1Privilege separation, or privsep, is method in OpenSSH by which 2operations that require root privilege are performed by a separate 3privileged monitor process. Its purpose is to prevent privilege 4escalation by containing corruption to an unprivileged process. 5More information is available at: 6 http://www.citi.umich.edu/u/provos/ssh/privsep.html 7 8Privilege separation is now enabled by default; see the 9UsePrivilegeSeparation option in sshd_config(5). 10 11On systems which lack mmap or anonymous (MAP_ANON) memory mapping, 12compression must be disabled in order for privilege separation to 13function. 14 15When privsep is enabled, during the pre-authentication phase sshd will 16chroot(2) to "/var/empty" and change its privileges to the "sshd" user 17and its primary group. sshd is a pseudo-account that should not be 18used by other daemons, and must be locked and should contain a 19"nologin" or invalid shell. 20 21You should do something like the following to prepare the privsep 22preauth environment: 23 24 # mkdir /var/empty 25 # chown root:sys /var/empty 26 # chmod 755 /var/empty 27 # groupadd sshd 28 # useradd -g sshd -c 'sshd privsep' -d /var/empty -s /bin/false sshd 29 30/var/empty should not contain any files. 31 32configure supports the following options to change the default 33privsep user and chroot directory: 34 35 --with-privsep-path=xxx Path for privilege separation chroot 36 --with-privsep-user=user Specify non-privileged user for privilege separation 37 38Privsep requires operating system support for file descriptor passing. 39Compression will be disabled on systems without a working mmap MAP_ANON. 40 41PAM-enabled OpenSSH is known to function with privsep on AIX, FreeBSD, 42HP-UX (including Trusted Mode), Linux, NetBSD and Solaris. 43 44On Cygwin, Tru64 Unix, OpenServer, and Unicos only the pre-authentication 45part of privsep is supported. Post-authentication privsep is disabled 46automatically (so you won't see the additional process mentioned below). 47 48Note that for a normal interactive login with a shell, enabling privsep 49will require 1 additional process per login session. 50 51Given the following process listing (from HP-UX): 52 53 UID PID PPID C STIME TTY TIME COMMAND 54 root 1005 1 0 10:45:17 ? 0:08 /opt/openssh/sbin/sshd -u0 55 root 6917 1005 0 15:19:16 ? 0:00 sshd: stevesk [priv] 56 stevesk 6919 6917 0 15:19:17 ? 0:03 sshd: stevesk@2 57 stevesk 6921 6919 0 15:19:17 pts/2 0:00 -bash 58 59process 1005 is the sshd process listening for new connections. 60process 6917 is the privileged monitor process, 6919 is the user owned 61sshd process and 6921 is the shell process. 62 63$Id: README.privsep,v 1.16 2005/06/04 23:21:41 djm Exp $ 64