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 authbind.

--depth levels

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

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).

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 context).

authbind is ineffective with setuid programs, because they do not honour LD_PRELOAD for security reasons. 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.

/usr/lib/authbind/libauthbind.so.1.0

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.

LD_PRELOAD

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).

AUTHBIND_LIB

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 entry.

AUTHBIND_LEVELS

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.