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source: vbox/trunk/src/VBox/Devices/Network/slirp/libalias/libalias.3@ 103068

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OSE fixes for NAT

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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/sys/netinet/libalias/libalias.3,v 1.58.8.1 2009/04/15 03:14:26 kensmith Exp $
27.\"
28.Dd October 1, 2006
29.Dt LIBALIAS 3
30.Os
31.Sh NAME
32.Nm libalias
33.Nd packet aliasing library for masquerading and network address translation
34.Sh SYNOPSIS
35.In sys/types.h
36.In netinet/in.h
37.In alias.h
38.Pp
39Function prototypes are given in the main body of the text.
40.Sh DESCRIPTION
41The
42.Nm
43library is a collection of functions for aliasing and de-aliasing of IP
44packets, intended for masquerading and network address translation (NAT).
45.Sh INTRODUCTION
46This library is a moderately portable set of functions designed to assist
47in the process of IP masquerading and network address translation.
48Outgoing packets from a local network with unregistered IP addresses can
49be aliased to appear as if they came from an accessible IP address.
50Incoming packets are then de-aliased so that they are sent to the correct
51machine on the local network.
52.Pp
53A certain amount of flexibility is built into the packet aliasing engine.
54In the simplest mode of operation, a many-to-one address mapping takes
55place between local network and the packet aliasing host.
56This is known as IP masquerading.
57In addition, one-to-one mappings between local and public addresses can
58also be implemented, which is known as static NAT.
59In between these extremes, different groups of private addresses can be
60linked to different public addresses, comprising several distinct
61many-to-one mappings.
62Also, a given public address and port can be statically redirected to a
63private address/port.
64.Pp
65The packet aliasing engine was designed to operate in user space outside
66of the kernel, without any access to private kernel data structure, but
67the source code can also be ported to a kernel environment.
68.Sh INITIALIZATION AND CONTROL
69One special function,
70.Fn LibAliasInit ,
71must always be called before any packet handling may be performed and
72the returned instance pointer passed to all the other functions.
73Normally, the
74.Fn LibAliasSetAddress
75function is called afterwards, to set the default aliasing address.
76In addition, the operating mode of the packet aliasing engine can be
77customized by calling
78.Fn LibAliasSetMode .
79.Pp
80.Ft "struct libalias *"
81.Fn LibAliasInit "struct libalias *"
82.Bd -ragged -offset indent
83This function is used to initialize
84internal data structures.
85When called the first time, a
86.Dv NULL
87pointer should be passed as an argument.
88The following mode bits are always set after calling
89.Fn LibAliasInit .
90See the description of
91.Fn LibAliasSetMode
92below for the meaning of these mode bits.
93.Pp
94.Bl -item -offset indent -compact
95.It
96.Dv PKT_ALIAS_SAME_PORTS
97.It
98.Dv PKT_ALIAS_USE_SOCKETS
99.It
100.Dv PKT_ALIAS_RESET_ON_ADDR_CHANGE
101.El
102.Pp
103This function will always return the packet aliasing engine to the same
104initial state.
105The
106.Fn LibAliasSetAddress
107function is normally called afterwards, and any desired changes from the
108default mode bits listed above require a call to
109.Fn LibAliasSetMode .
110.Pp
111It is mandatory that this function be called at the beginning of a program
112prior to any packet handling.
113.Ed
114.Pp
115.Ft void
116.Fn LibAliasUninit "struct libalias *"
117.Bd -ragged -offset indent
118This function has no return value and is used to clear any
119resources attached to internal data structures.
120.Pp
121This functions should be called when a program stops using the aliasing
122engine; it does, amongst other things, clear out any firewall holes.
123To provide backwards compatibility and extra security, it is added to
124the
125.Xr atexit 3
126chain by
127.Fn LibAliasInit .
128.Ed
129.Pp
130.Ft void
131.Fn LibAliasSetAddress "struct libalias *" "struct in_addr addr"
132.Bd -ragged -offset indent
133This function sets the source address to which outgoing packets from the
134local area network are aliased.
135All outgoing packets are re-mapped to this address unless overridden by a
136static address mapping established by
137.Fn LibAliasRedirectAddr .
138If this function is not called, and no static rules match, an outgoing
139packet retains its source address.
140.Pp
141If the
142.Dv PKT_ALIAS_RESET_ON_ADDR_CHANGE
143mode bit is set (the default mode of operation), then the internal aliasing
144link tables will be reset any time the aliasing address changes.
145This is useful for interfaces such as
146.Xr ppp 8 ,
147where the IP
148address may or may not change on successive dial-up attempts.
149.Pp
150If the
151.Dv PKT_ALIAS_RESET_ON_ADDR_CHANGE
152mode bit is set to zero, this function can also be used to dynamically change
153the aliasing address on a packet to packet basis (it is a low overhead call).
154.Pp
155It is mandatory that this function be called prior to any packet handling.
156.Ed
157.Pp
158.Ft unsigned int
159.Fn LibAliasSetMode "struct libalias *" "unsigned int flags" "unsigned int mask"
160.Bd -ragged -offset indent
161This function sets or clears mode bits
162according to the value of
163.Fa flags .
164Only bits marked in
165.Fa mask
166are affected.
167The following mode bits are defined in
168.In alias.h :
169.Bl -tag -width indent
170.It Dv PKT_ALIAS_LOG
171Enables logging into
172.Pa /var/log/alias.log .
173Each time an aliasing link is created or deleted, the log file is appended
174with the current number of ICMP, TCP and UDP links.
175Mainly useful for debugging when the log file is viewed continuously with
176.Xr tail 1 .
177.It Dv PKT_ALIAS_DENY_INCOMING
178If this mode bit is set, all incoming packets associated with new TCP
179connections or new UDP transactions will be marked for being ignored
180.Fn ( LibAliasIn
181returns
182.Dv PKT_ALIAS_IGNORED
183code)
184by the calling program.
185Response packets to connections or transactions initiated from the packet
186aliasing host or local network will be unaffected.
187This mode bit is useful for implementing a one-way firewall.
