1 Privilege separation, or privsep, is method in OpenSSH by which 2 operations that require root privilege are performed by a separate 3 privileged monitor process. Its purpose is to prevent privilege 4 escalation by containing corruption to an unprivileged process. 5 More information is available at: 6 http://www.citi.umich.edu/u/provos/ssh/privsep.html 7 8 Privilege separation is now enabled by default; see the 9 UsePrivilegeSeparation option in sshd_config(5). 10 11 On systems which lack mmap or anonymous (MAP_ANON) memory mapping, 12 compression must be disabled in order for privilege separation to 13 function. 14 15 When privsep is enabled, during the pre-authentication phase sshd will 16 chroot(2) to "/var/empty" and change its privileges to the "sshd" user 17 and its primary group. sshd is a pseudo-account that should not be 18 used by other daemons, and must be locked and should contain a 19 "nologin" or invalid shell. 20 21 You should do something like the following to prepare the privsep 22 preauth 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 32 configure supports the following options to change the default 33 privsep 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 38 Privsep requires operating system support for file descriptor passing. 39 Compression will be disabled on systems without a working mmap MAP_ANON. 40 41 PAM-enabled OpenSSH is known to function with privsep on AIX, FreeBSD, 42 HP-UX (including Trusted Mode), Linux, NetBSD and Solaris. 43 44 On Cygwin, Tru64 Unix, OpenServer, and Unicos only the pre-authentication 45 part of privsep is supported. Post-authentication privsep is disabled 46 automatically (so you won't see the additional process mentioned below). 47 48 Note that for a normal interactive login with a shell, enabling privsep 49 will require 1 additional process per login session. 50 51 Given 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 59 process 1005 is the sshd process listening for new connections. 60 process 6917 is the privileged monitor process, 6919 is the user owned 61 sshd process and 6921 is the shell process. 62 63 $Id: README.privsep,v 1.16 2005/06/04 23:21:41 djm Exp $ 64
