PAM_SSH(8) BSD System Manager's Manual PAM_SSH(8)
pam_ssh -- authentication and session management with SSH private keys
[service-name] module-type control-flag pam_ssh [options]
The SSH authentication service module for PAM provides functionality for two PAM categories: authentication and session management. In terms
of the module-type parameter, they are the ``auth'' and ``session'' features.
SSH Authentication Module
The SSH authentication component provides a function to verify the identity of a user (pam_sm_authenticate()), by prompting the user for a
passphrase and verifying that it can decrypt the target user's SSH key using that passphrase.
The following options may be passed to the authentication module:
use_first_pass If the authentication module is not the first in the stack, and a previous module obtained the user's password, that password
is used to authenticate the user. If this fails, the authentication module returns failure without prompting the user for a
password. This option has no effect if the authentication module is the first in the stack, or if no previous modules
obtained the user's password.
try_first_pass This option is similar to the use_first_pass option, except that if the previously obtained password fails, the user is
prompted for another password.
nullok Normally, keys with no passphrase are ignored for authentication purposes. If this option is set, keys with no passphrase
will be taken into consideration, allowing the user to log in with a blank password.
SSH Session Management Module
The SSH session management component provides functions to initiate (pam_sm_open_session()) and terminate (pam_sm_close_session()) sessions.
The pam_sm_open_session() function starts an SSH agent, passing it any private keys it decrypted during the authentication phase, and sets
the environment variables the agent specifies. The pam_sm_close_session() function kills the previously started SSH agent by sending it a
The following options may be passed to the session management module:
want_agent Start an agent even if no keys were decrypted during the authentication phase.
$HOME/.ssh/identity SSH1 RSA key
$HOME/.ssh/id_rsa SSH2 RSA key
$HOME/.ssh/id_dsa SSH2 DSA key
$HOME/.ssh/id_ecdsa SSH2 ECDSA key
ssh-agent(1), pam.conf(5), pam(8)
The pam_ssh module was originally written by Andrew J. Korty <firstname.lastname@example.org>. The current implementation was developed for the FreeBSD Project
by ThinkSec AS and NAI Labs, the Security Research Division of Network Associates, Inc. under DARPA/SPAWAR contract N66001-01-C-8035
(``CBOSS''), as part of the DARPA CHATS research program. This manual page was written by Mark R V Murray <markm@FreeBSD.org>.
The pam_ssh module implements what is fundamentally a password authentication scheme. Care should be taken to only use this module over a
secure session (secure TTY, encrypted session, etc.), otherwise the user's SSH passphrase could be compromised.
Additional consideration should be given to the use of pam_ssh. Users often assume that file permissions are sufficient to protect their SSH
keys, and thus use weak or no passphrases. Since the system administrator has no effective means of enforcing SSH passphrase quality, this
has the potential to expose the system to security risks.
December 16, 2011 BSD