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SSH, The Secure Shell: The Definitive Guide, 2nd Edition by Robert G. Byrnes, Richard E. Silverman, Daniel J. Barrett

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The SSH Agent

Each time you run ssh or scp with public-key authentication, you have to retype your passphrase. The first few times you might not mind, but eventually this retyping gets annoying. Wouldn’t it be nicer to identify yourself just once and have ssh and scp remember your identity until further notice (for example, until you log out), not prompting for your passphrase? In fact, this is just what an SSH agent does for you.

An agent is a program that keeps private keys in memory and provides authentication services to SSH clients. If you preload an agent with private keys at the beginning of a login session, your SSH clients won’t prompt for passphrases. Instead, they communicate with the agent as needed. The effects of the agent last until you terminate the agent, usually just before logging out. The agent program for both OpenSSH and Tectia is called ssh-agent.

Generally, you run a single ssh-agent in your local login session, before running any SSH clients. You can run the agent by hand, but people usually edit their login files (for example, ~/.login or ~/.xsession) to run the agent automatically. SSH clients communicate with the agent via a local socket or named pipe whose filename is stored in an environment variable, so all clients (and all other processes) within your login session have access to the agent. [6.3.4] To try the agent, type:

    $ ssh-agent $SHELL

where SHELL is the environment variable containing the name of your login shell. Alternatively, you could ...

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