1 .\" Hey, Emacs! This is an -*- nroff -*- source file.
2 .\" Authors: Ian Jackson
4 .\" authbind is Copyright (C) 1998 Ian Jackson
6 .\" This program is free software; you can redistribute it and/or modify
7 .\" it under the terms of the GNU General Public License as published by
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12 .\" but WITHOUT ANY WARRANTY; without even the implied warranty of
13 .\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
14 .\" GNU General Public License for more details.
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22 .TH AUTHBIND 1 "30th August 1998" "Debian Project" "Debian Linux manual"
24 authbind \- bind sockets to privileged ports without root
27 .RI [ options "] " program " [" argument " ...]"
30 allows a program which does not or should not run as root to bind to
31 low-numbered ports in a controlled way.
33 You must invoke the program using
34 .BR authbind ". " authbind
35 will set up some environment variables, including an
37 which will allow the program (including any subprocesses it may run)
38 to bind to low-numbered (<512) ports if the system is configured to
45 arranges for only the program which it directly invokes to be affected
46 by its special version of
50 then all programs which that program invokes directly or indirectly
51 will be affected, so long as they do not unset the environment
58 to affect programs which are
60 deep in the calling graph. The default is
63 Access to low numbered ports is controlled by permissions and contents
64 of files in a configuration area,
68 .BI /etc/authbind/byport/ port
69 is tested. If this file is accessible for execution to the calling
72 then binding to the port is authorised. If the file can be seen not
73 to exist (the existence check returns
75 then further tests will be used to find authorisation; otherwise,
76 binding is not authorised, and the
78 call will return with the
84 .RI ( "Permission denied" ).
86 Secondly, if that test fails to resolve the matter,
87 .BI /etc/authbind/byaddr/ addr , port
88 (any protocol) or failing that
89 .BI /etc/authbind/byaddr/ addr : port
91 is tested, in the same manner as above. (Here
96 Thirdly, if the question is still unresolved, the file
97 .BI /etc/authbind/byuid/ uid
98 will be opened and read. If the file does not exist then the binding
103 .RI ( "Operation not permitted" ", or " "Not owner" ).
104 If the file does exist it will be searched for a line of the form
106 .IR addrmin [\fB\-\fR addrmax ]\fB,\fR portmin \fB\-\fR portmax
107 .IB addr4 / length : portmin , portmax
109 matching the request.
110 The first form requires that the address lies in the
111 relevant range (inclusive at both ends).
112 The second form requires that the initial
116 match those in the proposed
118 call and is only available for IPv4.
120 be in any form acceptable to inet_pton. In both cases
121 the proposed port number must lie is in the inclusive range
122 specified. If such a line is found then the binding is authorised.
123 Otherwise it is not, and
127 .RI ( "No such file or directory" ).
132 is the (local) TCP or UDP port number, expressed as an unsigned
133 integer in the minimal non-zero number of digits, and
135 If a read error occurs, or the directory
137 cannot be accessed, then not only will
139 fail, but an error message will be printed to stderr. Unrecognised
141 .BI /etc/authbind/byuid/ uid
142 files are silently ignored (as are lines whose
144 has non-zero bits more than
146 from the top) or where
151 Authorising binding to ports from 512 to 1023 inclusive is
152 not recommended. Some protocols (including some versions of NFS)
153 authorise clients by seeing that they are using a port number in this
154 range. So by authorising a program to be a server for such a port,
155 you are also authorising it to impersonate the whole host for those
156 protocols. To make sure that this isn't done by accident,
157 if the port number requested is in the range 512-1023, authbind
158 will expect the permission files to have an additional
160 at the start of their leafname.
162 The shared library loaded using
166 system call. When a program invoked via
170 to bind a socket to a low-numbered TCP/IP port, and if the program
171 doesn't already have an effective uid of 0, the version of
175 forks and executes a setuid-root helper program. For non-TCP/IP
176 sockets, high-numbered ports, or programs which are already root,
178 passes the call to the original
180 system call, which is found using
186 Usually the normal C error handling mechanisms apply. If
188 cannot find the program it has been asked to execute it will print a
189 message to stderr and exit with code 255.
191 The helper program usually reports back to the shared library with an
192 exit status containing an
194 value which encodes whether the
196 was permitted and successful. This will be returned to the calling
197 program in the usual way.
199 In the case of apparent configuration or other serious errors the
200 library and/or the helper program may cause messages to be printed to
201 the program's stderr, was well as returning -1 from
205 currently only supports IPv4 and IPv6 sockets.
206 Programs which open other kinds
207 of sockets will not benefit from
209 but it won't get in their way.
215 installation specific to a particular C library. This version is for
216 GNU/Linux libc6 (glibc2).
219 may not operate correctly with multithreaded programs. It is
220 inherently very difficult (if not impossible) to perform the kind of
221 trickery that authbind does while preventing all undesirable
222 interactions between authbind's activities and those of (say) a
223 threading runtime system.
225 It is quite possible that
227 and other programs and facilities which use
229 may interfere with each other, causing unpredictable behaviour or even
232 is known sometimes not to work correctly with
234 for example (even supposing it could be determined what `correctly'
235 means in this context).
238 is ineffective with setuid programs, because they do not honour
240 references outside the system directories, for security reasons. (In
241 fact, setuid programs should not honour
244 Of course a setuid-root program does not need
246 but it might be useful to apply it to program which are setuid to
247 another user or setgid. If the author or builder of such a programs
248 wishes it to use authbind they could have it load the
250 library explicitly rather than via
253 Some programs may have trouble because
255 spawns a child process `under their feet', causing (for example) a
259 signal to be delivered. Unfortunately the Unix API does not make
260 it possible to deal with this problem in a sane way.
262 The access control configuration scheme is somewhat strange.
263 .SH FILES AND ENVIRONMENT VARIABLES
265 .I /usr/lib/authbind/libauthbind.so.1.0
266 The shared library which
268 causes to be loaded using
270 and which actually implements the diversion of
272 to an external program.
275 The variable used by the dynamic linker when starting dynamically
276 linked programs and deciding which shared libraries to load and
279 program to allow it to override the usual meaning of
285 to use its value as the path to the shared library to put in
287 instead of the compiled-in value. In any case, unless
291 will set this variable to the name of the library actually added to
293 so that the library can find and remove the right entry.
296 This variable is set by
298 to the number of levels left from the
302 option, minus one. It is decremented during
304 by the library on each program call, and the library will remove
307 when it reaches zero. The special value
314 .BR authbind\-helper (8),
319 and this manpage were written by Ian Jackson. They are
320 Copyright (C)1998,2012
321 by him and released under the GNU General Public Licence; there is NO
323 .B /usr/doc/authbind/copyright
325 .B /usr/doc/copyright/GPL