188.It Dv PKT_ALIAS_SAME_PORTS
189If this mode bit is set, the packet aliasing engine will attempt to leave
190the alias port numbers unchanged from the actual local port numbers.
191This can be done as long as the quintuple (proto, alias addr, alias port,
192remote addr, remote port) is unique.
193If a conflict exists, a new aliasing port number is chosen even if this
194mode bit is set.
195.It Dv PKT_ALIAS_USE_SOCKETS
196This bit should be set when the packet aliasing host originates network
197traffic as well as forwards it.
198When the packet aliasing host is waiting for a connection from an unknown
199host address or unknown port number (e.g.\& an FTP data connection), this
200mode bit specifies that a socket be allocated as a place holder to prevent
201port conflicts.
202Once a connection is established, usually within a minute or so, the socket
203is closed.
204.It Dv PKT_ALIAS_UNREGISTERED_ONLY
205If this mode bit is set, traffic on the local network which does not
206originate from unregistered address spaces will be ignored.
207Standard Class A, B and C unregistered addresses are:
208.Bd -literal -offset indent
20910.0.0.0 -> 10.255.255.255 (Class A subnet)
210172.16.0.0 -> 172.31.255.255 (Class B subnets)
211192.168.0.0 -> 192.168.255.255 (Class C subnets)
212.Ed
213.Pp
214This option is useful in the case that packet aliasing host has both
215registered and unregistered subnets on different interfaces.
216The registered subnet is fully accessible to the outside world, so traffic
217from it does not need to be passed through the packet aliasing engine.
218.It Dv PKT_ALIAS_RESET_ON_ADDR_CHANGE
219When this mode bit is set and
220.Fn LibAliasSetAddress
221is called to change the aliasing address, the internal link table of the
222packet aliasing engine will be cleared.
223This operating mode is useful for
224.Xr ppp 8
225links where the interface address can sometimes change or remain the same
226between dial-up attempts.
227If this mode bit is not set, the link table will never be reset in the event
228of an address change.
229.It Dv PKT_ALIAS_PUNCH_FW
230This option makes
231.Nm
232`punch holes' in an
233.Xr ipfirewall 4
234based firewall for FTP/IRC DCC connections.
235The holes punched are bound by from/to IP address and port; it will not be
236possible to use a hole for another connection.
237A hole is removed when the connection that uses it dies.
238To cater to unexpected death of a program using
239.Nm
240(e.g.\& kill -9),
241changing the state of the flag will clear the entire firewall range
242allocated for holes.
243This will also happen on the initial call to
244.Fn LibAliasSetFWBase .
245This call must happen prior to setting this flag.
246.It Dv PKT_ALIAS_REVERSE
247This option makes
248.Nm
249reverse the way it handles incoming and outgoing packets, allowing it
250to be fed with data that passes through the internal interface rather
251than the external one.
252.It Dv PKT_ALIAS_PROXY_ONLY
253This option tells
254.Nm
255to obey transparent proxy rules only.
256Normal packet aliasing is not performed.
257See
258.Fn LibAliasProxyRule
259below for details.
260.El
261.Ed
262.Pp
263.Ft void
264.Fn LibAliasSetFWBase "struct libalias *" "unsigned int base" "unsigned int num"
265.Bd -ragged -offset indent
266Set firewall range allocated for punching firewall holes (with the
267.Dv PKT_ALIAS_PUNCH_FW
268flag).
269The range will be cleared for all rules on initialization.
270.Ed
271.Pp
272.Ft void
273.Fn LibAliasSkinnyPort "struct libalias *" "unsigned int port"
274.Bd -ragged -offset indent
275Set the TCP port used by the Skinny Station protocol.
276Skinny is used by Cisco IP phones to communicate with
277Cisco Call Managers to set up voice over IP calls.
278If this is not set, Skinny aliasing will not be done.
279The typical port used by Skinny is 2000.
280.Ed
281.Sh PACKET HANDLING
282The packet handling functions are used to modify incoming (remote to local)
283and outgoing (local to remote) packets.
284The calling program is responsible for receiving and sending packets via
285network interfaces.
286.Pp
287Along with
288.Fn LibAliasInit
289and
290.Fn LibAliasSetAddress ,
291the two packet handling functions,
292.Fn LibAliasIn
293and
294.Fn LibAliasOut ,
295comprise minimal set of functions needed for a basic IP masquerading
296implementation.
297.Pp
298.Ft int
299.Fn LibAliasIn "struct libalias *" "char *buffer" "int maxpacketsize"
300.Bd -ragged -offset indent
301An incoming packet coming from a remote machine to the local network is
302de-aliased by this function.
303The IP packet is pointed to by
304.Fa buffer ,
305and
306.Fa maxpacketsize
307indicates the size of the data structure containing the packet and should
308be at least as large as the actual packet size.
309.Pp
310Return codes:
311.Bl -tag -width indent
312.It Dv PKT_ALIAS_OK
313The packet aliasing process was successful.
314.It Dv PKT_ALIAS_IGNORED
315The packet was ignored and not de-aliased.
316This can happen if the protocol is unrecognized, possibly an ICMP message
317type is not handled or if incoming packets for new connections are being
318ignored (if
319.Dv PKT_ALIAS_DENY_INCOMING
320mode bit was set by
321.Fn LibAliasSetMode ) .
322.It Dv PKT_ALIAS_UNRESOLVED_FRAGMENT
323This is returned when a fragment cannot be resolved because the header
324fragment has not been sent yet.
325In this situation, fragments must be saved with
326.Fn LibAliasSaveFragment
327until a header fragment is found.
328.It Dv PKT_ALIAS_FOUND_HEADER_FRAGMENT
329The packet aliasing process was successful, and a header fragment was found.
330This is a signal to retrieve any unresolved fragments with
331.Fn LibAliasGetFragment
332and de-alias them with
333.Fn LibAliasFragmentIn .
334.It Dv PKT_ALIAS_ERROR
335An internal error within the packet aliasing engine occurred.
336.El
337.Ed
338.Pp
339.Ft int
340.Fn LibAliasOut "struct libalias *" "char *buffer" "int maxpacketsize"
341.Bd -ragged -offset indent
342An outgoing packet coming from the local network to a remote machine is
343aliased by this function.
