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_passoption, 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
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.