authbind - bind sockets to privileged ports without root
authbind [options] program [argument ...]
authbind allows a program which does not or should not run as root to
bind to low-numbered ports in a controlled way.
You must invoke the program using authbind. authbind will set up some
environment variables, including an LD_PRELOAD, which will allow the
program (including any subprocesses it may run) to bind to low-numbered
(<512) ports if the system is configured to allow this.
--deep Normally, authbind arranges for only the program which it
directly invokes to be affected by its special version of
bind(2). If you specify --deep then all programs which that
program invokes directly or indirectly will be affected, so long
as they do not unset the environment variables set up by
Causes authbind to affect programs which are levels deep in the
calling graph. The default is --depth 1.
Access to low numbered ports is controlled by permissions and contents
of files in a configuration area, /etc/authbind.
Firstly, /etc/authbind/byport/port is tested. If this file is
accessible for execution to the calling user, according to access(2),
then binding to the port is authorised. If the file can be seen not to
exist (the existence check returns ENOENT) then further tests will be
used to find authorisation; otherwise, binding is not authorised, and
the bind call will return with the errno value from the access(2) call,
usually EACCES (Permission denied).
Secondly, if that test fails to resolve the matter,
/etc/authbind/byaddr/addr:port is tested, in the same manner as above.
Thirdly, if the question is still unresolved, the file
/etc/authbind/byuid/uid will be opened and read. If the file does not
exist then the binding is not authorised and bind will return EPERM
(Operation not permitted, or Not owner). If the file does exist it
will be searched for a line of the form
matching the request (ie, the initial length bits of addr match those
in the proposed bind call, and the proposed port number lies is in the
inclusive range specified. If such a line is found then the binding is
authorised. Otherwise it is not, and bind will fail with ENOENT (No
such file or directory).
In each case above,
port is the (local) TCP or UDP port number, expressed as an unsigned
integer in the minimal non-zero number of digits, and
addr is the (local) IP address, as a dotted quad.
If a read error occurs, or the directory /etc/authbind cannot be
accessed, then not only will bind fail, but an error message will be
printed to stderr. Unrecognised lines in /etc/authbind/byuid/uid files
are silently ignored (as are lines whose addr has non-zero bits more
than length from the top).
The shared library loaded using LD_PRELOAD overrides the bind(2) system
call. When a program invoked via authbind calls bind to bind a socket
to a low-numbered TCP/IP port, and if the program doesn’t already have
an effective uid of 0, the version of bind supposed by authbind forks
and executes a setuid-root helper program. For non-TCP/IP sockets,
high-numbered ports, or programs which are already root, authbind
passes the call to the original bind(2) system call, which is found
using dlsym(3) with the handle RTLD_NEXT.
Usually the normal C error handling mechanisms apply. If authbind
cannot find the program it has been asked to execute it will print a
message to stderr and exit with code 255.
The helper program usually reports back to the shared library with an
exit status containing an errno value which encodes whether the bind
was permitted and successful. This will be returned to the calling
program in the usual way.
In the case of apparent configuration or other serious errors the
library and/or the helper program may cause messages to be printed to
the program’s stderr, was well as returning -1 from bind.
authbind currently only supports IPv4 sockets. Programs which open
other kinds of sockets will not benefit from authbind, but it won’t get
in their way.
The use of LD_PRELOAD makes an authbind installation specific to a
particular C library. This version is for GNU/Linux libc6 (glibc2).
authbind may not operate correctly with multithreaded programs. It is
inherently very difficult (if not impossible) to perform the kind of
trickery that authbind does while preventing all undesirable
interactions between authbind’s activities and those of (say) a
threading runtime system.
It is quite possible that authbind and other programs and facilities
which use LD_PRELOAD may interfere with each other, causing
unpredictable behaviour or even core dumps. authbind is known
sometimes not to work correctly with fakeroot, for example (even
supposing it could be determined what ‘correctly’ means in this
authbind is ineffective with setuid programs, because they do not
honour LD_PRELOAD references outside the system directories, for
security reasons. (In fact, setuid programs should not honour
LD_PRELOAD at all.) Of course a setuid-root program does not need
authbind, but it might be useful to apply it to program which are
setuid to another user or setgid. If the author or builder of such a
programs wishes it to use authbind they could have it load the
libauthbind library explicitly rather than via LD_PRELOAD.
Some badly-written programs may have trouble because authbind spawns a
child process ‘under their feet’, causing (for example) a fork(2) to
happen and SIGCHLD signal to be delivered. Programs should not rely on
standard libraries not doing these things.
Ports from 512 to 1023 inclusive cannot be used with authbind because
that would create a security hole, in conjection with rshd.
The access control configuration scheme is somewhat strange.
FILES AND ENVIRONMENT VARIABLES
The shared library which authbind causes to be loaded using
LD_PRELOAD, and which actually implements the diversion of
bind(2) to an external program.
The variable used by the dynamic linker when starting
dynamically linked programs and deciding which shared libraries
to load and modifed by the authbind program to allow it to
override the usual meaning of bind(2).
If set, forces authbind to use its value as the path to the
shared library to put in LD_PRELOAD, instead of the compiled-in
value. In any case, unless --deep was specified, authbind will
set this variable to the name of the library actually added to
LD_PRELOAD, so that the library can find and remove the right
This variable is set by authbind to the number of levels left
from the --depth or --deep option, minus one. It is decremented
during _init by the library on each program call, and the
library will remove itself from the LD_PRELOAD when it reaches
zero. The special value y means --deep was specified.
bind(2), authbind-helper(8), dlsym(3), ld.so(8)
authbind and this manpage were written by Ian Jackson. They are
Copyright (C)1998 by him and released under the GNU General Public
Licence; there is NO WARRANTY. See /usr/doc/authbind/copyright and
/usr/doc/copyright/GPL for details.