PAM_SSH(8) | System Manager's Manual | PAM_SSH(8)

# NAME

pam_ssh — authentication and session management with SSH private keys

# SYNOPSIS

[service-name] module-type control-flag pam_ssh [options]

# DESCRIPTION

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 SIGTERM.

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.

# FILES

$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

# SEE ALSO

ssh-agent(1), pam.conf(5), pam(8)

# AUTHORS

The pam_ssh module was originally written by Andrew J. Korty <ajk@iu.edu>. 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>.

# SECURITY CONSIDERATIONS

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 | NetBSD 6.0