3 .\" $Id: fw.1,v 1.19 2004/04/08 01:36:25 mdw Exp $
7 .\" (c) 1999 Straylight/Edgeware
10 .\"----- Licensing notice ---------------------------------------------------
12 .\" This file is part of the `fw' port forwarder.
14 .\" `fw' is free software; you can redistribute it and/or modify
15 .\" it under the terms of the GNU General Public License as published by
16 .\" the Free Software Foundation; either version 2 of the License, or
17 .\" (at your option) any later version.
19 .\" `fw' is distributed in the hope that it will be useful,
20 .\" but WITHOUT ANY WARRANTY; without even the implied warranty of
21 .\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
22 .\" GNU General Public License for more details.
24 .\" You should have received a copy of the GNU General Public License
25 .\" along with `fw'; if not, write to the Free Software Foundation,
26 .\" Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
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30 .\" --- Indented paragraphs with right-aligned tags ---
34 \h'-\w'\fB\\$1\ \fP'u'\fB\\$1\ \fP\c
37 .\" --- Verbatim-oid typesetting ---
51 .\" --- Grammar markup ---
53 .\" This is mainly for the benefit of the automatic scripts which
54 .\" generate the grammar summary.
73 .\" --- Other bits of styling ---
89 .\"--------------------------------------------------------------------------
91 .TH fw 1 "1 July 1999" "Straylight/Edgeware" "fw port forwarder"
93 .\"--------------------------------------------------------------------------
98 .\"--------------------------------------------------------------------------
111 .\"--------------------------------------------------------------------------
116 program is a simple port forwarder. It supports a number of features
117 the author hasn't found in similar programs:
119 .I "Connection logging"
120 Each connection attempt to the forwarder is logged, giving the time of
121 the connection, the DNS-resolved hostname (if available), and the user
122 name resulting from an RFC931 lookup. These lookups are done
123 asynchronously to the main forwarder's operation.
126 Each forwarded port may have an access control list attached to it.
127 Only authorized hosts are allowed to connect. Access control checks are
128 performed by quick checks on the client's IP address.
130 .I "Nonblocking single-process design"
131 The internal structure of the server is completely nonblocking. The
132 connections don't block; the reading and writing don't block; the name
133 lookups don't block. This is all done in a single process, with the
134 single exception of the DNS resolver.
136 .I "Support for Unix-domain sockets"
137 Connections from and to Unix-domain sockets can be handled just as
138 easily as more normal Internet sockets. Access control doesn't work on
139 Unix domain sockets, though. (Yet.)
140 .SS "Command line options"
143 program understands a few simple command line options:
146 Displays a screen of help text on standard output and exits
149 .B "\-v, \-\-version"
150 Writes the version number to standard output and exits successfully.
153 Writes a terse usage summary to standard output and exits successfully.
155 .B "\-G, \-\-grammar"
156 Writes a summary of the configuration file grammar to standard output
157 and exits successfully.
159 .B "\-O, \-\-options"
160 Writes a summary of the source and target options to standard output and
163 .BI "\-f, \-\-file=" file
164 Read configuration information from
169 configuration file statement.
171 .B "\-d, \-\-daemon, \-\-fork"
172 Forks into the background after reading the configuration and
173 initializing properly.
175 .B "\-l, \-\-syslog, \-\-log"
176 Emit logging information to the system log, rather than standard error.
179 Don't output any logging information. This option is not recommended
180 for normal use, although it can make system call traces clearer so I use
183 .BI "\-s, \-\-setuid=" user
184 Change uid to that of
186 which may be either a user name or uid number, after initializing all
187 the sources. This will usually require elevated privileges.
189 .BI "\-g, \-\-setgid=" group
190 Change gid to that of
192 which may be either a group name or gid number, after initializing all
193 the sources. If the operating system understands supplementary groups
194 then the supplementary groups list is altered to include only
197 Any further command line arguments are interpreted as configuration
198 lines to be read. Configuration supplied in command line arguments has
199 precisely the same syntax as configuration in files. If there are no
200 configuration statements on the command line, and no
202 options were supplied, configuration is read from standard input, if
203 stdin is not a terminal.