344The IP packet is pointed to by
345.Fa buffer ,
346and
347.Fa maxpacketsize
348indicates the maximum packet size permissible should the packet length be
349changed.
350IP encoding protocols place address and port information in the encapsulated
351data stream which has to be modified and can account for changes in packet
352length.
353Well known examples of such protocols are FTP and IRC DCC.
354.Pp
355Return codes:
356.Bl -tag -width indent
357.It Dv PKT_ALIAS_OK
358The packet aliasing process was successful.
359.It Dv PKT_ALIAS_IGNORED
360The packet was ignored and not aliased.
361This can happen if the protocol is unrecognized, or possibly an ICMP message
362type is not handled.
363.It Dv PKT_ALIAS_ERROR
364An internal error within the packet aliasing engine occurred.
365.El
366.Ed
367.Sh PORT AND ADDRESS REDIRECTION
368The functions described in this section allow machines on the local network
369to be accessible in some degree to new incoming connections from the external
370network.
371Individual ports can be re-mapped or static network address translations can
372be designated.
373.Pp
374.Ft struct alias_link *
375.Fo LibAliasRedirectPort
376.Fa "struct libalias *"
377.Fa "struct in_addr local_addr"
378.Fa "u_short local_port"
379.Fa "struct in_addr remote_addr"
380.Fa "u_short remote_port"
381.Fa "struct in_addr alias_addr"
382.Fa "u_short alias_port"
383.Fa "u_char proto"
384.Fc
385.Bd -ragged -offset indent
386This function specifies that traffic from a given remote address/port to
387an alias address/port be redirected to a specified local address/port.
388The parameter
389.Fa proto
390can be either
391.Dv IPPROTO_TCP
392or
393.Dv IPPROTO_UDP ,
394as defined in
395.In netinet/in.h .
396.Pp
397If
398.Fa local_addr
399or
400.Fa alias_addr
401is zero, this indicates that the packet aliasing address as established
402by
403.Fn LibAliasSetAddress
404is to be used.
405Even if
406.Fn LibAliasSetAddress
407is called to change the address after
408.Fn LibAliasRedirectPort
409is called, a zero reference will track this change.
410.Pp
411If the link is further set up to operate for a load sharing, then
412.Fa local_addr
413and
414.Fa local_port
415are ignored, and are selected dynamically from the server pool, as described in
416.Fn LibAliasAddServer
417below.
418.Pp
419If
420.Fa remote_addr
421is zero, this indicates to redirect packets from any remote address.
422Likewise, if
423.Fa remote_port
424is zero, this indicates to redirect packets originating from any remote
425port number.
426Almost always, the remote port specification will be zero, but non-zero
427remote addresses can sometimes be useful for firewalling.
428If two calls to
429.Fn LibAliasRedirectPort
430overlap in their address/port specifications, then the most recent call
431will have precedence.
432.Pp
433This function returns a pointer which can subsequently be used by
434.Fn LibAliasRedirectDelete .
435If
436.Dv NULL
437is returned, then the function call did not complete successfully.
438.Pp
439All port numbers should be in network address byte order, so it is necessary
440to use
441.Xr htons 3
442to convert these parameters from internally readable numbers to network byte
443order.
444Addresses are also in network byte order, which is implicit in the use of the
445.Fa struct in_addr
446data type.
447.Ed
448.Pp
449.Ft struct alias_link *
450.Fo LibAliasRedirectAddr
451.Fa "struct libalias *"
452.Fa "struct in_addr local_addr"
453.Fa "struct in_addr alias_addr"
454.Fc
455.Bd -ragged -offset indent
456This function designates that all incoming traffic to
457.Fa alias_addr
458be redirected to
459.Fa local_addr .
460Similarly, all outgoing traffic from
461.Fa local_addr
462is aliased to
463.Fa alias_addr .
464.Pp
465If
466.Fa local_addr
467or
468.Fa alias_addr
469is zero, this indicates that the packet aliasing address as established by
470.Fn LibAliasSetAddress
471is to be used.
472Even if
473.Fn LibAliasSetAddress
474is called to change the address after
475.Fn LibAliasRedirectAddr
476is called, a zero reference will track this change.
477.Pp
478If the link is further set up to operate for a load sharing, then
479.Fa local_addr
480is ignored, and is selected dynamically from the server pool, as described in
481.Fn LibAliasAddServer
482below.
483.Pp
484If subsequent calls to
485.Fn LibAliasRedirectAddr
486use the same aliasing address, all new incoming traffic to this aliasing
487address will be redirected to the local address made in the last function
488call.
489New traffic generated by any of the local machines, designated in the
490several function calls, will be aliased to the same address.
491Consider the following example:
492.Bd -literal -offset indent
493LibAliasRedirectAddr(la, inet_aton("192.168.0.2"),
494 inet_aton("141.221.254.101"));
495LibAliasRedirectAddr(la, inet_aton("192.168.0.3"),
496 inet_aton("141.221.254.101"));
497LibAliasRedirectAddr(la, inet_aton("192.168.0.4"),
498 inet_aton("141.221.254.101"));
499.Ed
500.Pp
501Any outgoing connections such as
502.Xr telnet 1
503or
504.Xr ftp 1
505from 192.168.0.2, 192.168.0.3 and 192.168.0.4 will appear to come from
506141.221.254.101.
507Any incoming connections to 141.221.254.101 will be directed to 192.168.0.4.
508.Pp
509Any calls to
510.Fn LibAliasRedirectPort
511will have precedence over address mappings designated by
512.Fn LibAliasRedirectAddr .
513.Pp
514This function returns a pointer which can subsequently be used by
515.Fn LibAliasRedirectDelete .
516If
517.Dv NULL
518is returned, then the function call did not complete successfully.
519.Ed
520.Pp
521.Ft int
522.Fo LibAliasAddServer
523.Fa "struct libalias *"
524.Fa "struct alias_link *link"
525.Fa "struct in_addr addr"
526.Fa "u_short port"
527.Fc
528.Bd -ragged -offset indent
529This function sets the
530.Fa link
531up for Load Sharing using IP Network Address Translation (RFC 2391, LSNAT).
