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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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 .IB addr4 / length : portmin , portmax
111 matching the request.
112 The first form requires that the address lies in the
113 relevant range (inclusive at both ends).
114 The second form requires that the initial
118 match those in the proposed
120 call and is only available for IPv4.
122 be in any form acceptable to inet_pton. In both cases
123 the proposed port number must lie is in the inclusive range
124 specified. If such a line is found then the binding is authorised.
125 Otherwise it is not, and
129 .RI ( "No such file or directory" ).
134 is the (local) TCP or UDP port number, expressed as an unsigned
135 integer in the minimal non-zero number of digits, and
137 If a read error occurs, or the directory
139 cannot be accessed, then not only will
141 fail, but an error message will be printed to stderr. Unrecognised
143 .BI /etc/authbind/byuid/ uid
144 files are silently ignored (as are lines whose
146 has non-zero bits more than
148 from the top) or where
153 Authorising binding to ports from 512 to 1023 inclusive is
154 not recommended. Some protocols (including some versions of NFS)
155 authorise clients by seeing that they are using a port number in this
156 range. So by authorising a program to be a server for such a port,
157 you are also authorising it to impersonate the whole host for those
158 protocols. To make sure that this isn't done by accident,
159 if the port number requested is in the range 512-1023, authbind
160 will expect the permission files to have an additional
162 at the start of their leafname.
164 The shared library loaded using
168 system call. When a program invoked via
172 to bind a socket to a low-numbered TCP/IP port, and if the program
173 doesn't already have an effective uid of 0, the version of
177 forks and executes a setuid-root helper program. For non-TCP/IP
178 sockets, high-numbered ports, or programs which are already root,
180 passes the call to the original
182 system call, which is found using
188 Usually the normal C error handling mechanisms apply. If
190 cannot find the program it has been asked to execute it will print a
191 message to stderr and exit with code 255.
193 The helper program usually reports back to the shared library with an
194 exit status containing an
196 value which encodes whether the
198 was permitted and successful. This will be returned to the calling
199 program in the usual way.
201 In the case of apparent configuration or other serious errors the
202 library and/or the helper program may cause messages to be printed to
203 the program's stderr, was well as returning -1 from
207 currently only supports IPv4 and IPv6 sockets.
208 Programs which open other kinds
209 of sockets will not benefit from
211 but it won't get in their way.
217 installation specific to a particular C library. This version is for
218 GNU/Linux libc6 (glibc2).
221 may not operate correctly with multithreaded programs. It is
222 inherently very difficult (if not impossible) to perform the kind of
223 trickery that authbind does while preventing all undesirable
224 interactions between authbind's activities and those of (say) a
225 threading runtime system.
227 It is quite possible that
229 and other programs and facilities which use
231 may interfere with each other, causing unpredictable behaviour or even
234 is known sometimes not to work correctly with
236 for example (even supposing it could be determined what `correctly'
237 means in this context).
240 is ineffective with setuid programs, because they do not honour
242 references outside the system directories, for security reasons. (In
243 fact, setuid programs should not honour
246 Of course a setuid-root program does not need
248 but it might be useful to apply it to program which are setuid to
249 another user or setgid. If the author or builder of such a programs
250 wishes it to use authbind they could have it load the
252 library explicitly rather than via
255 Some programs may have trouble because
257 spawns a child process `under their feet', causing (for example) a
261 signal to be delivered. Unfortunately the Unix API does not make
262 it possible to deal with this problem in a sane way.
264 The access control configuration scheme is somewhat strange.
265 .SH FILES AND ENVIRONMENT VARIABLES
267 .I /usr/lib/authbind/libauthbind.so.1.0
268 The shared library which
270 causes to be loaded using
272 and which actually implements the diversion of
274 to an external program.
277 The variable used by the dynamic linker when starting dynamically
278 linked programs and deciding which shared libraries to load and
281 program to allow it to override the usual meaning of
287 to use its value as the path to the shared library to put in
289 instead of the compiled-in value. In any case, unless
293 will set this variable to the name of the library actually added to
295 so that the library can find and remove the right entry.
298 This variable is set by
300 to the number of levels left from the
304 option, minus one. It is decremented during
306 by the library on each program call, and the library will remove
309 when it reaches zero. The special value
316 .BR authbind\-helper (8),
321 and this manpage were written by Ian Jackson. They are
322 Copyright (C)1998,2012
323 by him and released under the GNU General Public Licence; there is NO
325 .B /usr/doc/authbind/copyright
327 .B /usr/doc/copyright/GPL