This is privbind 0.2, written by Shachar Shemesh with improvements by Amos
Shapira and Nadav Har'El.
Privbind is free software, released under the GNU General Public License
(see the attached file COPYING for details).
What is Privbind
Privbind is a tool for running a command as an unprivileged user, with
additional reserved port binding privileges.
Normally in Linux, only a superuser process can bind an Internet domain socket
with a privileged port (port numbers less than 1024). Accordingly, server
processes are typically run with superuser privileges, which can be dropped
after binding the reserved port. The privilege dropping has to be a part of the
Privbind can run a server, right from the start, under an unprivileged non-root
user with just one extra privilege: the server can bind to reserved ports.
Privbind is useful in several situations. It can be used when the server code
cannot be trusted enough to start it as root; It can be used when the server
is written in a language without the setuid(2) feature (e.g., Java); It can
also be used to run a server written to remain running as root, while all the
privileges it really needs is the ability to bind reserved ports.
The current privbind implementation (see manual page privbind.1 for details)
was only tested on Linux, using a 2.6 kernel.
Among other things, it assumes the LD_PRELOAD feature, and the SOCK_SEQPACKET
socket type which is a relatively recent addition to Linux
How privbind works
In theory, all one would need is to make use of Linux's capabilities(7) feature
and its CAP_NET_BIND_SERVICE. Unfortunately, Linux's capabilities feature is
totally useless because capability inheritance was never implemented!
Privbind uses a 100% user space approach for solving this issue. Privbind uses
the LD_PRELOAD method to override the running command's "bind" function.
Whenever the command tries to bind a privileged port, privbind intercepts the
call and forwards it to a process left behind, still running as root. This
process performs the bind on the command's behalf.
Forwarding of a socket to another process is done via a unix domain socket,
which supports sending a fd to another process.
privbind deviates from the accepted standard of "fork + exec", in that it is
the parent process, not the child, that does "exec". This is done so that the
command's return code (as well as actual return) be visible to the user
running privbind, and not privbind's return code, which is, for all intent and
Caveats and corner cases
Since privbind changes both the parent-child relationship and the code flow of
certain system functions, care must be taken to avoid the changes in semantics
from affecting the program run.
The most notable case for such interaction are signals. Two signals of note
should be pointed out. One is the case where privbind receives the "-n" option.
With this option, privbind's privileged process terminates after 'num'
privileged binds have been performed. This means that the child process
terminates while the parent is still running, causing the parent to receive a
SIGCHILD from a child it did not know it had. Privbind solves this problem by
performing a double-fork, and thus running with no parent.
The second case is when, as before, "-n" caused privbind to terminate prior to
the actual program terminating. If the program now attempts another bind, it is
possible that a SIGPIPE will be sent to the process. This signal, again, is one
not resulting from anything the program did, and may lead to trouble. The
solution is to send the message with the "MSG_NOSIGNAL" option, which prevents
SIGPIPE from being sent.
The third case of unintended consequences is when using privbind to run a
daemon. The standard instructions for writing daemons state that a daemon
should close, before starting to run, all of its open file descriptors.
Unfortunately, this also includes the fd used to communicate between privileged
and unprivileged processes. Privbind solves this problem by also intercepting
the "close" syscall, and making sure that our own fd is never closed.
For more information
Refer to the manual page, privbind(1).
The latest version of this software can be found in