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Setting up PAM sudo authentication, using ssh-agent, on 14.04.1-Ubuntu server LTS.

I'm still unable to successfully authenticate sudo, via the ssh-agent, using PAM.

Here are my relevant /var/log/auth.log entries...

Jun 17 11:31:16 host sudo[21318]: pam_ssh_agent_auth: Beginning pam_ssh_agent_auth for user userName
Jun 17 11:31:16 host sudo[21318]: pam_ssh_agent_auth: Attempting authentication: `userName' as `userName' using /etc/security/authorized_keys
Jun 17 11:31:16 host sudo[21318]: pam_ssh_agent_auth: Contacted ssh-agent of user userName (1000)
Jun 17 11:31:16 host sudo[21318]: pam_ssh_agent_auth: Failed Authentication: `userName' as `userName' using /etc/security/authorized_keys

As you can see, it successfully contacts the ssh-agent, but then authentication fails. PAM falls back to the next authentication method(s), and asks for the sudo/userName password, then I'm able to proceed. I'm trying to configure it, so that you don't need a sudo password, as long as you connect via ssh with an authorized key.

Here are the relevant files and their contents:

/etc/pam.d/sudo

#%PAM-1.0

auth       sufficient pam_ssh_agent_auth.so file=/etc/security/authorized_keys debug
auth       required   pam_env.so readenv=1 user_readenv=0
auth       required   pam_env.so readenv=1 envfile=/etc/default/locale user_readenv=0

@include common-auth
@include common-account
@include common-session-noninteractive

/etc/security/authorized_keys file information: This file contains 4 ssh-rsa public keys.

-rw-r--r-- 1 root root 1597 Jun 16 16:07 /etc/security/authorized_keys

/etc/sudoers

#
# This file MUST be edited with the 'visudo' command as root.
#
# Please consider adding local content in /etc/sudoers.d/ instead of
# directly modifying this file.
#
# See the man page for details on how to write a sudoers file.
#

Defaults        env_keep += SSH_AUTH_SOCK

Defaults        env_reset
Defaults        mail_badpass
Defaults        secure_path="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"

# Host alias specification

# User alias specification

# Cmnd alias specification

# User privilege specification
root    ALL=(ALL:ALL) ALL

# Members of the admin group may gain root privileges
%admin ALL=(ALL) ALL

# Allow members of group sudo to execute any command
%sudo   ALL=(ALL:ALL) ALL

# See sudoers(5) for more information on "#include" directives:

#includedir /etc/sudoers.d

And, for sanity's sake, you can see the SSH_AUTH_SOCK is indeed being "passed up the sudo chain" correctly...

printenv | grep SSH

SSH_AUTH_SOCK=/tmp/ssh-m9Ume3GOIP/agent.15964

sudo printenv | grep SSH

SSH_AUTH_SOCK=/tmp/ssh-m9Ume3GOIP/agent.15964

I ssh into the server via

ssh -A host@ip_address

I'll include any other information that may be helpful, just ask :)

I've been at this for over a day, and I've found dozens of "howtos" to setup PAM sudo authentication using ssh keys, and they're all similar... but I can't find anything that might shine light on why the PAM authentication fails, after successfully contacting/communicating with the ssh-agent.

Thanks in advance!

UPDATE

ssh-add on the client side, was the trick. I'm not an "ssh power user", but this gives me what I need to figure out the root cause. Thanks!

  • ssh-add -L will return something on your local computer and on the remote host? – Jakuje Jun 17 '16 at 18:13
  • That did the trick =D – Michael Claramunt Jun 17 '16 at 18:19
  • Not sure what you mean, but I suspect you don't have the agent running or you don't have the keys added. Is there anything unclear yet? – Jakuje Jun 17 '16 at 18:22
  • I simply ran ssh-add on the client, and that was able to load the private key into the agent, which allowed PAM to successfully authenticate on the server. I may still be missing something, but I'm currently researching "what that means." I'm not an "ssh power user", so I may still be missing something, but this gives me a jumping off point to further my research at least :) – Michael Claramunt Jun 17 '16 at 18:24
  • ssh-add -L on either system, prior to doing that, simply returned The agent has no identities. – Michael Claramunt Jun 17 '16 at 18:36
3

Configuration is OK, but you need to have some identities in your ssh-agent to be able to authorize the sudo operation. You can verify that your agent has some identities using

ssh-add -L

It should print the public keys in your agent and at least one of them should match the public key on server in /etc/security/authorized_keys.

If the agent does not have any identities, you need to add them on your computer, again using

ssh-add [path/to/key]

and insert your passphrase, if prompted.

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