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I've been wrestling with this particular issue most of the day, and have been making SOME progress, but I'm still a bit stuck. I would like to be able to authenticate an SSH session using a private key file and then use that same authentication to also authenticate sudo requests. The system I am connecting FROM is a Windows box using Putty and Pageant. The system I am connected TO is a Ubuntu box running 13.10 Saucy (32-bit), and specifically OpenSSH server. I have successfully generated a 4096-bit RSA key and am able to use that to connect to SSH sessions without issue.

To facilitate the sudo authentication, I am trying to install and use libpam-ssh-agent-auth. It's on SourceForge and is documented over here: http://pamsshagentauth.sourceforge.net/ There's a LOT of info and how-tos online regarding this library, but the info and guides are mostly outdated and / or distro-specific.

Some of my progress: I found that you must enable user agent forwarding AND have Pageant up and running in Putty to successfully get the the user agent to actually show up over on the Ubuntu server. I also had to add the "AllowAgentForwarding yes" to /etc/ssh/sshd_config, and the line "Defaults env_keep += SSH_AUTH_SOCK" to /etc/sudoers (after the other 3 Defaults lines). These changes appear to have allowed the SSH authentication socket to propagate up to the sudo commands, as tested in the terminal like this:

sudo echo "$SSH_AUTH_SOCK"

Prior to making these changes, I was getting no SSH_AUTH_SOCK at all, and then was getting it only in the normal command prompt, not in sudo. Make the changes documented above allowed the authentication socket to make it through to sudo.

The final step appears to be getting libpam-ssh-agent-auth installed and then making the appropriate configuration changes to /etc/pam.d/sudo. I have tried compiling it myself, and also installing precompiled .debs from two different PPAs as part of my quest to get this working (like I said, been trying all day ) This final step is what has me stuck. Basically, despite getting stuff apparently configured correctly, I'm getting no success.

It is my understanding that my test case should be to run a "sudo -K" and then sudo any command. The -K resets the sudo timer, forcing re-authentication on the next sudo attempt.

I have had no success.

My current /etc/pam.d/sudo file:


auth    required        pam_env.so      readenv=1 user_readenv=0
auth    required        pam_env.so      readenv=1 envfile=/etc/default/locale user_readenv=0
auth sufficient pam_ssh_agent_auth.so file=~/.ssh/authorized_keys
auth requisite pam_unix.so nullok_secure
@include common-auth
@include common-account
@include common-session-noninteractive

I figured this may be a good place to post about this stuff.

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1 Answer 1

I was able to resolve this issue on my own.

The key to troubleshooting this was examining the /var/log/auth.log file. Examining this log lead me to the error

pam_ssh_agent_auth: Authentication refused: bad ownership or modes for file

This lead me to the first issue: the file ownership and permissions must be set properly for SSH authentication to proceed. I specifically had to run a "chmod 700 -R ~/.ssh" This resolved that particular error, but then when I attempted to sudo, I was getting "the password is incorrect" three times, and then a sudo failure.

To resolve this I ended up altering the /etc/pam.d/sudo file, specifically commenting out everything but the line

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

This is far from ideal, but gives me a place to start from. Further edits to the /etc/pam.d/sudo file should allow for other kinds of authentication to also work.

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