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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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
94 Thirdly, if the question is still unresolved, the file
95 .BI /etc/authbind/byuid/ uid
96 will be opened and read. If the file does not exist then the binding
101 .RI ( "Operation not permitted" ", or " "Not owner" ).
102 If the file does exist it will be searched for a line of the form
104 .IR addrmin [\fB\-\fR addrmax ]\fB,\fR portmin \fB\-\fR portmax
105 .IB addr4 / length : portmin , portmax
107 matching the request.
108 The first form requires that the address lies in the
109 relevant range (inclusive at both ends).
110 The second form requires that the initial
114 match those in the proposed
116 call and is only available for IPv4.
118 be in any form acceptable to inet_pton. In both cases
119 the proposed port number must lie is in the inclusive range
120 specified. If such a line is found then the binding is authorised.
121 Otherwise it is not, and
125 .RI ( "No such file or directory" ).
130 is the (local) TCP or UDP port number, expressed as an unsigned
131 integer in the minimal non-zero number of digits, and
133 If a read error occurs, or the directory
135 cannot be accessed, then not only will
137 fail, but an error message will be printed to stderr. Unrecognised
139 .BI /etc/authbind/byuid/ uid
140 files are silently ignored (as are lines whose
142 has non-zero bits more than
144 from the top) or where
149 Authorising binding to ports from 512 to 1023 inclusive is
150 not recommended. Some protocols (including some versions of NFS)
151 authorise clients by seeing that they are using a port number in this
152 range. So by authorising a program to be a server for such a port,
153 you are also authorising it to impersonate the whole host for those
154 protocols. To make sure that this isn't done by accident,
155 if the port number requested is in the range 512-1023, authbind
156 will expect the permission files to have an additional
158 at the start of their leafname.
160 The shared library loaded using
164 system call. When a program invoked via
168 to bind a socket to a low-numbered TCP/IP port, and if the program
169 doesn't already have an effective uid of 0, the version of
173 forks and executes a setuid-root helper program. For non-TCP/IP
174 sockets, high-numbered ports, or programs which are already root,
176 passes the call to the original
178 system call, which is found using
184 Usually the normal C error handling mechanisms apply. If
186 cannot find the program it has been asked to execute it will print a
187 message to stderr and exit with code 255.
189 The helper program usually reports back to the shared library with an
190 exit status containing an
192 value which encodes whether the
194 was permitted and successful. This will be returned to the calling
195 program in the usual way.
197 In the case of apparent configuration or other serious errors the
198 library and/or the helper program may cause messages to be printed to
199 the program's stderr, was well as returning -1 from
203 currently only supports IPv4 and IPv6 sockets.
204 Programs which open other kinds
205 of sockets will not benefit from
207 but it won't get in their way.
213 installation specific to a particular C library. This version is for
214 GNU/Linux libc6 (glibc2).
217 may not operate correctly with multithreaded programs. It is
218 inherently very difficult (if not impossible) to perform the kind of
219 trickery that authbind does while preventing all undesirable
220 interactions between authbind's activities and those of (say) a
221 threading runtime system.
223 It is quite possible that
225 and other programs and facilities which use
227 may interfere with each other, causing unpredictable behaviour or even
230 is known sometimes not to work correctly with
232 for example (even supposing it could be determined what `correctly'
233 means in this context).
236 is ineffective with setuid programs, because they do not honour
238 references outside the system directories, for security reasons. (In
239 fact, setuid programs should not honour
242 Of course a setuid-root program does not need
244 but it might be useful to apply it to program which are setuid to
245 another user or setgid. If the author or builder of such a programs
246 wishes it to use authbind they could have it load the
248 library explicitly rather than via
251 Some programs may have trouble because
253 spawns a child process `under their feet', causing (for example) a
257 signal to be delivered. Unfortunately the Unix API does not make
258 it possible to deal with this problem in a sane way.
260 The access control configuration scheme is somewhat strange.
261 .SH FILES AND ENVIRONMENT VARIABLES
263 .I /usr/lib/authbind/libauthbind.so.1.0
264 The shared library which
266 causes to be loaded using
268 and which actually implements the diversion of
270 to an external program.
273 The variable used by the dynamic linker when starting dynamically
274 linked programs and deciding which shared libraries to load and
277 program to allow it to override the usual meaning of
283 to use its value as the path to the shared library to put in
285 instead of the compiled-in value. In any case, unless
289 will set this variable to the name of the library actually added to
291 so that the library can find and remove the right entry.
294 This variable is set by
296 to the number of levels left from the
300 option, minus one. It is decremented during
302 by the library on each program call, and the library will remove
305 when it reaches zero. The special value
312 .BR authbind\-helper (8),
317 and this manpage were written by Ian Jackson. They are
318 Copyright (C)1998,2012
319 by him and released under the GNU General Public Licence; there is NO
321 .B /usr/doc/authbind/copyright
323 .B /usr/doc/copyright/GPL