pam_ssh_agent_auth - Man Page

PAM module for granting permissions based on SSH agent requests

Description

This module provides authentication via ssh keys.  If an ssh-agent listening at SSH_AUTH_SOCK can successfully authenticate that it has the secret key for a public key in the specified file, authentication is granted.  If the public key originally used to authenticate at sshd matches an authorized key, authentication succeeds.  Otherwise authentication fails.

Configuration

Sudo

/etc/pam.d/sudo:
 auth   sufficient      pam_ssh_agent_auth.so file=/etc/security/authorized_keys
/etc/sudoers:

In older versions of sudo (< 1.8.5) it was necessary to set:
Defaults    env_keep += “SSH_AUTH_SOCK”

This configuration would permit anyone who has an SSH_AUTH_SOCK that manages the private key matching a public key in /etc/security/authorized_keys to execute sudo without having to enter a password. Note that the ssh-agent listening to SSH_AUTH_SOCK can either be local, or forwarded.

Unlike NOPASSWD, this still requires an authentication, it's just that the authentication is provided by ssh-agent, and not password entry.

SSHD

/etc/ssh/sshd_config:
 PubkeyAuthentication yes
 AuthenticationMethods publickey,keyboard-interactive:pam
 PasswordAuthentication no
 UsePAM yes
/etc/pam.d/sshd:
 auth required    pam_permit.so
 auth   sufficient      pam_ssh_agent_auth.so file=/etc/security/super_authorized_keys
 auth sufficient  pam_secondary_auth_method.so
 ...
 auth requisite   pam_deny.so

This configuration would permit anyone who originally authenticated to sshd with a public key also found in /etc/security/super_authorized_keys to log in without having to complete other PAM auth methods. Anyone whose ssh key was accepted initially by sshd but whose key is not in the allowlist must complete another secondary PAM module such as OTP or else be denied.

Arguments

file=<path to authorized_keys>

Specify the path to the authorized_keys file(s) you would like to use for authentication. Subject to tilde and % Expansions (below)

allow_user_owned_authorized_keys_file

A flag which enables authorized_keys files to be owned by the invoking user, instead of root. This flag is enabled automatically whenever the expansions %h or ~ are used.

authorized_keys_command=<path to executable>

Specify an external command to run, which should take a single argument, the username of the person being authenticated, and emit to its stdout a file in authorized_keys format. This is ideally suited for use with sssd's sss_ssh_authorizedkeys, for authenticating users via authorized_keys stored in ldap or other sssd supported security service.

authorized_keys_command_user=<username>

Specify a user to run the authorized_keys_command as. If this option is not specified, the authorized_keys_command will be run as the user being authenticated.

debug

A flag which enables verbose logging

sudo_service_name=<service name you compiled sudo to use>

(when compiled with --enable-sudo-hack)

Specify the service name to use to identify the service “sudo”. When the PAM_SERVICE identifier matches this  string, and if PAM_RUSER is not set, pam_ssh_agent_auth will attempt to identify the calling user from the  environment variable SUDO_USER.

This defaults to “sudo”.

Expansions

~  — same as in shells, a user's Home directory

Automatically enables allow_user_owned_authorized_keys_file if used in the context of ~/. If used as ~user/, it would expect the file to be owned by 'user', unless you explicitly set allow_user_owned_authorized_keys_file

%h — User's Home directory

Automatically enables allow_user_owned_authorized_keys_file

%H — The short-hostname

%u — Username

%f — FQDN

Examples

in /etc/pam.d/sudo

auth sufficient pam_ssh_agent_auth.so file=~/.ssh/authorized_keys

The default .ssh/authorized_keys file in a user's home-directory

auth sufficient pam_ssh_agent_auth.so file=%h/.ssh/authorized_keys

Same as above.

auth sufficient pam_ssh_agent_auth.so file=~fred/.ssh/authorized_keys

If the home-directory of user 'fred' was /home/fred, this would expand to /home/fred/.ssh/authorized_keys. In this case, we have not specified allow_user_owned_authorized_keys_file,  so this file must be owned by 'fred'.

auth sufficient pam_ssh_agent_auth.so file=/secure/%H/%u/authorized_keys allow_user_owned_authorized_keys_file

On a host named foobar.baz.com, and a user named fred, would expand to /secure/foobar/fred/authorized_keys. In this case, we specified allow_user_owned_authorized_keys_file,  so fred would be able to manage that authorized_keys file himself.

auth sufficient pam_ssh_agent_auth.so file=/secure/%f/%u/authorized_keys

On a host named foobar.baz.com, and a user named fred,  would expand to /secure/foobar.baz.com/fred/authorized_keys. In this case, we have not specified allow_user_owned_authorized_keys_file,  so this file must be owned by root.

auth [success=3 default=ignore] pam_ssh_agent_auth.so file=~/.ssh/authorized_keys debug

This pam.d config format allows for more control over how pam handles success and failure. In this example, we use success=3, which specifies that when this module succeeds, pam should jump over the next 3 auth modules and continue from there. This is useful, for instance, if /etc/pam.d/common-auth is included, and contains 3 “auth required” or similar module rules that we wish to skip, but we wish not to skip other auth rules.

For more information, please see http://linux.die.net/man/5/pam.d

Info

2021-03-09 v0.10.3 PAM