Provided by: privbind_1.1-1_i386
privbind - allow an unprivileged application to bind with reserved
privbind -u user [ -g group] [ -n num] [ -l path] command [ arguments
Normally in Linux, only a superuser process can bind an Internet domain
socket with a reserved port (port numbers less than 1024). Accordingly,
server processes are typically run with superuser privileges, which can
be dropped after binding the reserved port.
privbind can execute an application as an unprivileged user with just
one extra privilege: it can bind to reserved ports.
privbind is useful in several situations. It can be used when the
application is not trusted enough; It can be used when the server is
written in a language without the setuid(2) feature (e.g., Java(TM));
It can also be used to run applications which don’t manipulate their
own user id and need to be able to bind to a reserved port without
needing any other root privileges.
-u The -u option is mandatory, and specifies under which user to
run the given command. The user can be specified using either a
username or a numeric user id. It should be an unprivileged
-g Specifies the group to switch to when running the given command.
If this option is missing, then the given user’s default group
-n privbind’s default behaviour is to allow the application to call
bind(2) with reserved ports an unlimited number of times. In
order to do that (see "HOW IT WORKS" below), the privbind helper
process needs to wait for the application to exit before it
The -n num option tells privbind that it can assume that only
num binds need to be given elevated privileges. After this
number of bind(2) calls have been executed, privbind’s helper
process will exit, leaving behind only the unprivileged
-l Mostly for internal use during build. Gives the explicit path to
the LD_PRELOAD library.
-h Shows a short help screen, and exits.
Using technical jargon, privbind execs command as its main process,
running itself in the background (as a child of the application’s
process). The practical upshot of this, in layman’s terms, is that the
user never sees privbind’s exit status. When running privbind, the
process will exit whenever, and with whatever exit status, command
The above point should be particularly noted when using privbind to run
privbind has no SUID parts, and runs within the confines of a single
process. This serves to minimize the security implications of using
it. It is strongly advised that privbind not be made SUID, as this
would allow any user that can run it to run any process as any other
(non-root) user. At the moment privbind detects such a situation and
warns about it, but will continue with the execution.
HOW IT WORKS
In a nutshell, privbind works by starting two processes. One drops
privileges and runs (exec(2)) the command, the other remains as root.
Privbind makes sure to keep a unix domain socket connecting the two
Privbind uses LD_PRELOAD to intercept every call to bind(2) made by the
program. Calls that can be completed non-privileged are done so. Calls
that require root privileges are forwarded to the root process, that
carry them out on the program’s behalf.
A more detailed explanation is available in the README file.
privbind currently uses "SOCK_SEQPACKET" for communication between
privileged and non-privileged processes. This socket type is only
implemented on Linux kernel 2.6.4 and later, which makes privbind none
portable to older Linux kernels and many other non-Linux platforms.
The version of privbind described by this manual page is 1.0 (June 12,
Copyright (C) 2006-2007, Shachar Shemesh plus others. See the AUTHORS
privbind was written by Shachar Shemesh, with contributions from Amos
Shapira and Nadav Har’El.
privbind is free software, released under the GNU General Public
License (GPL). See the COPYING file for more information and the exact
The latest version of this software can be found in
Java is a registered trademark of Sun Microsystems.
su(1), sudo(8), capabilities(7), bind(2), setuid(2), ld.so(8), unix(7)