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Use OpenSSH’s ssh-agent to unlock a passphrase-protected private key once, then let SSH clients use that loaded identity while it remains available to the agent. Start or connect to an agent, add the key with ssh-add, and configure SSH to add a key automatically if that better fits your workflow.
How the SSH agent stops repeated passphrase prompts
An SSH agent holds identities for public-key authentication. It starts with no keys; ssh-add loads a private-key identity into it. You enter the key’s passphrase when adding it, and SSH clients connected to that agent can use the loaded identity without unlocking the key file for each authentication. The identity remains usable only while it is loaded and the agent is available.
SSH clients find the agent through their environment, especially SSH_AUTH_SOCK, which identifies the Unix-domain socket used to communicate with it. If a shell or program does not inherit the right environment, it may not reach the agent you expect. See the OpenBSD ssh-agent manual.
Load a key manually for your shell session
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Start an agent if your environment has not already provided one. In a shell login,
ssh-agentcan print commands to evaluate in the calling shell so that it receives the agent environment. Follow the syntax for your shell; do not start another agent if your environment already provides one.#1 Best Overall
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Add your private key, substituting its actual path if it is not the default:
ssh-add ~/.ssh/id_ed25519Enter the key’s passphrase when prompted. The agent now has that identity available to connected SSH clients.
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Connect as usual, for example with
ssh user@host. If the client is connected to the same agent and selects the loaded identity, it can authenticate without asking you to unlock the key again.Quick wins for a faster PC:
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Use ssh-add again when you need to load another identity. For the command’s behavior and options, see the OpenBSD ssh-add manual.
Choose between manual and automatic loading
Manual loading with ssh-add
With the manual approach, you decide when the key enters the agent and can load only the identities you need. It requires an explicit ssh-add command for a new agent session or after an identity is no longer loaded.
Automatic loading with AddKeysToAgent
To have SSH add a file-backed key to an agent when the key is loaded, set AddKeysToAgent yes for the relevant host in ~/.ssh/config:
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Host example
HostName example.org
User alice
IdentityFile ~/.ssh/id_ed25519
IdentitiesOnly yes
AddKeysToAgent yesWith this setting, the key may be added as part of using the matching SSH configuration rather than through a separate manual command. That is more convenient, but makes adding the configured key less deliberate. The current OpenBSD ssh_config manual documents AddKeysToAgent as defaulting to no; check the manual for your installed OpenSSH version because platform packages can differ.
Set key lifetime and use limits
An identity can remain available for the life of the agent unless you set a lifetime. In ssh_config, AddKeysToAgent can take a time interval to set a key lifetime; it can also use confirm to require confirmation on each use. For example, a finite interval trades some convenience for limiting how long the identity remains loaded. Choose a value that suits your work and trust in the devices and sessions where the agent is accessible; the manuals do not prescribe one universal duration.
Manual and automatic loading are separate from lifetime: either way, consider whether you want an identity available indefinitely for that agent session or only for a defined interval. Consult the installed version’s ssh_config options for accepted interval syntax and platform-specific behavior.
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Control which key a host uses
If the agent holds several identities, narrow what a host can use with IdentityFile and IdentitiesOnly yes. In the example above, those settings identify the intended key rather than offering every identity available through the agent.
You can also control agent selection with IdentityAgent: set it to a particular socket when a host should use a specific agent, or set it to none to disable agent use for that host. These choices are useful when a shell can access more than one agent or when a particular connection should not use one.
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Understand agent forwarding before enabling it
Agent forwarding lets a remote host use your local agent for onward authentication; it does not copy the private-key file to that host. That distinction does not make forwarding harmless: someone on the remote host who can access the forwarded socket may request authentication operations using identities loaded in your local agent, even though they cannot obtain the key material itself.
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Leave forwarding disabled unless you need it for a specific onward connection, and forward only to a remote machine you trust. Forwarding delegates access to agent operations for the duration and scope of that connection.
Troubleshoot a key that is not being used
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Check that the identity is loaded. If it was not added, use
ssh-add ~/.ssh/id_ed25519or the path to the intended private key. -
Check the agent connection. Confirm that
SSH_AUTH_SOCKis set in the shell running SSH and points to the expected agent environment. A process that did not inherit the right environment may be connected to no agent or a different one.Special offer. See more information about Outbyte and uninstall instructions. Please review EULA and Privacy policy. -
Check key selection. A client configuration may select a different identity. Set the intended
IdentityFileand useIdentitiesOnly yesfor the host when you want to constrain selection. -
Check the host-specific settings. Confirm that the relevant
Hostblock matches the destination and that its agent and identity options are appropriate. OpenSSH options and defaults can vary with the installed version.Quick Recap
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