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
8 .\" the Free Software Foundation; either version 2, or (at your option)
11 .\" This program is distributed in the hope that it will be useful,
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.
16 .\" You should have received a copy of the GNU General Public License
17 .\" along with this program; if not, write to the Free Software Foundation,
18 .\" Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
20 .TH AUTHBIND 1 "30th August 1998" "Debian Project" "Debian Linux manual"
22 authbind \- bind sockets to privileged ports without root
25 .RI [ options "] " program " [" argument " ...]"
28 allows a program which does not or should not run as root to bind to
29 low-numbered ports in a controlled way.
31 You must invoke the program using
32 .BR authbind ". " authbind
33 will set up some environment variables, including an
35 which will allow the program (including any subprocesses it may run)
36 to bind to low-numbered (<512) ports if the system is configured to
43 arranges for only the program which it directly invokes to be affected
44 by its special version of
48 then all programs which that program invokes directly or indirectly
49 will be affected, so long as they do not unset the environment
56 to affect programs which are
58 deep in the calling graph. The default is
61 Access to low numbered ports is controlled by permissions and contents
62 of files in a configuration area,
66 .BI /etc/authbind/byport/ port
67 is tested. If this file is accessible for execution to the calling
70 then binding to the port is authorised. If the file can be seen not
71 to exist (the existence check returns
73 then further tests will be used to find authorisation; otherwise,
74 binding is not authorised, and the
76 call will return with the
82 .RI ( "Permission denied" ).
84 Secondly, if that test fails to resolve the matter,
85 .BI /etc/authbind/byaddr/ addr , port
86 (any protocol) or failing that
87 .BI /etc/authbind/byaddr/ addr : port
89 is tested, in the same manner as above. Here
95 is the (local) TCP or UDP port number, expressed as an unsigned
96 integer in the minimal non-zero number of digits.
98 Thirdly, for IPv6 only: since the textual representation from
100 is complicated to predict, a variant of
102 is also tested which does not use the double colon abbreviation:
103 each 16-byte chunk expressed in the minimal nonzero number
104 of hex digits (i.e. with leading zeroes removed), the chunks
105 being separated by colons as is conventional.
107 Fourthly, if the question is still unresolved, the file
108 .BI /etc/authbind/byuid/ uid
109 will be opened and read. If the file does not exist then the binding
110 is not authorised and
114 .RI ( "Operation not permitted" ", or " "Not owner" ).
115 If the file does exist it will be searched for a line of the form
117 .IR addrmin [\fB\-\fR addrmax ]\fB,\fR portmin [\fB\-\fR portmax ]
118 .IR addr [\fB/\fR length ]\fB,\fR portmin [\fB\-\fR portmax ]
119 .IB addr4 / length : portmin , portmax
121 matching the request.
122 The first form requires that the address lies in the
123 relevant range (inclusive at both ends).
124 The second and third forms require that the initial
128 match those in the proposed
130 call. The third form is only available for IPv4 since IPv6 addresses
132 Addresses in the byuid file can
133 be in any form acceptable to inet_pton. In all cases
134 the proposed port number must lie is in the inclusive range
135 specified. If such a line is found then the binding is authorised.
136 Otherwise it is not, and
140 .RI ( "No such file or directory" ).
142 If a read error occurs, or the directory
144 cannot be accessed, then not only will
146 fail, but an error message will be printed to stderr. Unrecognised
148 .BI /etc/authbind/byuid/ uid
149 files are silently ignored, as are lines whose
151 has non-zero bits more than
153 from the top or where some
158 So for example an attempt by uid 432
159 to bind to port 80 of address [2620:106:e002:f00f::21]
160 would result in authbind calling
164 .B /etc/authbind/byport/80
166 .B /etc/authbind/byaddr/2620:106:e002:f00f::21,80
168 .B /etc/authbind/byaddr/2620:106:e002:f00f:0:0:0:21,80
170 If none of these files exist, authbind will read
172 .B /etc/authbind/byuid/432
174 and search for a line to permit
175 the relevant access; examples of lines which would do so are:
177 .B 2620:106:e002:f00f::21,80
182 Authorising binding to ports from 512 to 1023 inclusive is
183 not recommended. Some protocols (including some versions of NFS)
184 authorise clients by seeing that they are using a port number in this
185 range. So by authorising a program to be a server for such a port,
186 you are also authorising it to impersonate the whole host for those
189 To make sure that this isn't done by accident,
190 if the port number requested is in the range 512-1023, authbind
191 will expect the permission files to have an additional
193 at the start of their leafname.
195 The shared library loaded using
199 system call. When a program invoked via
203 to bind a socket to a low-numbered TCP/IP port, and if the program
204 doesn't already have an effective uid of 0, the version of
208 forks and executes a setuid-root helper program. For non-TCP/IP
209 sockets, high-numbered ports, or programs which are already root,
211 passes the call to the original
213 system call, which is found using
219 Usually the normal C error handling mechanisms apply. If
221 cannot find the program it has been asked to execute it will print a
222 message to stderr and exit with code 255.
224 The helper program usually reports back to the shared library with an
225 exit status containing an
227 value which encodes whether the
229 was permitted and successful. This will be returned to the calling
230 program in the usual way.
232 In the case of apparent configuration or other serious errors the
233 library and/or the helper program may cause messages to be printed to
234 the program's stderr, was well as returning -1 from
238 currently only supports IPv4 and IPv6 sockets.
239 Programs which open other kinds
240 of sockets will not benefit from
242 but it won't get in their way.
248 installation specific to a particular C library. This version is for
249 GNU/Linux libc6 (glibc2).
252 may not operate correctly with multithreaded programs. It is
253 inherently very difficult (if not impossible) to perform the kind of
254 trickery that authbind does while preventing all undesirable
255 interactions between authbind's activities and those of (say) a
256 threading runtime system.
258 It is quite possible that
260 and other programs and facilities which use
262 may interfere with each other, causing unpredictable behaviour or even
265 is known sometimes not to work correctly with
267 for example (even supposing it could be determined what `correctly'
268 means in this context).
271 is ineffective with setuid programs, because they do not honour
273 references outside the system directories, for security reasons. (In
274 fact, setuid programs should not honour
277 Of course a setuid-root program does not need
279 but it might be useful to apply it to program which are setuid to
280 another user or setgid. If the author or builder of such a programs
281 wishes it to use authbind they could have it load the
283 library explicitly rather than via
286 Some programs may have trouble because
288 spawns a child process `under their feet', causing (for example) a
292 signal to be delivered. Unfortunately the Unix API does not make
293 it possible to deal with this problem in a sane way.
295 The access control configuration scheme is somewhat strange.
296 .SH FILES AND ENVIRONMENT VARIABLES
298 .I /usr/lib/authbind/libauthbind.so.1.0
299 The shared library which
301 causes to be loaded using
303 and which actually implements the diversion of
305 to an external program.
308 The variable used by the dynamic linker when starting dynamically
309 linked programs and deciding which shared libraries to load and
312 program to allow it to override the usual meaning of
318 to use its value as the path to the shared library to put in
320 instead of the compiled-in value. In any case, unless
324 will set this variable to the name of the library actually added to
326 so that the library can find and remove the right entry.
329 This variable is set by
331 to the number of levels left from the
335 option, minus one. It is decremented during
337 by the library on each program call, and the library will remove
340 when it reaches zero. The special value
347 .BR authbind\-helper (8),
352 and this manpage were written by Ian Jackson. They are
353 Copyright (C)1998,2012
354 by him and released under the GNU General Public Licence; there is NO
356 .B /usr/doc/authbind/copyright
358 .B /usr/doc/copyright/GPL