532LSNAT operates as follows.
533A client attempts to access a server by using the server virtual address.
534The LSNAT router transparently redirects the request to one of the hosts
535in server pool, selected using a real-time load sharing algorithm.
536Multiple sessions may be initiated from the same client, and each session
537could be directed to a different host based on load balance across server
538pool hosts at the time.
539If load share is desired for just a few specific services, the configuration
540on LSNAT could be defined to restrict load share for just the services
541desired.
542.Pp
543Currently, only the simplest selection algorithm is implemented, where a
544host is selected on a round-robin basis only, without regard to load on
545the host.
546.Pp
547First, the
548.Fa link
549is created by either
550.Fn LibAliasRedirectPort
551or
552.Fn LibAliasRedirectAddr .
553Then,
554.Fn LibAliasAddServer
555is called multiple times to add entries to the
556.Fa link Ns 's
557server pool.
558.Pp
559For links created with
560.Fn LibAliasRedirectAddr ,
561the
562.Fa port
563argument is ignored and could have any value, e.g.\& htons(~0).
564.Pp
565This function returns 0 on success, \-1 otherwise.
566.Ed
567.Pp
568.Ft int
569.Fn LibAliasRedirectDynamic "struct libalias *" "struct alias_link *link"
570.Bd -ragged -offset indent
571This function marks the specified static redirect rule entered by
572.Fn LibAliasRedirectPort
573as dynamic.
574This can be used to e.g.\& dynamically redirect a single TCP connection,
575after which the rule is removed.
576Only fully specified links can be made dynamic.
577(See the
578.Sx STATIC AND DYNAMIC LINKS
579and
580.Sx PARTIALLY SPECIFIED ALIASING LINKS
581sections below for a definition of static vs.\& dynamic,
582and partially vs.\& fully specified links.)
583.Pp
584This function returns 0 on success, \-1 otherwise.
585.Ed
586.Pp
587.Ft void
588.Fn LibAliasRedirectDelete "struct libalias *" "struct alias_link *link"
589.Bd -ragged -offset indent
590This function will delete a specific static redirect rule entered by
591.Fn LibAliasRedirectPort
592or
593.Fn LibAliasRedirectAddr .
594The parameter
595.Fa link
596is the pointer returned by either of the redirection functions.
597If an invalid pointer is passed to
598.Fn LibAliasRedirectDelete ,
599then a program crash or unpredictable operation could result, so it is
600necessary to be careful using this function.
601.Ed
602.Pp
603.Ft int
604.Fn LibAliasProxyRule "struct libalias *" "const char *cmd"
605.Bd -ragged -offset indent
606The passed
607.Fa cmd
608string consists of one or more pairs of words.
609The first word in each pair is a token and the second is the value that
610should be applied for that token.
611Tokens and their argument types are as follows:
612.Bl -tag -width indent
613.It Cm type encode_ip_hdr | encode_tcp_stream | no_encode
614In order to support transparent proxying, it is necessary to somehow
615pass the original address and port information into the new destination
616server.
617If
618.Cm encode_ip_hdr
619is specified, the original destination address and port are passed
620as an extra IP option.
621If
622.Cm encode_tcp_stream
623is specified, the original destination address and port are passed
624as the first piece of data in the TCP stream in the format
625.Dq Li DEST Ar IP port .
626.It Cm port Ar portnum
627Only packets with the destination port
628.Ar portnum
629are proxied.
630.It Cm server Ar host Ns Op : Ns Ar portnum
631This specifies the
632.Ar host
633and
634.Ar portnum
635that the data is to be redirected to.
636.Ar host
637must be an IP address rather than a DNS host name.
638If
639.Ar portnum
640is not specified, the destination port number is not changed.
641.Pp
642The
643.Ar server
644specification is mandatory unless the
645.Cm delete
646command is being used.
647.It Cm rule Ar index
648Normally, each call to
649.Fn LibAliasProxyRule
650inserts the next rule at the start of a linear list of rules.
651If an
652.Ar index
653is specified, the new rule will be checked after all rules with lower
654indices.
655Calls to
656.Fn LibAliasProxyRule
657that do not specify a rule are assigned rule 0.
658.It Cm delete Ar index
659This token and its argument MUST NOT be used with any other tokens.
660When used, all existing rules with the given
661.Ar index
662are deleted.
663.It Cm proto tcp | udp
664If specified, only packets of the given protocol type are matched.
665.It Cm src Ar IP Ns Op / Ns Ar bits
666If specified, only packets with a source address matching the given
667.Ar IP
668are matched.
669If
670.Ar bits
671is also specified, then the first
672.Ar bits
673bits of
674.Ar IP
675are taken as a network specification, and all IP addresses from that
676network will be matched.
677.It Cm dst Ar IP Ns Op / Ns Ar bits
678If specified, only packets with a destination address matching the given
679.Ar IP
680are matched.
681If
682.Ar bits
683is also specified, then the first
684.Ar bits
685bits of
686.Ar IP
687are taken as a network specification, and all IP addresses from that
688network will be matched.
689.El
690.Pp
691This function is usually used to redirect outgoing connections for
692internal machines that are not permitted certain types of internet
693access, or to restrict access to certain external machines.
694.Ed
695.Pp
696.Ft struct alias_link *
697.Fo LibAliasRedirectProto
698.Fa "struct libalias *"
699.Fa "struct in_addr local_addr"
700.Fa "struct in_addr remote_addr"
701.Fa "struct in_addr alias_addr"
702.Fa "u_char proto"
703.Fc
704.Bd -ragged -offset indent
705This function specifies that any IP packet with protocol number of
706.Fa proto
707from a given remote address to an alias address be
708redirected to a specified local address.
709.Pp
710If
711.Fa local_addr
712or
713.Fa alias_addr
714is zero, this indicates that the packet aliasing address as established
715by
716.Fn LibAliasSetAddress
717is to be used.
718Even if
719.Fn LibAliasSetAddress
720is called to change the address after
721.Fn LibAliasRedirectProto
722is called, a zero reference will track this change.
