1 .\" Hey, Emacs! This is an -*- nroff -*- source file.
2 .\" Authors: Ian Jackson
4 .\" authbind is Copyright (C) 1998 Ian Jackson
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7 .\" it under the terms of the GNU General Public License as published by
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14 .\" GNU General Public License for more details.
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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
93 Since this is not completely predictable for IPv6,
96 is also tested which does not contain any ommitted zeroes or colons.
98 Thirdly, if the question is still unresolved, the file
99 .BI /etc/authbind/byuid/ uid
100 will be opened and read. If the file does not exist then the binding
101 is not authorised and
105 .RI ( "Operation not permitted" ", or " "Not owner" ).
106 If the file does exist it will be searched for a line of the form
108 .IR addrmin [\fB\-\fR addrmax ]\fB,\fR portmin [\fB\-\fR portmax ]
109 .IR addr [\fB/\fR length ]\fB,\fR portmin [\fB\-\fR portmax ]
110 .IB addr4 / length : portmin , portmax
112 matching the request.
113 The first form requires that the address lies in the
114 relevant range (inclusive at both ends).
115 The second and third forms require that the initial
119 match those in the proposed
121 call. The third form is only available for IPv4 since IPv6 addresses
123 Addresses in the byuid file can
124 be in any form acceptable to inet_pton. In all cases
125 the proposed port number must lie is in the inclusive range
126 specified. If such a line is found then the binding is authorised.
127 Otherwise it is not, and
131 .RI ( "No such file or directory" ).
136 is the (local) TCP or UDP port number, expressed as an unsigned
137 integer in the minimal non-zero number of digits, and
139 If a read error occurs, or the directory
141 cannot be accessed, then not only will
143 fail, but an error message will be printed to stderr. Unrecognised
145 .BI /etc/authbind/byuid/ uid
146 files are silently ignored (as are lines whose
148 has non-zero bits more than
150 from the top) or where
155 Authorising binding to ports from 512 to 1023 inclusive is
156 not recommended. Some protocols (including some versions of NFS)
157 authorise clients by seeing that they are using a port number in this
158 range. So by authorising a program to be a server for such a port,
159 you are also authorising it to impersonate the whole host for those
160 protocols. To make sure that this isn't done by accident,
161 if the port number requested is in the range 512-1023, authbind
162 will expect the permission files to have an additional
164 at the start of their leafname.
166 The shared library loaded using
170 system call. When a program invoked via
174 to bind a socket to a low-numbered TCP/IP port, and if the program
175 doesn't already have an effective uid of 0, the version of
179 forks and executes a setuid-root helper program. For non-TCP/IP
180 sockets, high-numbered ports, or programs which are already root,
182 passes the call to the original
184 system call, which is found using
190 Usually the normal C error handling mechanisms apply. If
192 cannot find the program it has been asked to execute it will print a
193 message to stderr and exit with code 255.
195 The helper program usually reports back to the shared library with an
196 exit status containing an
198 value which encodes whether the
200 was permitted and successful. This will be returned to the calling
201 program in the usual way.
203 In the case of apparent configuration or other serious errors the
204 library and/or the helper program may cause messages to be printed to
205 the program's stderr, was well as returning -1 from
209 currently only supports IPv4 and IPv6 sockets.
210 Programs which open other kinds
211 of sockets will not benefit from
213 but it won't get in their way.
219 installation specific to a particular C library. This version is for
220 GNU/Linux libc6 (glibc2).
223 may not operate correctly with multithreaded programs. It is
224 inherently very difficult (if not impossible) to perform the kind of
225 trickery that authbind does while preventing all undesirable
226 interactions between authbind's activities and those of (say) a
227 threading runtime system.
229 It is quite possible that
231 and other programs and facilities which use
233 may interfere with each other, causing unpredictable behaviour or even
236 is known sometimes not to work correctly with
238 for example (even supposing it could be determined what `correctly'
239 means in this context).
242 is ineffective with setuid programs, because they do not honour
244 references outside the system directories, for security reasons. (In
245 fact, setuid programs should not honour
248 Of course a setuid-root program does not need
250 but it might be useful to apply it to program which are setuid to
251 another user or setgid. If the author or builder of such a programs
252 wishes it to use authbind they could have it load the
254 library explicitly rather than via
257 Some programs may have trouble because
259 spawns a child process `under their feet', causing (for example) a
263 signal to be delivered. Unfortunately the Unix API does not make
264 it possible to deal with this problem in a sane way.
266 The access control configuration scheme is somewhat strange.
267 .SH FILES AND ENVIRONMENT VARIABLES
269 .I /usr/lib/authbind/libauthbind.so.1.0
270 The shared library which
272 causes to be loaded using
274 and which actually implements the diversion of
276 to an external program.
279 The variable used by the dynamic linker when starting dynamically
280 linked programs and deciding which shared libraries to load and
283 program to allow it to override the usual meaning of
289 to use its value as the path to the shared library to put in
291 instead of the compiled-in value. In any case, unless
295 will set this variable to the name of the library actually added to
297 so that the library can find and remove the right entry.
300 This variable is set by
302 to the number of levels left from the
306 option, minus one. It is decremented during
308 by the library on each program call, and the library will remove
311 when it reaches zero. The special value
318 .BR authbind\-helper (8),
323 and this manpage were written by Ian Jackson. They are
324 Copyright (C)1998,2012
325 by him and released under the GNU General Public Licence; there is NO
327 .B /usr/doc/authbind/copyright
329 .B /usr/doc/copyright/GPL