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.\" Authors: Ian Jackson
.\" 
.\" authbind is Copyright (C) 1998 Ian Jackson
.\"
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.\"
.TH AUTHBIND 1 "30th August 1998" "Debian Project" "Debian Linux manual"
.SH NAME 
authbind \- bind sockets to privileged ports without root
.SH SYNOPSIS
.BI authbind " program"
.RI [ argument " ...]"
.SH DESCRIPTION
.B authbind
allows a program which does not or should not run as root to bind to
low-numbered ports in a controlled way.
.PP
You must invoke the program using
.BR authbind ".  " authbind
will set up some environment variables, including an
.BR LD_PRELOAD ,
which will allow the program (including any subprocesses it may run)
to bind to low-numbered (<1024) ports if the system is configured to
allow this.
.SH ACCESS CONTROL
Access to low numbered ports is controlled by permissions and contents
of files in a configuration area,
.BR /etc/authbind .
.PP
Firstly,
.BI /etc/authbind/byport/ port
is tested.  If this file is accessible for execution to the calling
user, according to
.BR access (2),
then binding to the port is authorised.  If the file can be seen not
to exist (the existence check returns
.BR ENOENT )
then further tests will be used to find authorisation; otherwise,
binding is not authorised, and the
.B bind
call will return with the
.B errno
value from the
.BR access (2)
call, usually
.B EACCES
.RI ( "Permission denied" ).
.PP
Secondly, if that test fails to resolve the matter,
.BI /etc/authbind/byaddr/ addr : port
is tested, in the same manner as above.
.PP
Thirdly, if the question is still unresolved, the file
.BI /etc/authbind/byuid/ uid
will be opened and read.  If the file does not exist then the binding
is not authorised and
.B bind
will return
.B EPERM
.RI ( "Operation not permitted" ", or " "Not owner" ).
If the file does exist it will be searched for a line of the form
.nf
.IB		addr / length : min\-port , max\-port
.fi
matching the request (ie, the initial
.I length
bits of
.I addr
match those in the proposed
.B 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
.B bind
will fail with
.B ENOENT
.RI ( "No such file or directory" ).
.PP
In each case above,
.TP
.I port
is the (local) TCP or UDP port number, expressed as an unsigned
integer in the minimal non-zero number of digits, and
.TP
.I addr
is the (local) IP address, as a dotted quad.
.PP
If a read error occurs, or the directory
.B /etc/authbind
cannot be accessed, then not only will
.B bind
fail, but an error message will be printed to stderr.  Unrecognised
lines in
.BI /etc/authbind/byuid/ uid
files are silently ignored (as are lines whose
.I addr
has non-zero bits more than
.I length
from the top).
.SH MECHANISM
The shared library loaded using
.B LD_PRELOAD
overrides the
.BR bind (2)
system call.  When a program invoked via
.B authbind
calls
.B 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
.B bind
supposed by
.B authbind
forks and executes a setuid-root helper program.  For non-TCP/IP
sockets, high-numbered ports, or programs which are already root,
.B authbind
passes the call to the original
.BR bind (2)
system call, which is found using
.BR dlsym (3)
with the handle
.BR RTLD_NEXT .
.PP
.SH ERROR HANDLING
Usually the normal C error handling mechanisms apply.  If
.B authbind
cannot find the program it has been asked to execute it will print a
message to stderr and exit with code 255.
.PP
The helper program usually reports back to the shared library with an
exit status containing an
.B errno
value which encodes whether the
.B bind
was permitted and successful.  This will be returned to the calling
program in the usual way.
.PP
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
.BR bind .
.SH BUGS
.B authbind
currently only supports IPv4 sockets.  Programs which open other kinds
of sockets will not benefit from
.BR authbind ,
but it won't get in their way.
.PP
The use of
.B LD_PRELOAD
makes an
.B authbind
installation specific to a particular C library.  This version is for
GNU/Linux libc6 (glibc2).
.PP
It is quite possible that
.B authbind
and other programs and facilities which use
.B LD_PRELOAD
may interfere with each other, causing unpredictable behaviour or even
core dumps.
.B authbind
is known sometimes not to work correctly with
.BR fakeroot ,
for example (even supposing it could be determined what `correctly'
means in this context).
.PP
.B authbind
is ineffective with setuid programs, because they do not honour
.B LD_PRELOAD
for security reasons.  Of course a setuid-root program does not need
.BR 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
.B libauthbind
library explicitly rather than via
.BR LD_PRELOAD .
.PP
Some badly-written programs may have trouble because
.B authbind
spawns a child process `under their feet', causing (for example) a
.BR fork (2)
to happen and
.B SIGCHLD
signal to be delivered.  Programs should not rely on standard
libraries not doing these things.
.PP
The access control configuration scheme is somewhat strange.
.SH FILES AND ENVIRONMENT VARIABLES
.TP
.I /usr/lib/authbind/libauthbind.so.1.0
The shared library which
.B authbind
causes to be loaded using
.BR LD_PRELOAD ,
and which actually implements the diversion of
.BR bind (2)
to an external program.
.TP
.I 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
.B authbind
program to allow it to override the usual meaning of
.BR bind (2) .
.TP
.I AUTHBIND_LIB
If set, forces
.B authbind
to use its value as the path to the shared library to put in
.BR LD_PRELOAD ,
instead of the compiled-in value.
.TP
.I AUTHBIND_NESTED
Do not set this variable.  It is set to
.B 1
by
.B libauthbind
when it invokes the helper program.  This allows detection of the
situation where the helper has not been installed setuid, which would
otherwise lead to infinite recursion.
.SH SEE ALSO
.BR bind (2),
.BR authbind\-helper (8),
.BR dlsym (3),
.BR ld.so (8)
.SH AUTHOR
.B 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
.B /usr/doc/authbind/copyright
and
.B /usr/doc/copyright/GPL
for details.