3 .\" $Id: fw.1,v 1.3 1999/07/30 06:49:00 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.
28 .\" ---- Revision history ---------------------------------------------------
31 .\" Revision 1.3 1999/07/30 06:49:00 mdw
32 .\" Minor tidying and typo correction.
34 .\" Revision 1.2 1999/07/26 23:31:04 mdw
35 .\" Document lots of new features and syntax.
38 .\"----- Various bits of fancy styling --------------------------------------
40 .\" --- Indented paragraphs with right-aligned tags ---
44 \h'-\w'\fB\\$1\ \fP'u'\fB\\$1\ \fP\c
47 .\" --- Verbatim-oid typesetting ---
61 .\" --- Other bits of styling ---
77 .\"--------------------------------------------------------------------------
79 .TH fw 1 "1 July 1999" fw
81 .\"--------------------------------------------------------------------------
86 .\"--------------------------------------------------------------------------
95 .\"--------------------------------------------------------------------------
100 program is a simple port forwarder. It supports a number of features
101 the author hasn't found in similar programs:
103 .I "Connection logging"
104 Each connection attempt to the forwarder is logged, giving the time of
105 the connection, the DNS-resolved hostname (if available), and the user
106 name resulting from an RFC931 lookup. These lookups are done
107 asynchronously to the main forwarder's operation.
110 Each forwarded port may have an access control list attached to it.
111 Only authorized hosts are allowed to connect. Access control checks are
112 performed by quick checks on the client's IP address.
114 .I "Nonblocking single-process design"
115 The internal structure of the server is completely nonblocking. The
116 connections don't block; the reading and writing don't block; the name
117 lookups don't block. This is all done in a single process, with the
118 single exception of the DNS resolver.
120 .I "Support for Unix-domain sockets"
121 Connections from and to Unix-domain sockets can be handled just as
122 easily as more normal Internet sockets. Access control doesn't work on
123 Unix domain sockets, though. (Yet.)
124 .SS "Command line options"
127 program understands a few simple command line options:
130 Displays a screen of help text on standard output and exits
133 .B "\-v, \-\-version"
134 Writes the version number to standard output and exits successfully.
137 Writes a terse usage summary to standard output and exits successfully.
139 .BI "\-f, \-\-file=" file
140 Read configuration information from
145 configuration file statement.
147 .B "\-d, \-\-daemon, \-\-fork"
148 Forks into the background after reading the configuration and
149 initializing properly.
152 Don't output any logging information. This option is not recommended
153 for normal use, although it can make system call traces clearer so I use
156 Any further command line arguments are interpreted as configuration
157 lines to be read. Configuration supplied in command line arguments has
158 precisely the same syntax as configuration in files. If there are no
159 configuration statements on the command line, and no
161 options were supplied, configuration is read from standard input, if
162 stdin is not a terminal.
164 .\"--------------------------------------------------------------------------
165 .SH "CONFIGURATION LANGUAGE"
169 program has a fairly sophisticated configuration language to let you
170 describe which things should be forwarded where and what special
171 features there should be.
172 .SS "Lexical structure"
173 There are four types of characters.
175 .I "word constituent characters"
176 Word constituent characters are gathered together into words.
177 Depending on its surrounding context, a word might act as a keyword or a
178 string. All alphanumerics are word constituents, as is the hyphen
180 Other characters may change their status in future versions.
182 .I "self-delimiting characters"
183 Self-delimiting characters always stand alone. They act as punctuation,
184 shaping the sequence of words into more complex grammatical forms. The
197 are self-delimiting. Note that while some characters, e.g.,
201 require escaping by the shell, they are strictly optional in the grammar
202 and can be omitted in quick hacks at the shell prompt.
204 .I "whitespace characters"
205 Whitespace characters separate words but are otherwise ignored. All
206 `normal' whitespace characters (e.g., space, tab and newline) are
207 considered to be whitespace for these purposes.
209 .I "special characters"
210 There are three special characters. The
212 character, if it appears at the start of a word, introduces a
214 which extends to the end of the current line or command-line argument.
215 Within a word, it behaves like a normal word-constituent character. The
218 escapes the following character causing it to be interpreted as a word
219 constituent regardless of its normal type. The double-quote
221 escapes all characters other than backslashes up to the next
222 double-quote and causes them to be regarded as word constituents. Note
223 that you don't have to quote a whole word. The backslash can escape a
224 quote character allowing you to insert it into a word if really
228 The overall syntax looks a bit like this:
267 If you prefer, the keyword
275 .SS "Sources and targets"
276 Forwarding is set up by attaching
280 Sources are things which are capable of
282 one end of a data flow on their own, while targets are things which are
283 capable of setting up the other end on demand. In the case of a TCP
284 port forwarder, the part which listens for incoming client connections
285 is the source, while the part which sets up outgoing connections to the
286 destination server is the target.
290 does is set up a collection of sources and targets based on your
291 configuration file so that when a source decides to initiate a data
292 flow, it tells its target to set its end up, and then squirts data back
293 and forth between the two until there's no more.