205 .\"--------------------------------------------------------------------------
206 .SH "CONFIGURATION LANGUAGE"
210 program has a fairly sophisticated configuration language to let you
211 describe which things should be forwarded where and what special
212 features there should be.
213 .SS "Lexical structure"
214 There are four types of characters.
216 .I "word constituent characters"
217 Word constituent characters are gathered together into words.
218 Depending on its surrounding context, a word might act as a keyword or a
219 string. All alphanumerics are word constituents, as is the hyphen
221 Other characters may change their status in future versions.
223 .I "self-delimiting characters"
224 Self-delimiting characters always stand alone. They act as punctuation,
225 shaping the sequence of words into more complex grammatical forms. The
238 are self-delimiting. Note that while some characters, e.g.,
242 require escaping by the shell, they are mostly optional in the grammar
243 and can tend to be omitted in quick hacks at the shell prompt.
245 .I "whitespace characters"
246 Whitespace characters separate words but are otherwise ignored. All
247 `normal' whitespace characters (e.g., space, tab and newline) are
248 considered to be whitespace for these purposes.
250 .I "special characters"
251 There are three special characters. The
253 character, if it appears at the start of a word, introduces a
255 which extends to the end of the current line or command-line argument.
256 Within a word, it behaves like a normal word-constituent character. The
259 escapes the following character causing it to be interpreted as a word
260 constituent regardless of its normal type. The double-quote
262 escapes all characters other than backslashes up to the next
263 double-quote and causes them to be regarded as word constituents. Note
264 that you don't have to quote a whole word. The backslash can escape a
265 quote character allowing you to insert it into a word if really
269 The overall syntax looks a bit like this:
308 If you prefer, the keyword
316 .SS "Sources and targets"
317 Forwarding is set up by attaching
321 Sources are things which are capable of
323 one end of a data flow on their own, while targets are things which are
324 capable of setting up the other end on demand. In the case of a TCP
325 port forwarder, the part which listens for incoming client connections
326 is the source, while the part which sets up outgoing connections to the
327 destination server is the target.
331 does is set up a collection of sources and targets based on your
332 configuration file so that when a source decides to initiate a data
333 flow, it tells its target to set its end up, and then squirts data back
334 and forth between the two until there's no more.
338 they stay around indefinitely setting up multiple attachments to
341 they set up one connection and then disappear. If all the sources
342 defined are transient, then
344 will quit when no more active sources remain and all connections have
349 program is fairly versatile. It allows you to attach any supported type
350 of source to any supported type of target. This will, I hope, be the
351 case in all future versions.
357 depend on the source or target type, and are therefore described in the
358 sections specific to the various types.
360 .SS "Options structure"
361 Most of the objects that
363 knows about (including sources and targets, but also other more specific
364 things such as socket address types) can have their behaviour modified
367 The options available at a particular point in the configuration depend
370 A global option, outside of a
372 has no context unless it is explicitly qualified, and affects global
373 behaviour. A local option, applied to a source or target in a
375 has the context of the type of source or target to which it is applied,
376 and affects only that source or target.
378 Note that it's important to distinguish between an option's context
379 (which is affected by its qualification) and its local or global
380 status. No matter how qualified, a global option will always control
381 default options for objects, and a local option will only affect a
382 specific source or target.
384 The syntax for qualifying options is like this:
409 Thus, you may qualify either an individual option or a sequence of
410 options. The two are equivalent; for example,
419 exec.rlimit.core = 0;
422 For each option, there is a sequence of prefixes which maximally qualify
423 that option. An option prefixed with this sequence is
424 .IR "fully qualified" .
425 In actual use, some or all of those prefixes may be omitted. However,
426 it's possible for the option to become
428 if you do this. For example, the option
433 .BR socket.unix.fattr.owner .
434 In this case, the ambiguity is benign: a local option will have as its
435 context an appropriate source or target, and both global options
436 actually control the same default. However, the option
442 which have separate defaults, and which one you actually get depends on
443 the exact implementation of
445 option parser. (Currently this would resolve to
447 although this may change in a later version.)
449 In this manual, options are usually shown in their fully-qualified form.
451 .SS "File attributes for created files: `fattr'"
456 sources and targets can create new filesystem objects. The
458 options allow control over the attributes of the newly-created objects.
463 use the same set of defaults, so a prefix of
465 is good enough for setting global options, and the implicit context
466 disambiguates local options.
