README.privsep

Privilege separation, or privsep, is an experimental feature in
OpenSSH in which operations that require root privilege are performed
by a separate privileged monitor process.  Its purpose is to prevent
privilege escalation by containing corruption to an unprivileged
process.  More information is available at:
	http://www.citi.umich.edu/u/provos/ssh/privsep.html

Privilege separation is not enabled by default, and may be enabled by
specifying "UsePrivilegeSeparation yes" in sshd_config; see the
UsePrivilegeSeparation option in sshd(8).

When privsep is enabled, the pre-authentication sshd process will
chroot(2) to "/var/empty" and change its privileges to the "sshd" user
and its primary group.  You should do something like the following to
prepare the privsep preauth environment:

	# mkdir /var/empty
	# chown root:sys /var/empty
	# chmod 755 /var/empty
	# groupadd sshd
	# useradd -g sshd sshd

If you are on UnixWare 7 or OpenUNIX 8 do this additional step.
	# ln /usr/lib/.ns.so /usr/lib/ns.so.1

/var/empty should not contain any files.

configure supports the following options to change the default
privsep user and chroot directory:

  --with-privsep-path=xxx Path for privilege seperation chroot
  --with-privsep-user=user Specify non-privileged user for privilege separation

Privsep requires operating system support for file descriptor passing
and mmap(MAP_ANON).

PAM-enabled OpenSSH is known to function with privsep on Linux and
Solaris 8.  It does not function on HP-UX with a trusted system
configuration.  PAMAuthenticationViaKbdInt does not function with
privsep.

Note that for a normal interactive login with a shell, enabling privsep
will require 1 additional process per login session.

Given the following process listing (from HP-UX):

     UID   PID  PPID  C    STIME TTY       TIME COMMAND
    root  1005     1  0 10:45:17 ?         0:08 /opt/openssh/sbin/sshd -u0
    root  6917  1005  0 15:19:16 ?         0:00 sshd: stevesk [priv]
 stevesk  6919  6917  0 15:19:17 ?         0:03 sshd: stevesk@2
 stevesk  6921  6919  0 15:19:17 pts/2     0:00 -bash

process 1005 is the sshd process listening for new connections.
process 6917 is the privileged monitor process, 6919 is the user owned
sshd process and 6921 is the shell process.

$Id: README.privsep,v 1.4 2002/05/14 03:50:38 tim Exp $