723.Pp
724If
725.Fa remote_addr
726is zero, this indicates to redirect packets from any remote address.
727Non-zero remote addresses can sometimes be useful for firewalling.
728.Pp
729If two calls to
730.Fn LibAliasRedirectProto
731overlap in their address specifications, then the most recent call
732will have precedence.
733.Pp
734This function returns a pointer which can subsequently be used by
735.Fn LibAliasRedirectDelete .
736If
737.Dv NULL
738is returned, then the function call did not complete successfully.
739.Ed
740.Sh FRAGMENT HANDLING
741The functions in this section are used to deal with incoming fragments.
742.Pp
743Outgoing fragments are handled within
744.Fn LibAliasOut
745by changing the address according to any applicable mapping set by
746.Fn LibAliasRedirectAddr ,
747or the default aliasing address set by
748.Fn LibAliasSetAddress .
749.Pp
750Incoming fragments are handled in one of two ways.
751If the header of a fragmented IP packet has already been seen, then all
752subsequent fragments will be re-mapped in the same manner the header
753fragment was.
754Fragments which arrive before the header are saved and then retrieved
755once the header fragment has been resolved.
756.Pp
757.Ft int
758.Fn LibAliasSaveFragment "struct libalias *" "char *ptr"
759.Bd -ragged -offset indent
760When
761.Fn LibAliasIn
762returns
763.Dv PKT_ALIAS_UNRESOLVED_FRAGMENT ,
764this function can be used to save the pointer to the unresolved fragment.
765.Pp
766It is implicitly assumed that
767.Fa ptr
768points to a block of memory allocated by
769.Xr malloc 3 .
770If the fragment is never resolved, the packet aliasing engine will
771automatically free the memory after a timeout period.
772[Eventually this function should be modified so that a callback function
773for freeing memory is passed as an argument.]
774.Pp
775This function returns
776.Dv PKT_ALIAS_OK
777if it was successful and
778.Dv PKT_ALIAS_ERROR
779if there was an error.
780.Ed
781.Pp
782.Ft char *
783.Fn LibAliasGetFragment "struct libalias *" "char *buffer"
784.Bd -ragged -offset indent
785This function can be used to retrieve fragment pointers saved by
786.Fn LibAliasSaveFragment .
787The IP header fragment pointed to by
788.Fa buffer
789is the header fragment indicated when
790.Fn LibAliasIn
791returns
792.Dv PKT_ALIAS_FOUND_HEADER_FRAGMENT .
793Once a fragment pointer is retrieved, it becomes the calling program's
794responsibility to free the dynamically allocated memory for the fragment.
795.Pp
796The
797.Fn LibAliasGetFragment
798function can be called sequentially until there are no more fragments
799available, at which time it returns
800.Dv NULL .
801.Ed
802.Pp
803.Ft void
804.Fn LibAliasFragmentIn "struct libalias *" "char *header" "char *fragment"
805.Bd -ragged -offset indent
806When a fragment is retrieved with
807.Fn LibAliasGetFragment ,
808it can then be de-aliased with a call to
809.Fn LibAliasFragmentIn .
810The
811.Fa header
812argument is the pointer to a header fragment used as a template, and
813.Fa fragment
814is the pointer to the packet to be de-aliased.
815.Ed
816.Sh MISCELLANEOUS FUNCTIONS
817.Ft void
818.Fn LibAliasSetTarget "struct libalias *" "struct in_addr addr"
819.Bd -ragged -offset indent
820When an incoming packet not associated with any pre-existing aliasing link
821arrives at the host machine, it will be sent to the address indicated by a
822call to
823.Fn LibAliasSetTarget .
824.Pp
825If this function is called with an
826.Dv INADDR_NONE
827address argument, then all new incoming packets go to the address set by
828.Fn LibAliasSetAddress .
829.Pp
830If this function is not called, or is called with an
831.Dv INADDR_ANY
832address argument, then all new incoming packets go to the address specified
833in the packet.
834This allows external machines to talk directly to internal machines if they
835can route packets to the machine in question.
836.Ed
837.Pp
838.Ft int
839.Fn LibAliasCheckNewLink "struct libalias *"
840.Bd -ragged -offset indent
841This function returns a non-zero value when a new aliasing link is created.
842In circumstances where incoming traffic is being sequentially sent to
843different local servers, this function can be used to trigger when
844.Fn LibAliasSetTarget
845is called to change the default target address.
846.Ed
847.Pp
848.Ft u_short
849.Fn LibAliasInternetChecksum "struct libalias *" "u_short *buffer" "int nbytes"
850.Bd -ragged -offset indent
851This is a utility function that does not seem to be available elsewhere and
852is included as a convenience.
853It computes the internet checksum, which is used in both IP and
854protocol-specific headers (TCP, UDP, ICMP).
855.Pp
856The
857.Fa buffer
858argument points to the data block to be checksummed, and
859.Fa nbytes
860is the number of bytes.
861The 16-bit checksum field should be zeroed before computing the checksum.
862.Pp
863Checksums can also be verified by operating on a block of data including
864its checksum.
865If the checksum is valid,
866.Fn LibAliasInternetChecksum
867will return zero.
868.Ed
869.Pp
870.Ft int
871.Fn LibAliasUnaliasOut "struct libalias *" "char *buffer" "int maxpacketsize"
872.Bd -ragged -offset indent
873An outgoing packet, which has already been aliased,
874has its private address/port information restored by this function.
875The IP packet is pointed to by
876.Fa buffer ,
877and
878.Fa maxpacketsize
879is provided for error checking purposes.
880This function can be used if an already-aliased packet needs to have its
881original IP header restored for further processing (e.g.\& logging).
882.Ed
883.Sh AUTHORS
884.An Charles Mott Aq cm@linktel.net ,
885versions 1.0 - 1.8, 2.0 - 2.4.
886.An Eivind Eklund Aq eivind@FreeBSD.org ,
887versions 1.8b, 1.9 and 2.5.
888Added IRC DCC support as well as contributing a number of architectural
889improvements; added the firewall bypass for FTP/IRC DCC.