468 The following file attribute options are supported:
469 .OS "File attribute options (`fattr')"
470 .IB prefix .fattr.mode
474 Sets the permissions mode for a new file. The
476 argument may be either an octal number or a
478 string which acts on the default permissions established by the
481 setting. The characters
485 do not have to be quoted within the mode string.
487 .OS "File attribute options (`fattr')"
488 .IB prefix .fattr.owner
492 Sets the owner for newly created files. On non-broken systems you will
493 need to be the superuser to set the owner on a file. The
495 may either be a numeric uid or a username. The default is not to change
496 the owner of the file once it's created. The synonyms
500 are accepted in place of
503 .OS "File attribute options (`fattr')"
504 .IB prefix .fattr.group
508 Sets the group for newly created files. You will usually need to be a
509 member of the group in question order to set the group of a file. The
511 may either be a numeric gid or a group name. The default is not to
512 change the group of the file once it's created. The synonym
514 is accepted in place of
518 .SS "The `file' source and target types"
521 source and target allow data to move to and from objects other
522 than sockets within the Unix filesystem. (Unix-domain sockets are
529 is used as a source, it is set up immediately.
533 sources and targets is like this:
534 .GS "File sources and targets"
567 .RB [[ : ] file [ : ]]
597 specification describes two files, the first to be used as input, the
598 second to be used as output, each described by an
601 If none of the keywords
606 are given, the type of an
608 is deduced from its nature: if it matches one of the strings
612 or begins with a digit, it's considered to be a file descriptor;
613 otherwise it's interpreted as a filename.
617 spec describes a file by its name within the filesystem. It is opened
618 when needed and closed again after use. For output files, the precise
619 behaviour is controlled by options described below.
623 spec attaches the input or output of the source or target to
628 spec uses an existing open file descriptor, given either by number or a
629 symbolic name. The name
631 refers to standard input (file descriptor 0 on normal systems) and
633 refers to standard output (file descriptor 1). The names work in
634 exactly the same way as the equivalent file descriptor numbers.
638 is omitted, the input
640 is used for both input and output. Exception: if the input refers to
641 standard input then the output will refer to standard output instead.
645 options apply equally to sources and targets. The options are as
652 Whether to create the output file if it doesn't exist. If
654 (the default), an error is reported if the file doesn't exist. If
656 the file is created if it doesn't exist.
661 .BR no | truncate | append
663 Controls the behaviour if the output file already exists. If
665 an error is reported. If
667 (the default), the existing file is replaced by the new data. If
669 the new data is appended to the file.
676 source and target also accept
678 options for controlling the attributes of the created file.
681 Under no circumstances will
683 create a file through a `dangling' symbolic link.
685 .SS "The `exec' source and target types"
688 source and target execute programs and allow access to their standard
689 input and output streams. Both source and target have the same syntax,
691 .GS "Exec source and target"
731 If a single word is given, it is a
733 and will be passed to the Bourne shell for execution. If a
734 bracket-enclosed sequence of words is given, it is considered to be a
735 list of arguments to pass to the program: if a
737 is also supplied, it names the file containing the program to execute;
738 otherwise the file named by the first argument
742 Note that the shell command or program name string must, if present,
743 have any delimiter characters (including
747 quoted; this is not required in the
751 The standard input and output of the program are forwarded to the other
752 end of the connection. The standard error stream is caught by
758 source and target both understand the same set of options. The list of
759 options supported is as follows:
