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, pam_ssh provides functionality for two PAM categories: authentication and session management. In terms of the module-type parameter, they are the “auth” and “session” features. It also provides null functions for the remaining categories.

SSH Authentication Module¶

The SSH authentication component verifies the identity of a user by prompting the user for a passphrase and verifying that it can decrypt at least one of the user's SSH login keys using that passphrase. The following options may be passed to the authentication module:

debug

syslog(3) debugging information at LOG_DEBUG level.

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 decrypt the user's SSH login keys. If this fails, then the authentication module returns failure without prompting the user for a passphrase.

try_first_pass

Similar to the use_first_pass option, except that if the previously obtained password fails to decrypt any of the SSH login keys, then the user is prompted for a SSH passphrase.

nullok

Allow SSH keys with no passphrase.

If neither use_first_pass nor try_first_pass is specified, pam_ssh will unconditionally ask for an SSH passphrase. In addition to the above authentication procedure, all standard SSH keys (identity, id_rsa, id_dsa, id_ecdsa) for which the obtained password matches will be decrypted. The now deprecated name allow_blank_passphrase for nullok is kept for compatibility reasons.

SSH Session Management Module¶

The SSH session management component initiates sessions by starting an SSH agent, passing it any SSH login keys it decrypted during the authentication phase, and sets the environment variables accordingly; the environment variable TMPDIR, which can be set through the pam_tmpdir module for example, is also honoured by being passed to the SSH agent. The SSH session management component terminates the session by killing the previously started SSH agent by sending it a SIGTERM. The following options may be passed to the session management module:

debug

syslog(3) debugging information at LOG_DEBUG level.

INFORMATION LEAKS¶

Be careful with the using the try_first_pass option when pam_ssh is the first authentication module because it will then leak information about existing users without login keys: such users will not be asked for a specific SSH passphrase, whereas non-existing users and existing users with login keys will be asked for a passphrase.

FILES¶

$HOME/.ssh/identity

standard SSH1/SSH RSA key decrypted by pam_ssh

$HOME/.ssh/id_rsa

standard SSH RSA key decrypted by pam_ssh

$HOME/.ssh/id_dsa

standard SSH DSA key decrypted by pam_ssh

$HOME/.ssh/id_ecdsa

standard SSH ECDSA key decrypted by pam_ssh

$HOME/.ssh/login-keys.d

location of (possibly symbolic links to) SSH RSA1/RSA/DSA/ECDSA keys decrypted by pam_ssh (keys with .disabled or .frozen as suffix are ignored)

/var/log/auth.log

usual log file for syslog(3)

SEE ALSO¶

ssh-agent(1), syslog(3), pam.conf(5), pam(8), pam_tmp

AUTHORS¶

Andrew J. Korty <ajk@iu.edu> wrote pam_ssh. Dag-Erling Smorgrav wrote the original OpenPAM support code. Mark R V Murray wrote the original version of this manual page. Jens Peter Secher introduced the login-key concept.