1 Privilege separation, or privsep, is an experimental feature in
2 OpenSSH in which operations that require root privilege are performed
3 by a separate privileged monitor process. Its purpose is to prevent
4 privilege escalation by containing corruption to an unprivileged
5 process. More information is available at:
6 http://www.citi.umich.edu/u/provos/ssh/privsep.html
8 Privilege separation is not enabled by default, and may be enabled by
9 specifying "UsePrivilegeSeparation yes" in sshd_config; see the
10 UsePrivilegeSeparation option in sshd(8).
12 When privsep is enabled, the pre-authentication sshd process will
13 chroot(2) to "/var/empty" and change its privileges to the "sshd" user
14 and its primary group. You should do something like the following to
15 prepare the privsep preauth environment:
18 # chown root:sys /var/empty
19 # chmod 755 /var/empty
21 # useradd -g sshd sshd
23 If you are on UnixWare 7 or OpenUNIX 8 do this additional step.
24 # ln /usr/lib/.ns.so /usr/lib/ns.so.1
26 /var/empty should not contain any files.
28 configure supports the following options to change the default
29 privsep user and chroot directory:
31 --with-privsep-path=xxx Path for privilege separation chroot
32 --with-privsep-user=user Specify non-privileged user for privilege separation
34 Privsep requires operating system support for file descriptor passing
37 PAM-enabled OpenSSH is known to function with privsep on Linux and
38 Solaris 8. It does not function on HP-UX with a trusted system
39 configuration. PAMAuthenticationViaKbdInt does not function with
42 Note that for a normal interactive login with a shell, enabling privsep
43 will require 1 additional process per login session.
45 Given the following process listing (from HP-UX):
47 UID PID PPID C STIME TTY TIME COMMAND
48 root 1005 1 0 10:45:17 ? 0:08 /opt/openssh/sbin/sshd -u0
49 root 6917 1005 0 15:19:16 ? 0:00 sshd: stevesk [priv]
50 stevesk 6919 6917 0 15:19:17 ? 0:03 sshd: stevesk@2
51 stevesk 6921 6919 0 15:19:17 pts/2 0:00 -bash
53 process 1005 is the sshd process listening for new connections.
54 process 6917 is the privileged monitor process, 6919 is the user owned
55 sshd process and 6921 is the shell process.