765 Whether to log the start and end of executed programs. If
767 (the default), a log message is emitted when the program is started
768 listing its process id, and another is emitted when the program finishes
769 giving its process id and exit status. If
771 these messages are not emitted. However the standard error stream is
774 abbreviation is accepted as a synonym for
782 Sets the current directory from which the the program should be run.
783 The default is not to change directory. The synonyms
788 are accepted in place of
796 Sets the root directory for the program, using the
798 system call. You must be the superuser for this option to work. The
799 default is not to set a root directory. The synonym
801 is accepted in place of
809 Sets the user (real and effective uid) to run the program as. This will
810 usually require superuser privileges to work. The default is not to
811 change uid. The synonym
813 is accepted in place of
821 Sets the group (real and effective gid) to run the program as. If
822 running with superuser privileges, the supplementary groups list is
823 cleared at the same time. The default is not to change gid (or clear
824 the supplementary groups list). The synonym
826 is accepted in place of
830 .BI exec.rlimit. limit \c
831 .RB [ .hard | .soft ]
835 Set resource limits for the program. The
837 may be one of the resource limit names described in
839 in lower-case and without the
847 is a number, followed optionally by
849 to multiply by 1024 (2\*(ss10\*(se),
851 to multiply by 1048576 (2\*(ss20\*(se), or
853 to multiply by 1073741824 (2\*(ss30\*(se); purists can use upper-case
854 versions of these if they want. If
858 was specified, only the hard or soft limit is set; otherwise both are
859 set to the same value. Only the superuser can raise the hard limit.
860 The soft limit cannot be set above the hard limit.
865 Clears the program's environment.
873 from the program's environment. It is not an error if no variable named
878 .BR exec.env. [ set ]
887 in the program's environment, possibly replacing the existing value.
890 may be omitted if the
892 qualifier is present.
895 Note that environment variable modifications are performed in order,
896 global modifications before local ones.
898 .SS "The `socket' source and target types"
901 source and target provide access to network services. Support is
902 currently provided for TCP/IP and Unix-domain sockets, although other
903 address types can be added with reasonable ease.
905 The syntax for socket sources and targets is:
906 .GS "Socket source and target"
918 .RB [ socket [ .\& ]]
926 .RB [ socket [ .\& ]]
933 The syntax of the source and target addresses depend on the address
934 types, which are described below. The default address type, if no
939 Socket sources support options; socket targets do not. The source
940 options provided are:
945 .BR unlimited | one-shot
947 Controls the behaviour of the source when it receives connections. A
949 limits the number of simultaneous connections. The value
953 removes any limit on the number of connections possible. The value
955 will remove the socket source after a single successful connection.
956 (Connections refused by access control systems don't count here.)
957 The default is to apply a limit of 256 concurrent connections. Use of
960 option is not recommended.
967 Sets the maximum of the kernel incoming connection queue for this socket
968 source. This is the number given to the
970 system call. The default is 5.
977 Whether to log incoming connections. If
979 (the default) incoming connections are logged, together with information
980 about the client (where available) and whether the connection was
981 accepted or refused. If
983 log messages are not generated.
986 Address types also provide their own options.
988 .SS "The `inet' socket address type"
991 address type provides access to TCP ports. The
993 source and target addresses have the following syntax:
994 .GS "Socket source and target"
1021 may be given as a port number or a service name from the
1023 file (or YP map if you do that sort of thing). A
1025 may be a textual hostname or a numerical IP address.
1029 source address accepts the following options:
1030 .OS "Socket options"
1031 .B socket.inet.source.addr
1036 Specify the IP address on which to listen for incoming connections. The
1039 which means to listen on all addresses, though it may be useful to
1040 specify this explicitly, if the global setting is different.
1042 .OS "Socket options"
1043 .BR socket.inet.source. [ allow | deny ]
1049 Adds an entry to the source's access control list. If only one
1051 is given, the entry applies only to that address; if two are given, the
1052 first is a network address and the second is a netmask either in
1053 dotted-quad format or a simple number of bits (e.g.,
1057 mean the same), and the entry applies to any address which, when masked
1058 by the netmask, is equal to the masked network address.
1060 .OS "Socket options"
1061 .BR socket.inet.source. [ allow | deny ]
1064 Accept or reject connections from low-numbered `privileged' ports, in
1067 .OS "Socket options"
1068 .B socket.inet.dest.addr
1073 Specify the IP address to bind the local socket to when making an
1074 outbound connection. The default is
1076 which means to use whichever address the kernel thinks is most
1077 convenient. This option is useful if the destination is doing
1078 host-based access control and your server is multi-homed.
