pam_ssh - Man Page

authentication and session management with SSH private keys

Synopsis

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

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 (or authentication) keys using that passphrase.

The user's SSH login keys must be either located or symbolically linked into the per-user dedicated folder ~/.ssh/login-keys.d/ in the user's home directory.

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 to 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 an 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.

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 launching an SSH agent, passing it any user's SSH login keys successfully decrypted during the authentication phase and any additional user SSH session keys successfully decrypted, and sets dedicated environment variables accordingly; the environment variable TMPDIR, which can be set through the pam_tmpdir module for instance, is also honoured by being passed to the SSH agent.

The SSH session management component terminates the session by killing the previously launched SSH agent by sending it a SIGTERM.

The traditional SSH keys ~/.ssh/id_rsa, ~/.ssh/id_dsa, ~/.ssh/id_ecdsa, and ~/.ssh/id_ed25519 are considered as the default SSH session keys. Nonetheless, extra user SSH session keys can be either located or symbolically linked into the per-user dedicated folder ~/.ssh/session-keys.d/ in the user's home directory.

Provided that they have been successfully decrypted, the SSH session management passes to the launched SSH agent first the session SSH keys in lexical order, second the login SSH keys in lexical order, and finally the traditional SSH keys in the reverse order cited above. Since the SSH agent keeps in its memory for each passed key its first position (but its last comment), each SSH key rank can be easily overwritten with an appropriate symbolic link placed in ~/.ssh/session-keys.d/ or ~/.ssh/login-keys.d/; this is especially true for the traditional SSH keys. The involved lexical order is performed against the SSH key file basenames (according to the C/POSIX locale character collation rules). Because actually their basenames are passed as comments, their effective order might be easily checked with an appropriate SSH agent helper as ssh-add(3). As final remark, keep in mind that the SSH agent may place itself SSH keys with protocol 1 before SSH keys with protocol 2.

The following option 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

~/.ssh/: This directory is the default per-user location for all user-specific SSH configuration and SSH authentication information as expected by SSH and its friends.
~/.ssh/id_rsa
~/.ssh/id_dsa
~/.ssh/id_ecdsa
~/.ssh/id_ed25519: Contains the traditional private key for authentication. These files contain sensitive data and should be readable by the user but not accessible by others: any traditional private key file that is accessible by others is simply ignored. While the SSH authentication component ignores the traditional private keys, the SSH session management component passes any successfully decrypted traditional key to the launched SSH agent.
~/.ssh/login-keys.d/: This directory is the dedicated per-user location for files or symbolic links to files that contains SSH private keys considered by the SSH authentication component. pam_ssh ignores any private key file that is accessible by others or that possesses .disabled or .frozen as suffix. Each login key successfully decrypted is passed by the SSH session management component to the launched SSH agent.
~/.ssh/session-keys.d/: This directory is the dedicated per-user location for files or symbolic links to files that contains (extra) SSH private keys considered only by the SSH session management component. pam_ssh ignores any private key file that is accessible by others or that possesses .disabled or .frozen as suffix. Each session key successfully decrypted is passed to the lauched SSH agent.
/run/pam_ssh/<user>*: ssh-agent environment information. The files are owned by the superuser but readable by the users. The location is Fedora specific, in the original package these files are in $HOME/.ssh/agent-*

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. Refreshed for Debian by Jerome Benoit <calculus@rezozer.net>. pam_ssh is currently maintained by Wolfgang Rosenauer.

Info

January 14, 2019