890.An Erik Salander Aq erik@whistle.com
891added support for PPTP and RTSP.
892.An Junichi Satoh Aq junichi@junichi.org
893added support for RTSP/PNA.
894.An Ruslan Ermilov Aq ru@FreeBSD.org
895added support for PPTP and LSNAT as well as general hacking.
896.An Paolo Pisati Aq piso@FreeBSD.org
897made the library modular, moving support for all
898protocols (except for IP, TCP and UDP) to external modules.
899.Sh ACKNOWLEDGMENTS
900Listed below, in approximate chronological order, are individuals who
901have provided valuable comments and/or debugging assistance.
902.Pp
903.Bd -ragged -offset indent
904.An -split
905.An Gary Roberts
906.An Tom Torrance
907.An Reto Burkhalter
908.An Martin Renters
909.An Brian Somers
910.An Paul Traina
911.An Ari Suutari
912.An Dave Remien
913.An J. Fortes
914.An Andrzej Bialecki
915.An Gordon Burditt
916.Ed
917.Sh CONCEPTUAL BACKGROUND
918This section is intended for those who are planning to modify the source
919code or want to create somewhat esoteric applications using the packet
920aliasing functions.
921.Pp
922The conceptual framework under which the packet aliasing engine operates
923is described here.
924Central to the discussion is the idea of an
925.Em aliasing link
926which describes the relationship for a given packet transaction between
927the local machine, aliased identity and remote machine.
928It is discussed how such links come into existence and are destroyed.
929.Ss ALIASING LINKS
930There is a notion of an
931.Em aliasing link ,
932which is a 7-tuple describing a specific translation:
933.Bd -literal -offset indent
934(local addr, local port, alias addr, alias port,
935 remote addr, remote port, protocol)
936.Ed
937.Pp
938Outgoing packets have the local address and port number replaced with the
939alias address and port number.
940Incoming packets undergo the reverse process.
941The packet aliasing engine attempts to match packets against an internal
942table of aliasing links to determine how to modify a given IP packet.
943Both the IP header and protocol dependent headers are modified as necessary.
944Aliasing links are created and deleted as necessary according to network
945traffic.
946.Pp
947Protocols can be TCP, UDP or even ICMP in certain circumstances.
948(Some types of ICMP packets can be aliased according to sequence or ID
949number which acts as an equivalent port number for identifying how
950individual packets should be handled.)
951.Pp
952Each aliasing link must have a unique combination of the following five
953quantities: alias address/port, remote address/port and protocol.
954This ensures that several machines on a local network can share the
955same aliasing IP address.
956In cases where conflicts might arise, the aliasing port is chosen so that
957uniqueness is maintained.
958.Ss STATIC AND DYNAMIC LINKS
959Aliasing links can either be static or dynamic.
960Static links persist indefinitely and represent fixed rules for translating
961IP packets.
962Dynamic links come into existence for a specific TCP connection or UDP
963transaction or ICMP ECHO sequence.
964For the case of TCP, the connection can be monitored to see when the
965associated aliasing link should be deleted.
966Aliasing links for UDP transactions (and ICMP ECHO and TIMESTAMP requests)
967work on a simple timeout rule.
968When no activity is observed on a dynamic link for a certain amount of time
969it is automatically deleted.
970Timeout rules also apply to TCP connections which do not open or close
971properly.
972.Ss PARTIALLY SPECIFIED ALIASING LINKS
973Aliasing links can be partially specified, meaning that the remote address
974and/or remote port are unknown.
975In this case, when a packet matching the incomplete specification is found,
976a fully specified dynamic link is created.
977If the original partially specified link is dynamic, it will be deleted
978after the fully specified link is created, otherwise it will persist.
979.Pp
980For instance, a partially specified link might be
981.Bd -literal -offset indent
982(192.168.0.4, 23, 204.228.203.215, 8066, 0, 0, tcp)
983.Ed
984.Pp
985The zeros denote unspecified components for the remote address and port.
986If this link were static it would have the effect of redirecting all
987incoming traffic from port 8066 of 204.228.203.215 to port 23 (telnet)
988of machine 192.168.0.4 on the local network.
989Each individual telnet connection would initiate the creation of a distinct
990dynamic link.
991.Ss DYNAMIC LINK CREATION
992In addition to aliasing links, there are also address mappings that can be
993stored within the internal data table of the packet aliasing mechanism.
994.Bd -literal -offset indent
995(local addr, alias addr)
996.Ed
997.Pp
998Address mappings are searched when creating new dynamic links.
999.Pp
1000All outgoing packets from the local network automatically create a dynamic
1001link if they do not match an already existing fully specified link.
1002If an address mapping exists for the outgoing packet, this determines
1003the alias address to be used.
1004If no mapping exists, then a default address, usually the address of the
1005packet aliasing host, is used.
1006If necessary, this default address can be changed as often as each individual
1007packet arrives.
1008.Pp
1009The aliasing port number is determined such that the new dynamic link does
1010not conflict with any existing links.
1011In the default operating mode, the packet aliasing engine attempts to set
1012the aliasing port equal to the local port number.
1013If this results in a conflict, then port numbers are randomly chosen until
1014a unique aliasing link can be established.
1015In an alternate operating mode, the first choice of an aliasing port is also
1016random and unrelated to the local port number.
1017.Sh MODULAR ARCHITECTURE (AND Xr ipfw 4 Sh SUPPORT)
1018One of the latest improvements to
1019.Nm
1020was to make its support
1021for new protocols independent from the rest of the library, giving it
1022the ability to load/unload support for new protocols at run-time.
1023To achieve this feature, all the code for protocol handling was moved
1024to a series of modules outside of the main library.
1025These modules are compiled from the same sources but work in
1026different ways, depending on whether they are compiled to work inside a kernel
1027or as part of the userland library.
1028.Ss LIBALIAS MODULES IN KERNEL LAND
1029When compiled for the kernel,
1030.Nm
1031modules are plain KLDs recognizable with the
1032.Pa alias_
1033prefix.
1034.Pp
1035To add support for a new protocol, load the corresponding module.