297 they stay around indefinitely setting up multiple attachments to
300 they set up one connection and then disappear. If all the sources
301 defined are transient, then
303 will quit when no more active sources remain and all connections have
308 program is fairly versatile. It allows you to attach any supported type
309 of source to any supported type of target. This will, I hope, be the
310 case in all future versions.
316 depend on the source or target type, and are therefore described in the
317 sections specific to the various types.
319 .SS "Options structure"
320 Most of the objects that
322 knows about (including sources and targets, but also other more specific
323 things such as socket address types) can have their behaviour modified
326 The options available at a particular point in the configuration depend
329 A global option, outside of a
331 has no context unless it is explicitly qualified, and affects global
332 behaviour. Local options, applied to a source or target in a
334 has the context of the type of source or target to which it is applied,
335 and affects only that source or target.
337 Note that it's important to distinguish between an option's context
338 (which is affected by its qualification) and its local or global
339 status. No matter how qualified, a global option will always control
340 default options for objects, and a local option will only affect a
341 specific source or target.
343 The syntax for qualifying options is like this:
368 Thus, you may qualify either an individual option or a sequence of
369 options. The two are equivalent; for example,
378 exec.rlimit.core = 0;
381 For each option, there is a sequence of prefixes which maximally qualify
382 that option. An option prefixed with this sequence is
383 .IR "fully qualified" .
384 In actual use, some or all of those prefixes may be omitted. However,
385 it's possible for the option to become
387 if you do this. For example, the option
392 .BR socket.unix.fattr.owner .
393 In this case, the ambiguity is benign: a local option will have as its
394 context an appropriate source or target, and both global options
395 actually control the same default. However, the option
401 which have separate defaults, and which one you actually get depends on
402 the exact implementation of
404 option parser. (Currently this would resolve to
406 although this may change in a later version.)
408 In this manual, options are usually shown in their fully-qualified form.
410 .SS "The `file' source and target types"
413 source and target allow data to move to and from objects other
414 than sockets within the Unix filesystem. (Unix-domain sockets are
421 is used as a source, it is set up immediately.
425 sources and targets is like this:
459 .RB [[ : ] file [ : ]]
485 .RB [[ : ] null [ : ]]
489 specification describes two files, the first to be used as input, the
490 second to be used as output, each described by an
493 If none of the keywords
498 are given, the type of an
500 is deduced from its nature: if it matches one of the strings
504 or begins with a digit, it's considered to be a file descriptor;
505 otherwise it's interpreted as a filename.
509 spec describes a file by its name within the filesystem. It is opened
510 when needed and closed again after use. For output files, the precise
511 behaviour is controlled by options described below.
515 spec attaches the input or output of the source or target to
520 spec uses an existing open file descriptor, given either by number or a
521 symbolic name. The name
523 refers to standard input (file descriptor 0 on normal systems) and
525 refers to standard output (file descriptor 1). The names work in
526 exactly the same way as the equivalent file descriptor numbers.
530 is omitted, the input
532 is used for both input and output. Exception: if the input refers to
533 standard input then the output will refer to standard output instead.
537 options apply equally to sources and targets. The options are as
544 Whether to create the output file if it doesn't exist. If
546 (the default), an error is reported if the file doesn't exist. If
548 the file is created if it doesn't exist.
553 .BR no | truncate | append
555 Controls the behaviour if the output file already exists. If
557 an error is reported. If
559 (the default), the existing file is replaced by the new data. If
561 the new data is appended to the file.
564 Under no circumstances will
566 create a file through a `dangling' symbolic link.
570 source and target also accept
572 options for controlling the attributes of the created file. The prefix
573 for setting file attributes is
576 .SS "File attributes for created files `fattr'"
581 sources and targets can create new filesystem objects. The
583 options allow control over the attributes of the newly-created objects.
588 use the same set of defaults, so a prefix of
590 is good enough for setting global options, and the implicit context
591 disambiguates local options.
593 The following file attribute options are supported:
595 .IB prefix .fattr.mode
599 Sets the permissions mode for a new file. The
601 argument may be either an octal number or a
603 string which acts on the default permissions established by the
612 characters that will need to be escaped or quoted.
615 .IB prefix .fattr.owner
619 Sets the owner for newly created files. On non-broken systems you will
620 need to be the superuser to set the owner on a file. The
622 may either be a numeric uid or a username. The default is not to change
623 the owner of the file once it's created. The synonyms
627 are accepted in place of
631 .IB prefix .fattr.group
635 Sets the group for newly created files. You will usually need to be a
636 member of the group in question order to set the group of a file. The
638 may either be a numeric gid or a group name. The default is not to
639 change the group of the file once it's created. The synonym
641 is accepted in place of
645 .SS "The `exec' source and target types"
648 source and target execute programs and allow access to their standard
649 input and output streams. Both source and target have the same syntax,
691 If a single word is given, it is a
693 and will be passed to the Bourne shell for execution. If a
694 bracket-enclosed sequence of words is given, it is considered to be a
695 list of arguments to pass to the program: if a
697 is also supplied, it names the file containing the program to execute;
698 otherwise the file named by the first argument
702 The standard input and output of the program are forwarded to the other
703 end of the connection. The standard error stream is caught by
709 source and target both understand the same set of options. The list of
710 options supported is as follows:
716 Whether to log the start and end of executed programs. If
718 (the default), a log message is emitted when the program is started
719 listing its process id, and another is emitted when the program finishes
720 giving its process id and exit status. If
722 these messages are not emitted. However the standard error stream is
725 abbreviation is accepted as a synonym for
733 Sets the current directory from which the the program should be run.
734 The default is not to change directory. The synonyms
739 are accepted in place of
747 Sets the root directory for the program, using the
749 system call. You must be the superuser for this option to work. The
750 default is not to set a root directory. The synonyms
755 are accepted in place of
763 Sets the user (real and effective uid) to run the program as. This will
764 usually require superuser privileges to work. The default is not to
765 change uid. The synonym
767 is accepted in place of
775 Sets the group (real and effective gid) to run the program as. If
776 running with superuser privileges, the supplementary groups list is
777 cleared at the same time. The default is not to change gid (or clear
778 the supplementary groups list). The synonym
780 is accepted in place of
784 .BI exec.rlimit. limit \c
785 .RB [ .hard | .soft ]
789 Set resource limits for the program. The
791 may be one of the resource limit names described in
793 in lower-case and without the
801 is a number, followed optionally by
803 to multiply by 1024 (2\*(ss10\*(se),
805 to multiply by 1048576 (2\*(ss20\*(se), or
807 to multiply by 1073741824 (2\*(ss30\*(se); purists can use upper-case
808 versions of these if they want. If
812 was specified, only the hard or soft limit is set; otherwise both are
813 set to the same value. Only the superuser can raise the hard limit.
814 The soft limit cannot be set above the hard limit.
819 Clears the program's environment.
827 from the program's environment. It is not an error if no variable named
832 .BR exec.env. [ set ]
841 in the program's environment, possibly replacing the existing value.
844 may be omitted if the
846 qualifier is present.
849 Note that environment variable modifications are performed in order,
850 global modifications before local ones.
852 .SS "The `socket' source and target types"
855 source and target provide access to network services. Support is
856 currently provided for TCP/IP and Unix-domain sockets, although other
857 address types can be added with reasonable ease.
859 The syntax for socket sources and targets is:
887 The syntax of the source and target addresses depend on the address
888 types, which are described below. The default address type, if no
893 Socket sources support options; socket targets do not. The source
894 options provided are:
900 Limits the number of simultaneous connections to this socket to the
902 given. The default is 256.
909 Whether to log incoming connections. If
911 (the default) incoming connections are logged, together with information
912 about the client (where available) and whether the connection was
913 accepted or refused. If
915 log messages are not generated.
918 Address types also provide their own options.
920 .SS "The `inet' socket address type"
923 address type provides access to TCP ports. The
925 source and target addresses have the following syntax:
953 may be given as a port number or a service name from the
955 file (or YP map if you do that sort of thing). A
957 may be a textual hostname or a numerical IP address.
961 source address accepts the following options:
963 .BR socket.inet. [ allow | deny ]
969 Adds an entry to the source's access control list. If only one
971 is given, the entry applies only to that address; if two are given, the
972 first is a network address and the second is a netmask either in
973 dotted-quad format or a simple number of bits (e.g.,
977 mean the same), and the entry applies to any address which, when masked
978 by the netmask, is equal to the masked network address.
981 control rules are examined in the order: local entries first, then
982 global ones, each in the order given in the configuration file. The
983 first matching entry is used. If no entries match, the behaviour is the
985 of the last entry tried. If there are no entries defined, the default
986 is to allow all clients.
989 .SS "The `unix' socket address type"
992 address type allows access to Unix-domain sockets. The syntax for
994 source and target addresses is like this:
1010 source address accepts
1012 options to control the attributes of the socket file created. Sockets
1015 exits normally (which it will do if it runs out of sources or
1016 connections, or if killed by SIGINT or SIGTERM).
1018 To forward the local port 25 to a main mail server:
1020 from 25 to mailserv:25
1022 To attach a fortune server to a Unix-domain socket:
1024 from unix:/tmp/fortunes
1025 to exec [/usr/games/fortune] { user nobody }
1027 To fetch a fortune from the server:
1029 from file stdin, stdout to unix:/tmp/fortunes
1034 from stdin, null to null, stdout
1037 .\"--------------------------------------------------------------------------
1040 The syntax for IP addresses and filenames is nasty. The requirement
1041 that textual permissions strings be quoted is probably nastier.
1043 IPv6 is not supported yet. It's probably not a major piece of work to
1046 Please inform me of any security problems you think you've identified in
1047 this program. I take security very seriously, and I will fix security
1048 holes as a matter of priority when I find out about them. I will be
1049 annoyed if I have to read about problems on Bugtraq because they weren't
1052 .\"--------------------------------------------------------------------------
1055 Mark Wooding, <mdw@nsict.org>
1057 .\"----- That's all, folks --------------------------------------------------
~mdw / fwd / blob