I am new to ssh keys. Can anyone explain how the ssh-agent bash and ssh-add works?

I need to understand its internals in future.

  • Have you tried the top Google hit for any of these terms? – Daniel Beck May 16 '11 at 19:37
  • Yes, I did. I didn't get any useful information. My bad – maneeshshetty May 16 '11 at 20:06
up vote 35 down vote accepted

An agent is a program that keeps your keys in memory so that you only need to unlock them once, instead of every time. ssh-agent does this for SSH keys.

The usual methods for starting ssh-agent are:

  • eval `ssh-agent` – this runs the agent in background, and sets the apropriate environment variables for the current shell instance.

    (ssh-agent, when started with no arguments, outputs commands to be interpreted by your shell.)

  • exec ssh-agent bash – starts a new instance of the bash shell, replacing the current one.

    (With one or more arguments, ssh-agent doesn't output anything, but starts the specified command: in this case, the bash shell, but technically it could be anything.)

    The second method is sometimes preferred, since it automatically kills ssh-agent when you close the terminal window. (When starting it with eval, the agent would remain running, but inaccessible.)

However, this only starts an empty agent. To actually make it useful, you need to use ssh-add, which unlocks your keys (usually ~/.ssh/id_*) and loads them into the agent, making them accessible to ssh or sftp connections.

  • Is there a way to start ssh-agent across multiple bash sessions, once? – Asim Jan 3 at 12:15

Additionally, you may want to add some keys at session start.

Edit your ~/.bashrc file, and add :

ssh-add &>/dev/null || eval `ssh-agent` &>/dev/null  # start ssh-agent if not present
[ $? -eq 0 ] && {                                     # ssh-agent has started
ssh-add ~/.ssh/your_private.key1 &>/dev/null        # Load key 1
ssh-add ~/.ssh/your_private.key2 &>/dev/null        # Load key 2
}

Check your keys with ssh-add -l

You can stop the current ssh-agent session with ssh-agent -k

Something to know about ssh-agent and .bashrc is don't load too many keys. The default number of tries for ssh daemon is limited to 6. This can been modified in /etc/ssh/sshd_config with the MaxAuthTries value.

  • 1
    What about the private key password when launching ssh-add with "&" ? Are you assuming the private key files are unprotected ? – Luciano Aug 7 '17 at 18:57

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