1036For example:
1037.Pp
1038.Dl "kldload alias_ftp"
1039.Pp
1040When support for a protocol is no longer needed, its module can be unloaded:
1041.Pp
1042.Dl "kldunload alias_ftp"
1043.Ss LIBALIAS MODULES IN USERLAND
1044Due to the differences between kernel and userland (no KLD mechanism,
1045many different address spaces, etc.), we had to change a bit how to
1046handle module loading/tracking/unloading in userland.
1047.Pp
1048While compiled for a userland
1049.Nm ,
1050all the modules are plain libraries, residing in
1051.Pa /usr/lib ,
1052and recognizable with the
1053.Pa libalias_
1054prefix.
1055.Pp
1056There is a configuration file,
1057.Pa /etc/libalias.conf ,
1058with the following contents (by default):
1059.Bd -literal -offset indent
1060/usr/lib/libalias_cuseeme.so
1061/usr/lib/libalias_ftp.so
1062/usr/lib/libalias_irc.so
1063/usr/lib/libalias_nbt.so
1064/usr/lib/libalias_pptp.so
1065/usr/lib/libalias_skinny.so
1066/usr/lib/libalias_smedia.so
1067.Ed
1068.Pp
1069This file contains the paths to the modules that
1070.Nm
1071will load.
1072To load/unload a new module, just add its path to
1073.Pa libalias.conf
1074and call
1075.Fn LibAliasRefreshModules
1076from the program.
1077In case the application provides a
1078.Dv SIGHUP
1079signal handler, add a call to
1080.Fn LibAliasRefreshModules
1081inside the handler, and everytime you want to refresh the loaded modules,
1082send it the
1083.Dv SIGHUP
1084signal:
1085.Pp
1086.Dl "kill -HUP <process_pid>"
1087.Ss MODULAR ARCHITECURE: HOW IT WORKS
1088The modular architecture of
1089.Nm
1090works similar whether it is running inside the
1091kernel or in userland.
1092From
1093.Pa alias_mod.c :
1094.Bd -literal
1095/* Protocol and userland module handlers chains. */
1096LIST_HEAD(handler_chain, proto_handler) handler_chain ...
1097\&...
1098SLIST_HEAD(dll_chain, dll) dll_chain ...
1099.Ed
1100.Pp
1101.Va handler_chain
1102keep tracks of all the protocol handlers loaded, while
1103.Va ddl_chain
1104takes care of userland modules loaded.
1105.Pp
1106.Va handler_chain
1107is composed of
1108.Vt "struct proto_handler"
1109entries:
1110.Bd -literal
1111struct proto_handler {
1112 u_int pri;
1113 int16_t dir;
1114 uint8_t proto;
1115 int (*fingerprint)(struct libalias *la,
1116 struct ip *pip, struct alias_data *ah);
1117 int (*protohandler)(struct libalias *la,
1118 struct ip *pip, struct alias_data *ah);
1119 LIST_ENTRY(proto_handler) entries;
1120};
1121.Ed
1122.Pp
1123where:
1124.Bl -inset
1125.It Va pri
1126is the priority assigned to a protocol handler, lower
1127is better.
1128.It Va dir
1129is the direction of packets: ingoing or outgoing.
1130.It Va proto
1131says at which protocol this packet belongs: IP, TCP or UDP.
1132.It Va fingerprint
1133points to the fingerprint function while protohandler points
1134to the protocol handler function.
1135.El
1136.Pp
1137The
1138.Va fingerprint
1139function has the double of scope of checking if the
1140incoming packet is found and if it belongs to any categories that this
1141module can handle.
1142.Pp
1143The
1144.Va protohandler
1145function actually manipulates
1146the packet to make
1147.Nm
1148correctly NAT it.
1149.Pp
1150When a packet enters
1151.Nm ,
1152if it meets a module hook,
1153.Va handler_chain
1154is searched to see if there is an handler that matches
1155this type of a packet (it checks protocol and direction of packet), then if
1156more than one handler is found, it starts with the module with
1157the lowest priority number: it calls the
1158.Va fingerprint
1159function and interprets the result.
1160.Pp
1161If the result value is equal to 0 then it calls the protocol handler
1162of this handler and returns.
1163Otherwise, it proceeds to the next eligible module until the
1164.Va handler_chain
1165is exhausted.
1166.Pp
1167Inside
1168.Nm ,
1169the module hook looks like this:
1170.Bd -literal -offset indent
1171struct alias_data ad = {
1172 lnk,
1173 &original_address,
1174 &alias_address,
1175 &alias_port,
1176 &ud->uh_sport, /* original source port */
1177 &ud->uh_dport, /* original dest port */
1178 256 /* maxpacketsize */
1179};
1180
1181\&...
1182
1183/* walk out chain */
1184err = find_handler(IN, UDP, la, pip, &ad);
1185.Ed
1186.Pp
1187All data useful to a module are gathered together in an
1188.Vt alias_data
1189structure, then
1190.Fn find_handler
1191is called.
1192The
1193.Fn find_handler
1194function is responsible for walking out the handler
1195chain, it receives as input parameters:
1196.Bl -tag -width indent
1197.It Fa IN
1198direction
1199.It Fa UDP
1200working protocol
1201.It Fa la
1202pointer to this instance of libalias
1203.It Fa pip
1204pointer to a
1205.Vt "struct ip"
1206.It Fa ad
1207pointer to
1208.Vt "struct alias_data"
1209(see above)
1210.El
1211.Pp
1212In this case,
1213.Fn find_handler
1214will search only for modules registered for
1215supporting INcoming UDP packets.
1216.Pp
1217As was mentioned earlier,
1218.Nm
1219in userland is a bit different, cause
1220care has to be taken of module handling too (avoiding duplicate load of
1221module, avoiding module with same name, etc.) so
1222.Va dll_chain
1223was introduced.
1224.Pp
1225.Va dll_chain
1226contains a list of all userland
1227.Nm
1228modules loaded.
