Using ssh-agent for auto login with public keys in Linux

Most of the time when you are using a Desktop Environment in Linux this is already implemented, all you have to do is add AddKeysToAgent yes to your /etc/ssh/ssh_config and you are good to go, the keys will be added to the ssh-agent and can be reused.

In this tutorial we will setup ssh-agent from scratch, but first let’s see how it works.

How ssh-agent works?

The first step is to run the ssh-agent.

We have to use the eval keyword, because the output looks like below and we must set and export the SSH_AUTH_SOCK and SSH_AGENT_PID variables, which will be later used by ssh-add.

SSH_AUTH_SOCK=/tmp/ssh-4Ao0M59fzyhD/agent.12706; export SSH_AUTH_SOCK;
SSH_AGENT_PID=12707; export SSH_AGENT_PID;
echo Agent pid 12707;

List keys, the output is correct, this means that ssh-add can connect to the ssh-agent socket.
Now you can log into your machines, the keys will be added.

The agent has no identities.

If your output looks like below, the variables are not exported correctly, make sure you run ssh-agent using the eval keyword.

Could not open a connection to your authentication agent.

Implementation

Now that we know how ssh-agent works, it would be easy to add this to your ~/.bashrc file, but it would not be a good choice because you will end up with a lot of ssh-agent services running.

I created a script which will first check if there is a running ssh-agent and make sure that we are not running multiple agents.

running_agent_user=$(pgrep -u $USER ssh-agent | wc -l)
tmp_ssh_agent="/tmp/${USER}_ssh_agent"
# timeout in seconds, lifetime = 0 (not a good idea if you are using a server)
timeout_ssh_agent=10800

function start_ssh_agent() {
    echo "Starting ssh-agent"
    ssh-agent -t $timeout_ssh_agent > $tmp_ssh_agent
    chmod 600 $tmp_ssh_agent
    eval $(cat $tmp_ssh_agent)
}

if [ $running_agent_user == 1  ]
then
    echo "ssh-agent already running, setting up the environment variables"
    eval $(cat $tmp_ssh_agent)
elif [ $running_agent_user == 0 ]
then
    echo "ssh-agent is not running"
    start_ssh_agent
else
    echo "Multiple ssh-agent services are running, stopping all the agents"
    kill $(pgrep -u $USER ssh-agent)
    start_ssh_agent
fi

Download from GitHub ssh-agent.bashrc

You can setup a timeout value for ssh-agent, I used 3 hours in my script.. feel free to modify it.
The default value for timeout is forever, so your keys will be kept until you restart the ssh-agent or the timeout value expires.

Add the script with the source keyword to your ~/.bashrc or global bashrc found in /etc.

source /path_to_your_script/ssh-agent.bashrc > /dev/null

With this implementation every user will have it’s own ssh-agent, good luck!