macOS: use ssh-agent from nix-env

README.md

This recipe allows you to use the ssh-agent from nix-env with the auto generated SSH_AUTH_SOCK in /private/tmp. It works because disabling com.openssh.ssh-agent doesn't stop launchd from allocating SSH_AUTH_SOCK. com.nix.ssh-agent spawns a shell to remove the socket and then execute the homebrew ssh-agent.

1. Install openssh from nix: nix-env --install --attr nikpkgs.openssh
2. Disable the built in macOS agent: launchctl disable gui/$UID/com.openssh.ssh-agent
3. Copy com.nix.ssh-agent.plist into ~/Library/LaunchAgents
4. Enable the new agent: launchctl bootstrap gui/$UID ~/Library/LaunchAgents/com.nix.ssh-agent.plist
5. Ensure that the correct agent is running:

   % launchctl list | grep ssh
   -	0	com.openssh.ssh-agent
   23986	0	com.nix.ssh-agent
   

com.nix.ssh-agent.plist

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
	<key>Label</key>
	<string>com.nix.ssh-agent</string>
	<key>ProgramArguments</key>
	<array>
		<string>/bin/sh</string>
		<string>-c</string>
		<!-- We reuse SSH_AUTH_SOCK from com.openssh.ssh-agent -->
		<string>rm -f $SSH_AUTH_SOCK; exec $HOME/.nix-profile/bin/ssh-agent -D -a $SSH_AUTH_SOCK</string>
	</array>
	<key>RunAtLoad</key>
	<true/>
</dict>
</plist>

I get

/Users/ahelwer/Library/LaunchAgents/com.homebrew.ssh-agent.plist: Invalid property list
Bootstrap failed: 109: Invalid property list

I'm on Ventura 13.0.