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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).
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
PAM-enabled OpenSSH is known to function with privsep on AIX, FreeBSD,
HP-UX (including Trusted Mode), Linux, NetBSD and Solaris.
On Cygwin, Tru64 Unix, OpenServer, and Unicos only the pre-authentication
part of privsep is supported. Post-authentication privsep is disabled
automatically (so you won't see the additional process mentioned below).
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.