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An SSH agent is a local process that holds private-key identities and performs authentication operations for SSH clients. The ssh-add command loads keys into an agent that is already running; it is not the agent itself. This lets SSH use a key without repeatedly opening its private-key file. The OpenBSD manuals describe the standard OpenSSH behavior, though defaults and setup can differ by operating system and version.
How an SSH agent and ssh-add work together
The agent usually runs as part of a login or desktop session and starts with no identities loaded. SSH clients locate it through the SSH_AUTH_SOCK environment variable, which identifies the agent’s communication socket. When you run ssh-add, it sends the requested key to that agent; if the private key is encrypted, you enter its passphrase locally.
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In a shell session that already has an agent, a typical sequence is:
ssh-add ~/.ssh/id_ed25519
ssh-add -lThe first command requests that the key be loaded. The second checks which identities the agent currently represents. Consult the OpenBSD ssh-agent(1) manual and OpenBSD ssh-add(1) manual for the documented behavior; installed versions may differ.
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Adding and managing keys
Load a key
Give ssh-add a key-file path to request that specific identity, for example ssh-add ~/.ssh/id_ed25519. With no file argument, the current OpenBSD manual says it tries its documented default identity filenames and a matching certificate where available. Those defaults are version-sensitive and should not be assumed to match every platform.
Inspect or remove identities
| Command | Effect |
|---|---|
ssh-add -l |
Lists fingerprints of identities represented by the agent. |
ssh-add -L |
Lists public-key parameters for identities represented by the agent. |
ssh-add -d ~/.ssh/id_ed25519 |
Removes the specified identity. |
ssh-add -D |
Removes all identities from the agent. |
Limit when or how a key can be used
ssh-add -t 1h ~/.ssh/id_ed25519 requests that the identity remain available for at most the specified lifetime; one hour is only an example. ssh-add -c ~/.ssh/id_ed25519 marks it for confirmation before authentication use. Destination constraints can be requested with ssh-add -h to restrict permitted host paths. The manual says these constraints were added in OpenSSH 8.9 and require cooperating SSH clients and servers for constrained keys over forwarding.
What to do if ssh-add cannot reach the agent
If ssh-add reports that it cannot connect to the authentication agent, either no agent is available to the shell or SSH_AUTH_SOCK does not point to its socket. OpenBSD documents two setup patterns: run a command under ssh-agent, or evaluate the environment commands printed by ssh-agent -s. Exact startup configuration varies among operating systems and login environments, so use the installed system’s manual and session setup rather than assuming one universal command sequence.
What agent forwarding does—and does not do
Agent forwarding lets a remote SSH session send authentication requests through the connection to the agent on your original machine. The private-key file and its passphrase are not sent to the remote host. However, a process on that host that can access the forwarded agent may ask it to perform authentication operations while access remains available. Forward only to hosts you trust. Destination constraints can narrow permitted paths when supported by cooperating clients and servers, but they do not remove every risk from a compromised forwarded session.
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Optional hardware-backed keys
OpenSSH also supports FIDO authenticator keys. The OpenBSD ssh-add manual documents -K for loading resident keys from a FIDO authenticator and -S for selecting an authenticator middleware library. FIDO support is an optional key source, not a requirement for using an SSH agent; the agent applies restrictions to FIDO signatures by default.
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