How to start and use ssh-agent as systemd service?

1. How to start ssh-agent as systemd service? There are some suggestions in the net, but they are not complete.

2. How to add automatically unencrypted keys if ssh-agent service was started successfully? Probably, adding keys from the list of ~/.ssh/.session-keys would be good.
3. How to set SSH_AUTH_SOCK in any login session afterwards? The most correct way is to push it from ssh-agent service to systemd-logind service (have no idea if it’s ever possible). The plain naive way is just add it to /etc/profile.

Answers:

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Method 1

To create a systemd ssh-agent service, you need to create a file in ~/.config/systemd/user/ssh-agent.service because ssh-agent is user isolated.

[Unit]
Description=SSH key agent

[Service]
Type=simple
Environment=SSH_AUTH_SOCK=%t/ssh-agent.socket
ExecStart=/usr/bin/ssh-agent -D -a $SSH_AUTH_SOCK

[Install]
WantedBy=default.target

Finally enable and start this service.

systemctl --user enable ssh-agent
systemctl --user start ssh-agent

This will instruct the ssh client to always add the key to a running agent, so there’s no need to ssh-add it beforehand.

Note that when you create the ~/.ssh/config file you may need to run:

chmod 600 ~/.ssh/config
or
chown $USER ~/.ssh/config

Otherwise, you might receive the Bad owner or permissions on ~/.ssh/config error.

Method 2

This is not supported if you are using centos 7 because it will not support the --user flag of systemctl. See this centos bug report, Systemd User Support is Broken on Delivery

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0