.TH privbind 1 "22 May 2007" "Privbind 0.1" ""
privbind \- allow an unprivileged application to bind with reserved ports.
[ \fB\-g\fR \fIgroup\fR]
[ \fB\-n\fR \fInum\fR]
[ \fB\-l\fR \fIpath\fR]
\fIcommand\fR [ \fIarguments\fR .\|.\|. ]
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.
can execute an application as an unprivileged
user with just one extra privilege: it can bind to reserved ports.
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
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 (non-root) user.
Specifies the group to switch to when running the given command. If this
option is missing, then the given user's default group is used.
.BR 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
helper process needs to wait for the application to exit before it terminates.
The \fB\-n \fInum\fR option tells
that it can assume that only \fInum\fR binds need to be given elevated privileges.
After this number of bind(2) calls have been executed,
.BR privbind 's
helper process will exit, leaving behind only the unprivileged application
Mostly for internal use during build. Gives the explicit path to the LD_PRELOAD
Shows a short help screen, and exits.
.SH "EXIT STATUS"
Using technical jargon,
execs \fIcommand\fR 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
\fBprivbind\fR's exit status. When running \fBprivbind\fR, the process will exit
whenever, and with whatever exit status, \fIcommand\fR does.
The above point should be particularly noted when using \fBprivbind\fR to run daemons.
.SH "SECURITY CONSIDERATIONS"
\fBprivbind\fR 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 \fBprivbind\fR \fInot\fR 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.
.SH "HOW IT WORKS"
In a nutshell, \fBprivbind\fR works by starting two processes. One drops
privileges and runs (exec(2)) the command, the other remains as root.
\fBPrivbind\fR makes sure to keep a unix domain socket connecting the two processes.
\fBPrivbind\fR 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.
\fBprivbind\fR 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
described by this manual page is 1.0 (June 12, 2007)
Copyright (C) 2006-2007, Shachar Shemesh plus others. See the AUTHORS file.
was written by Shachar Shemesh, with contributions from Amos Shapira and
is free software, released under the GNU General Public License (GPL).
See the COPYING file for more information and the exact license terms.
The latest version of this software can be found in
is a registered trademark of Sun Microsystems.
.SH "SEE ALSO"
.BR su (1),
.BR sudo (8),
.BR capabilities (7),
.BR bind (2),
.BR setuid (2),
.BR ld.so (8),
.BR unix (7)