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man page of pam_ssh

pam_ssh: authentication and session management with SSH private keys


pam_ssh - authentication and session management with SSH private keys


The SSH authentication service module for PAM, pam_ssh provides functionality for two PAM categories: authentication and session management. 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, then 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. try_first_pass has no effect if pam_ssh is the first module on the stack, or if no previous modules obtained the user's password. allow_blank_passphrase 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) for which the obtained password matches will be decrypted. 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 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.


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.


$HOME/.ssh/identity $HOME/.ssh/id_rsa $HOME/.ssh/id_dsa OpenSSH DSA/RSA keys decrypted by pam_ssh. $HOME/.ssh/login-keys.d/ Location of (possibly symbolic links to) OpenSSH DSA/RSA keys used for authentication and decrypted by pam_ssh. /var/log/auth.log Usual log file for syslog(3)
ssh-agent(1), syslog(3), pam.conf(5), pam(8).
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. PAM_SSH(8)

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