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SSH_AUTH_SOCK tells an SSH client where to find the authentication agent’s Unix-domain socket. It is not a private-key path. To diagnose connection problems, check the variable in the exact shell or process where SSH fails, then use ssh-add -L to determine whether the agent is reachable and has identities loaded.
Start with the failing process
Run these checks in the same terminal, IDE, container, or other process context that launches the failing SSH command:
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printf '%sn' "$SSH_AUTH_SOCK"shows the socket pathname available to that process. An empty result means the process has no socket path in its environment.- If a path is printed, run
ssh-add -L. It lists identities the agent exposes; if the agent cannot be contacted,ssh-adddocuments exit status 2 for that failure. - If the agent responds but the intended identity is missing, add it with
ssh-add path/to/key, then runssh-add -Lagain.
These checks distinguish three different conditions: no agent socket in the process environment, an unreachable agent, and a reachable agent without the needed identity. A printed path alone does not prove that the socket is live. See the OpenSSH ssh-add(1) manual and OpenSSH ssh-agent(1) manual.
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The failing process has not inherited a socket pathname. Check how the agent is started and how its environment reaches the shell or application that runs SSH. A value set in another terminal does not automatically appear in this one.
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OpenSSH documents two general approaches: run a child command under ssh-agent, or have the agent print environment-setting commands and evaluate them in the calling shell. For a Bourne-style shell, the documented form is:
eval "$(ssh-agent -s)"
Use syntax appropriate to your actual shell; that command is not universal shell syntax. If an agent is already running in another session, arrange for the failing process to receive that agent’s actual environment rather than guessing a socket pathname. Startup and session management vary by operating system and agent manager, so there is no single restart command that applies everywhere.
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If the socket path exists but the agent cannot be contacted
Run ssh-add -L in the failing process’s environment. If it cannot reach the agent, verify that the socket belongs to a running agent and that the process can access it. The variable can contain a pathname even when the socket is stale or inaccessible.
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Do not replace the value with a guessed path. Have the agent or session manager establish its real socket path and pass that environment to the process that needs it. Socket locations and startup behavior can differ across OpenSSH versions and platforms; consult the installed agent manual and identity-management manual.
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If the agent responds but has no usable key
An agent can be reachable while exposing no identities: OpenSSH agents start with no private keys loaded. Use ssh-add -L to see what the agent currently offers. If the intended key is absent, add it and check again:
ssh-add path/to/key
On Apple systems, GitHub’s guide documents ssh-add --apple-use-keychain for Apple’s standard ssh-add. That option is implementation-specific, and older macOS releases used different flags; do not assume it works with every SSH implementation. See GitHub’s guide to SSH key passphrases.
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If agent forwarding is not working on a remote host
Forwarding is a separate step from local key loading. Confirm first that the local agent exposes the intended identity. Then check the client’s forwarding setting, server policy, and the environment on the remote host.
Check the client setting
OpenSSH’s client default is ForwardAgent no. A host-specific entry in SSH configuration or a command-line option can change the effective setting. The client configuration accepts yes, no, a socket path, or an environment-variable name for ForwardAgent. Consult the OpenSSH ssh_config(5) manual for the syntax and behavior supported by your installed version.
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Check server policy and the remote environment
The server’s AllowAgentForwarding setting controls whether it permits forwarding. Its documented default is yes, but the server’s actual configuration may override that default. See the OpenSSH sshd_config(5) manual.
After connecting, check SSH_AUTH_SOCK on the remote host. If the variable is empty there, the remote session does not have a forwarded socket path. GitHub’s SSH agent forwarding guide also recommends checking the remote environment and testing the target authentication again. For Git-over-SSH, make sure the repository remote uses an SSH URL: an HTTPS connection does not use the SSH agent.
Forwarding is a trust decision
Forwarding does not transmit the private-key file, but it lets a remote host request authentication operations from the local agent while the forwarded connection is available. OpenSSH warns that a user able to access the forwarded socket can use the identities loaded in the agent to authenticate. Forward only to hosts you trust, and leave forwarding disabled where it is not needed. The OpenSSH manual states: “Agent forwarding should be enabled with caution.”
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