRC DCC support as well as contributing a number of architectural
855improvements; added the firewall bypass for FTP/IRC DCC.
856.An Erik Salander Aq erik@whistle.com
857added support for PPTP and RTSP.
858.An Junichi Satoh Aq junichi@junichi.org
859added support for RTSP/PNA.
860.Sh ACKNOWLEDGMENTS
861Listed below, in approximate chronological order, are individuals who
862have provided valuable comments and/or debugging assistance.
863.Pp
864.Bd -ragged -offset indent
865.An -split
866.An Gary Roberts
867.An Tom Torrance
868.An Reto Burkhalter
869.An Martin Renters
870.An Brian Somers
871.An Paul Traina
872.An Ari Suutari
873.An Dave Remien
874.An J. Fortes
875.An Andrzej Bialecki
876.An Gordon Burditt
877.Ed
878.Sh CONCEPTUAL BACKGROUND
879This section is intended for those who are planning to modify the source
880code or want to create somewhat esoteric applications using the packet
881aliasing functions.
882.Pp
883The conceptual framework under which the packet aliasing engine operates
884is described here.
885Central to the discussion is the idea of an
886.Em aliasing link
887which describes the relationship for a given packet transaction between
888the local machine, aliased identity and remote machine.
889It is discussed how such links come into existence and are destroyed.
890.Ss ALIASING LINKS
891There is a notion of an
892.Em aliasing link ,
893which is a 7-tuple describing a specific translation:
894.Bd -literal -offset indent
895(local addr, local port, alias addr, alias port,
896 remote addr, remote port, protocol)
897.Ed
898.Pp
899Outgoing packets have the local address and port number replaced with the
900alias address and port number.
901Incoming packets undergo the reverse process.
902The packet aliasing engine attempts to match packets against an internal
903table of aliasing links to determine how to modify a given IP packet.
904Both the IP header and protocol dependent headers are modified as necessary.
905Aliasing links are created and deleted as necessary according to network
906traffic.
907.Pp
908Protocols can be TCP, UDP or even ICMP in certain circumstances.
909(Some types of ICMP packets can be aliased according to sequence or ID
910number which acts as an equivalent port number for identifying how
911individual packets should be handled.)
912.Pp
913Each aliasing link must have a unique combination of the following five
914quantities: alias address/port, remote address/port and protocol.
915This ensures that several machines on a local network can share the
916same aliasing IP address.
917In cases where conflicts might arise, the aliasing port is chosen so that
918uniqueness is maintained.
919.Ss STATIC AND DYNAMIC LINKS
920Aliasing links can either be static or dynamic.
921Static links persist indefinitely and represent fixed rules for translating
922IP packets.
923Dynamic links come into existence for a specific TCP connection or UDP
924transaction or ICMP ECHO sequence.
925For the case of TCP, the connection can be monitored to see when the
926associated aliasing link should be deleted.
927Aliasing links for UDP transactions (and ICMP ECHO and TIMESTAMP requests)
928work on a simple timeout rule.
929When no activity is observed on a dynamic link for a certain amount of time
930it is automatically deleted.
931Timeout rules also apply to TCP connections which do not open or close
932properly.
933.Ss PARTIALLY SPECIFIED ALIASING LINKS
934Aliasing links can be partially specified, meaning that the remote address
935and/or remote port are unknown.
936In this case, when a packet matching the incomplete specification is found,
937a fully specified dynamic link is created.
938If the original partially specified link is dynamic, it will be deleted
939after the fully specified link is created, otherwise it will persist.
940.Pp
941For instance, a partially specified link might be
942.Bd -literal -offset indent
943(192.168.0.4, 23, 204.228.203.215, 8066, 0, 0, tcp)
944.Ed
945.Pp
946The zeros denote unspecified components for the remote address and port.
947If this link were static it would have the effect of redirecting all
948incoming traffic from port 8066 of 204.228.203.215 to port 23 (telnet)
949of machine 192.168.0.4 on the local network.
950Each individual telnet connection would initiate the creation of a distinct
951dynamic link.
952.Ss DYNAMIC LINK CREATION
953In addition to aliasing links, there are also address mappings that can be
954stored within the internal data table of the packet aliasing mechanism.
955.Bd -literal -offset indent
956(local addr, alias addr)
957.Ed
958.Pp
959Address mappings are searched when creating new dynamic links.
960.Pp
961All outgoing packets from the local network automatically create a dynamic
962link if they do not match an already existing fully specified link.
963If an address mapping exists for the outgoing packet, this determines
964the alias address to be used.
965If no mapping exists, then a default address, usually the address of the
966packet aliasing host, is used.
967If necessary, this default address can be changed as often as each individual
968packet arrives.
969.Pp
970The aliasing port number is determined such that the new dynamic link does
971not conflict with any existing links.
972In the default operating mode, the packet aliasing engine attempts to set
973the aliasing port equal to the local port number.
974If this results in a conflict, then port numbers are randomly chosen until
975a unique aliasing link can be established.
976In an alternate operating mode, the first choice of an aliasing port is also
977random and unrelated to the local port number.
0b84df5c
SW
978.Sh BUGS
979PPTP aliasing does not work when more than one internal client
980connects to the same external server at the same time, because
981PPTP requires a single TCP control connection to be established
982between any two IP addresses.