1080 .OS "Socket options"
1081 .B socket.inet.dest.priv-port
1085 Make a privileged connection (i.e., from a low-numbered port) to the
1086 target. This only works if
1088 was started with root privileges. However, it still works if
1092 privileges after initialization (the
1094 option). Before dropping privileges,
1096 forks off a separate process which continues to run with root
1097 privileges, and on demand passes sockets bound to privileged ports and
1098 connected to the appropriate peer back to the main program. The
1099 privileged child only passes back sockets connected to peer addresses
1100 named in the configuration; even if the
1102 process is compromised, it can't make privileged connections to other
1103 addresses. Note that because of this privilege separation, it's also
1104 not possible to reconfigure
1106 to make privileged connections to different peer addresses later by
1107 changing configuration files and sending the daemon a
1111 The access control rules are examined in the order: local entries first,
1112 then global ones, each in the order given in the configuration file.
1113 The first matching entry is used. If no entries match, the behaviour is
1116 of the last entry tried. If there are no entries defined, the default
1117 is to allow all clients.
1119 .SS "The `unix' socket address type"
1122 address type allows access to Unix-domain sockets. The syntax for
1124 source and target addresses is like this:
1125 .GS "Socket source and target"
1134 The following options are supported by the
1136 source address type:
1137 .OS "Socket options"
1138 .BR socket.unix.fattr. *
1142 source address accepts
1144 options to control the attributes of the socket file created.
1147 Sockets are removed if
1149 exits normally (which it will do if it runs out of sources or
1152 shuts down in a clean way).
1154 To forward the local port 25 to a main mail server:
1156 from 25 to mailserv:25
1158 To attach a fortune server to a Unix-domain socket:
1160 from unix:/tmp/fortunes
1161 to exec [/usr/games/fortune] { user nobody }
1163 To fetch a fortune from the server:
1165 from file stdin, stdout to unix:/tmp/fortunes
1170 from file stdin, null to file null, stdout
1172 .sp -1 \" undo final space
1174 .\"--------------------------------------------------------------------------
1175 .SH "SIGNAL HANDLING"
1179 program responds to various signals when it's running. If it receives
1186 shutdown: it removes all of its sources, and will exit when no more
1187 connections are running. (Note that if the disposition
1191 does not re-enable the signal. You'll have to send
1199 shutdown: it removes all sources and extant connections and closes down
1200 more-or-less immediately.
1202 Finally, if any configuration files (other than standard input) were
1205 on its command line using the
1209 signal may be sent to instruct
1211 to reload its configuration. Any existing connections are allowed to
1212 run their course. If no such configuration files are available,
1214 just logs a message about the signal and continues.
1216 .\"--------------------------------------------------------------------------
1217 .SH "GRAMMAR SUMMARY"
1281 .SS "File source and target"
1308 .RB [[ : ] fd [ : ]]
1310 .RB | stdin | stdout
1314 .RB [[ : ] file [ : ]]
1340 .RB [ : ] null [ : ]
1342 .SS "Exec source and target"
1382 .SS "Socket source and target"
1394 .RB [ socket [ .\& ]]
1402 .RB [ socket [ .\& ]]
1441 .\"--------------------------------------------------------------------------
1442 .SH "OPTION SUMMARY"
1444 .SS "File attributes (`fattr')"
1445 .IB prefix .fattr.mode
1449 .IB prefix .fattr.owner
1453 .IB prefix .fattr.group
1464 .BR no | truncate | append
1489 .BI exec.rlimit. limit \c
1490 .RB [ .hard | .soft ]
1499 .BR exec.env. [ set ]
1504 .SS "Socket options"
1508 .BR unlimited | one-shot
1518 .BR socket.inet.source. [ allow | deny ]
1524 .BR socket.inet.source. [ allow | deny ]
1527 .B socket.inet.source.addr
1532 .B socket.inet.dest.addr
1537 .B socket.inet.dest.priv-port
1541 .BR socket.unix.fattr. *
1543 .\"--------------------------------------------------------------------------
1546 The syntax for IP addresses and filenames is nasty.
1548 IPv6 is not supported yet. Because of
1550 socket address architecture, it's probably not a major piece of work to
1553 Please inform me of any security problems you think you've identified in
1554 this program. I take security very seriously, and I will fix security
1555 holes as a matter of priority when I find out about them. I will be
1556 annoyed if I have to read about problems on Bugtraq because they weren't
1559 The program is too complicated, and this manual page is too long.
1561 .\"--------------------------------------------------------------------------
1564 Mark Wooding, <mdw@nsict.org>
1566 .\"----- That's all, folks --------------------------------------------------
~mdw / fwd / blob