1229.Pp
1230When an application calls
1231.Fn LibAliasRefreshModules ,
1232.Nm
1233first unloads all the loaded modules, then reloads all the modules listed in
1234.Pa /etc/libalias.conf :
1235for every module loaded, a new entry to
1236.Va dll_chain
1237is added.
1238.Pp
1239.Va dll_chain
1240is composed of
1241.Vt "struct dll"
1242entries:
1243.Bd -literal
1244struct dll {
1245 /* name of module */
1246 char name[DLL_LEN];
1247 /*
1248 * ptr to shared obj obtained through
1249 * dlopen() - use this ptr to get access
1250 * to any symbols from a loaded module
1251 * via dlsym()
1252 */
1253 void *handle;
1254 struct dll *next;
1255};
1256.Ed
1257.Bl -inset
1258.It Va name
1259is the name of the module
1260.It Va handle
1261is a pointer to the module obtained through
1262.Xr dlopen 3
1263.El
1264Whenever a module is loaded in userland, an entry is added to
1265.Va dll_chain ,
1266then every protocol handler present in that module
1267is resolved and registered in
1268.Va handler_chain .
1269.Ss HOW TO WRITE A MODULE FOR LIBALIAS
1270There is a module (called
1271.Pa alias_dummy.[ch] )
1272in
1273.Nm
1274that can be used as a skeleton for future work, here we analyse some parts of that
1275module.
1276From
1277.Pa alias_dummy.c :
1278.Bd -literal
1279struct proto_handler handlers [] = {{666, IN|OUT, UDP|TCP,
1280 &fingerprint, &protohandler}};
1281.Ed
1282.Pp
1283The variable
1284.Va handlers
1285is the
1286.Dq "most important thing"
1287in a module
1288cause it describes the handlers present and lets the outside world use
1289it in an opaque way.
1290.Pp
1291It must ALWAYS be present in every module, and it MUST retain
1292the name
1293.Va handlers ,
1294otherwise attempting to load a module in userland will fail and
1295complain about missing symbols: for more information about module
1296load/unload, please refer to
1297.Fn LibAliasRefreshModules ,
1298.Fn LibAliasLoadModule
1299and
1300.Fn LibAliasUnloadModule
1301in
1302.Pa alias.c .
1303.Pp
1304.Va handlers
1305contains all the
1306.Vt proto_handler
1307structures present in a module.
1308.Bd -literal
1309static int
1310mod_handler(module_t mod, int type, void *data)
1311{
1312 int error;
1313
1314 switch (type) {
1315 case MOD_LOAD:
1316 error = 0;
1317 attach_handlers(handlers);
1318 break;
1319 case MOD_UNLOAD:
1320 error = 0;
1321 detach_handlers(handlers;
1322 break;
1323 default:
1324 error = EINVAL;
1325 }
1326 return (error);
1327}
1328.Ed
1329When running as KLD,
1330.Fn mod_handler
1331register/deregister the module using
1332.Fn attach_handlers
1333and
1334.Fn detach_handlers ,
1335respectively.
1336.Pp
1337Every module must contain at least 2 functions: one fingerprint
1338function and a protocol handler function.
1339.Bd -literal
1340#ifdef _KERNEL
1341static
1342#endif
1343int
1344fingerprint(struct libalias *la, struct ip *pip, struct alias_data *ah)
1345{
1346
1347\&...
1348}
1349
1350#ifdef _KERNEL
1351static
1352#endif
1353int
1354protohandler(struct libalias *la, struct ip *pip,
1355 struct alias_data *ah)
1356{
1357
1358\&...
1359}
1360.Ed
1361and they must accept exactly these input parameters.
1362.Ss PATCHING AN APPLICATION FOR USERLAND LIBALIAS MODULES
1363To add module support into an application that uses
1364.Nm ,
1365the following simple steps can be followed.
1366.Bl -enum
1367.It
1368Find the main file of an application
1369(let us call it
1370.Pa main.c ) .
1371.It
1372Add this to the header section of
1373.Pa main.c ,
1374if not already present:
1375.Pp
1376.Dl "#include <signal.h>"
1377.Pp
1378and this just after the header section:
1379.Pp
1380.Dl "static void signal_handler(int);"
1381.It
1382Add the following line to the init function of an application or,
1383if it does not have any init function, put it in
1384.Fn main :
1385.Pp
1386.Dl "signal(SIGHUP, signal_handler);"
1387.Pp
1388and place the
1389.Fn signal_handler
1390function somewhere in
1391.Pa main.c :
1392.Bd -literal -offset indent
1393static void
1394signal_handler(int sig)
1395{
1396
1397 LibAliasRefreshModules();
1398}
1399.Ed
1400.Pp
1401Otherwise, if an application already traps the
1402.Dv SIGHUP
1403signal, just add a call to
1404.Fn LibAliasRefreshModules
1405in the signal handler function.
1406.El
1407For example, to patch
1408.Xr natd 8
1409to use
1410.Nm
1411modules, just add the following line to
1412.Fn RefreshAddr "int sig __unused" :
1413.Pp
1414.Dl "LibAliasRefreshModules()"
1415.Pp
1416recompile and you are done.
1417.Ss LOGGING SUPPORT IN KERNEL LAND
1418When working as KLD,
1419.Nm
1420now has log support that
1421happens on a buffer allocated inside
1422.Vt "struct libalias"
1423(from
1424.Pa alias_local.h ) :
1425.Bd -literal
1426struct libalias {
1427 ...
1428
1429 /* log descriptor */
1430#ifdef KERNEL_LOG
1431 char *logDesc; /*
1432 * ptr to an auto-malloced
1433 * memory buffer when libalias
1434 * works as kld
1435 */
1436#else
1437 FILE *logDesc; /*
1438 * ptr to /var/log/alias.log
1439 * when libalias runs as a
1440 * userland lib
1441 */
1442#endif
1443
1444 ...
1445}
1446.Ed
1447so all applications using
1448.Nm
1449will be able to handle their
1450own logs, if they want, accessing
1451.Va logDesc .
1452Moreover, every change to a log buffer is automatically added to
1453.Xr syslog 3
1454with the
1455.Dv LOG_SECURITY
1456facility and the
1457.Dv LOG_INFO
1458level.
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