SSH Agent Forwarding

SSH keys backed by hardware security keys with verify-required need a PIN on every use. By default, the ssh-agent cannot prompt for the PIN when a request comes from a remote host, so agent forwarding fails with agent refused operation.

The fix is to install ssh-askpass, which gives the agent a GUI dialog to collect the PIN on your local machine, even when the signing request originates from a remote VM.

Setup

Install ssh-askpass

macOS

brew tap theseal/ssh-askpass
brew install ssh-askpass

Configure the environment

Add to your shell rc file (~/.zshrc, ~/.bashrc, etc.):

export SSH_ASKPASS=ssh-askpass
export SSH_ASKPASS_REQUIRE=force

The path varies by distro — run which ssh-askpass to confirm it resolves. On Debian/Ubuntu with ssh-askpass-gnome, the system typically symlinks /usr/bin/ssh-askpass automatically.

Then restart the agent:

pkill ssh-agent
source ~/.zshrc   # or ~/.bashrc

Remote VM

1. Copy your signing public key to the VM:

scp ~/.ssh/id_ed25519_sk_rk_sign-SERIAL.pub VMHOST:~/.ssh/

2. On the VM, configure git to use the forwarded agent for commit signing:

git config --global gpg.format ssh
git config --global user.signingKey ~/.ssh/id_ed25519_sk_rk_sign-SERIAL.pub
git config --global commit.gpgsign true

You SHOULD NOT add an IdentityFile directive in the VM's ~/.ssh/config — the forwarded agent handles authentication.

3. Ensure sshd allows forwarding on every hop:

AllowAgentForwarding yes

How it works

VM: git commit
  -> forwarded agent -> local machine ssh-agent
  -> ssh-askpass pops up GUI PIN dialog on local machine
  -> enter PIN + touch YubiKey
  -> signed commit on VM

The PIN never leaves your local machine. The YubiKey never leaves your local machine. The VM only sees the signed result.

Verify

# From your local terminal, through the VM
ssh VMHOST 'ssh -T git@github.com'

# On the VM
ssh-add -l                              # Should show your keys
git commit --allow-empty -m "test"      # PIN dialog on local machine + touch
git log --show-signature -1             # Good signature

Troubleshooting