source: trunk/third/openssh/README.privsep @ 18759

Privilege separation, or privsep, is method in OpenSSH by 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 now enabled by default; see the
UsePrivilegeSeparation option in sshd_config(5).

On systems which lack mmap or anonymous (MAP_ANON) memory mapping, 
compression must be disabled in order for privilege separation to 
function.

When privsep is enabled, during the pre-authentication phase sshd will
chroot(2) to "/var/empty" and change its privileges to the "sshd" user
and its primary group.  sshd is a pseudo-account that should not be
used by other daemons, and must be locked and should contain a
"nologin" or invalid shell.

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 -c 'sshd privsep' -d /var/empty -s /bin/false sshd

/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 separation chroot
  --with-privsep-user=user Specify non-privileged user for privilege separation

Privsep requires operating system support for file descriptor passing.
Compression will be disabled on systems without a working mmap MAP_ANON.

PAM-enabled OpenSSH is known to function with privsep on Linux.  
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 1.1.1.1 2003-02-05 19:03:14 zacheiss Exp $