How to enable SSH agent forwarding

Forwarding an SSH agent lets a remote session authenticate to another host with a key that stays on the workstation. It fits bastion and jump-host handoffs, but it also gives the trusted remote host a live signing path back to the local agent while the session is open.

The OpenSSH client enables agent forwarding with ForwardAgent yes in a matching Host block, or with -A for a single command. A forwarded session exposes an SSH_AUTH_SOCK path on the remote host; programs there can ask the local agent to sign authentication challenges, but they cannot read the private key file itself.

Keep forwarding host-specific and load only the identities needed for the onward connection. The server-side AllowAgentForwarding setting can further restrict which bastion accounts receive forwarded sockets, and the final proof is a remote ssh-add -l output that matches the local agent before an onward login succeeds.

Steps to enable SSH agent forwarding securely:

List keys loaded in the local ssh-agent.
```
$ ssh-add -l
256 SHA256:G9x8RpzHGWETUZiMicVJWorguVdQRuvL/1GwuBUBrl0 user@workstation (ED25519)
```
If the agent reports The agent has no identities., load the key that the internal host already trusts before enabling forwarding.

Related: How to add an SSH key to ssh-agent
Create the per-user SSH configuration directory.
```
$ mkdir -p ~/.ssh
```
Restrict the per-user SSH configuration directory.
```
$ chmod 700 ~/.ssh
```
Open the per-user SSH client configuration file.
```
$ vi ~/.ssh/config
```
Add a host-specific block for the trusted bastion.
```
Host bastion
    HostName bastion.example.net
    User ops
    IdentityFile ~/.ssh/id_ed25519
    IdentitiesOnly yes
    ForwardAgent yes
```
Do not place ForwardAgent yes in a broad Host * block because every matched remote host would receive a live agent socket while sessions remain open.
Tool: SSH Client Config Editor
Restrict the client configuration file.
```
$ chmod 600 ~/.ssh/config
```
OpenSSH may ignore client configuration files that are writable by other users.
Confirm the saved client alias enables agent forwarding.
```
$ ssh -G bastion | grep "^forwardagent "
forwardagent yes
```
ssh -G prints the effective client settings after Host and Match processing.

Related: How to show SSH client configuration
Check whether the bastion server permits agent forwarding for the intended account.
```
$ sudo sshd -T -C "user=ops,host=bastion.example.net,addr=203.0.113.10" | grep "^allowagentforwarding "
allowagentforwarding yes
```
Replace ops and the addr value with the account and client source address that should receive the forwarded agent.
Open the sshd configuration on the bastion when server policy needs a narrower allow list.
```
$ sudoedit /etc/ssh/sshd_config
```
Keep an existing root session or console path open before changing /etc/ssh/sshd_config on a remote bastion.
Add a restrictive server policy that allows agent forwarding only for the selected account.
```
AllowAgentForwarding no

Match User ops
    AllowAgentForwarding yes
```
Place unconditional server directives before the first Match block. Later Match blocks apply only to sessions that meet their conditions.
Validate the sshd configuration syntax.
```
$ sudo sshd -t
```
No output means the server configuration parsed successfully.

Related: How to test SSH server configuration
Reload the SSH service on the bastion.
```
$ sudo systemctl reload ssh
```
Use sudo systemctl reload sshd on systems where the OpenSSH server unit is named sshd.

Related: How to manage the SSH server service with systemctl
List forwarded identities from the bastion session.
```
$ ssh bastion "ssh-add -l"
256 SHA256:G9x8RpzHGWETUZiMicVJWorguVdQRuvL/1GwuBUBrl0 user@workstation (ED25519)
```
The fingerprint should match the local ssh-add -l output because the remote host is querying the forwarded agent.
Use the forwarded agent to reach the internal host from the bastion.
```
$ ssh bastion "ssh app.internal.example hostname"
app.internal.example
```
A compromised bastion can request signatures from the forwarded agent while the session remains open, so close the session when the onward work is finished.