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I want to run ssh-agent (with maximum lifetime option), but not add any keys at startup, but instead add them on demand.

Like first time I login to some server it should ask for passphrase, next time (unless I waited for more than a hour) it should connect cleanly:

ssh server1
Enter passphrase for key '/home/vi/.ssh/id_dsa':
server1> ...

ssh server2
server2> # no passphrase this time

# wait for lifetime

ssh server2
Enter passphrase for key '/home/vi/.ssh/id_dsa':

I don't want to manually remember about running 'ssh-add' each time. (e.g. entered passphrase for just for ssh and "Oh, it hasn't remembered, need to retype").

How to configure ssh to automatically add key to ssh-agent if user provided the passphrase?

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4 Answers 4

109

ssh supports adding a key to the agent on first use (since version 7.2).  You can enable that feature by putting the following into ~/.ssh/config:

AddKeysToAgent yes

This also works when using derivative tools, such as git.

From the 7.2 changelog:

  • ssh(1): Add an AddKeysToAgent client option which can be set to 'yes', 'no', 'ask', or 'confirm', and defaults to 'no'.  When enabled, a private key that is used during authentication will be added to ssh-agent if it is running (with confirmation enabled if set to 'confirm').
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  • This requires a ssh-agent to be running. Refer to stackoverflow.com/a/24347344/4573065 + its comments for a good way to start it (only once).
    – ST-DDT
    Jul 10, 2018 at 9:23
  • 4
    you may want to make sure your ~/.ssh/config file has the right permissions with chmod 600 ~/.ssh/config
    – Kerem
    Jan 3, 2020 at 18:56
  • This feature is so undesirable. I want to add the key to the agent for my git repo, not the key that I use to connect to the sandbox and if not for this conf option I would see no reason for them to be the same key. Aug 10, 2023 at 16:37
  • @Evan Carroll this, or any option, in ~/.ssh/config applies only to host names as typed at the ssh command line (or configured into git as a remote to use) that match the host line preceding them. The entries on that host line can be aliases with the actual host name listed on a 'hostname' line; so you can even have different sets of options for the same server if that should be desirable. See linuxize.com/post/using-the-ssh-config-file or Google "SSH Config Syntax" and dig around. Aug 18, 2023 at 19:08
  • 1
    @EvanCaroll, oh you are wanting to preload other keys, probably for use with ForwardAgent so you can connect to B(git) from inside your connection to A(sandbox). Yes that would be a different sort of extremely handy. I don't think this can be done without scripting around ssh-agent and ssh-add. You might checkout this answer and the options it lists, such as ssh-ident, though they might not be Windows compatible if that is an issue. Aug 18, 2023 at 23:47
18

You could cheat and put something like alias ssh='ssh-add -l || ssh-add && ssh' on your .bashrc / .profile. This first runs ssh-add -l, which can return 0 (there are keys on agent), 1 (no keys) or 2 (no agent running); if it returns 0, ssh will run; if 1, ssh-add will run and then ssh; if 2, ssh-add will fail and ssh won't be run. Replace the && with ; if you want ssh to run even when there's no agent running.

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  • 4
    It will request passphrase to the key even when further ssh command does not use it.
    – Vi.
    Sep 9, 2011 at 15:02
  • @Vi. true, but rare are the commands that don't use them (unless you're connecting to a server that uses keyboard-interactive). The key will be in the agent whenever you need them afterwards though. Also, the alias fortunately (or unfortunately) doesn't change backend uses of ssh (such as with git or rsync).
    – Jessidhia
    Apr 3, 2012 at 12:05
  • 2
    (thinking about writing auto-call-ssh-add patch for ssh client)
    – Vi.
    Apr 3, 2012 at 17:48
  • Based on this answer you can create an aliases for a specific hosts: alias ssh-hostname='(ssh-add -l | grep hostname > /dev/null) || ssh-add ~/.ssh/id_rsa_hostname && ssh -p 12345 username@hostname'. You could even put that in a loop for all your ssh keys and generate aliases. A dirty hack, but it works.
    – dset0x
    Apr 19, 2013 at 16:04
5

Until auto-call-ssh-add is supported by ssh, I added this in my .bashrc, based on Kovensky proposal:

ssh-add -l >/dev/null || alias ssh='ssh-add -l >/dev/null || ssh-add && unalias ssh; ssh'

The alias is created only if the identity is not added, and the alias destroys itself once run.

This way the regular ssh command is used after the identity has been added.

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  • Will not work in my case because of ssh-add also have timeout configured, but the idea is good enough.
    – Vi.
    Sep 8, 2012 at 12:04
  • @Vi. How would a timeout affect this? It seems to work great for me.
    – lanrat
    May 4, 2014 at 19:22
  • 1
    @lanrat, This checks if key is present in ssh-agent and configures alias depending on the presence. But due to timeout (ssh-add -t ...) the key added to ssh-agent may go away abruptly, but and the alias will stay as it the key is still in memory.
    – Vi.
    May 5, 2014 at 9:18
2

I'm using the following shell function:

ssh() {
    local possible_keys=($(/usr/bin/env ssh -G $@ | grep '^identityfile' \
                           | cut -d " " -f 2- | sed -e "s|^~|$HOME|"))
    for k in $possible_keys; do
        if [[ -f $k ]]; then
            local fingerprint=$(ssh-keygen -lf $k)
            ssh-add -l | grep -q "$fingerprint" || ssh-add $k
        fi
    done
    /usr/bin/env ssh $@
    return $?
}

It first resolves the configuration for the host I'm trying to connect to, then adds possible keys for that host to the ssh-agent if they're not added yet and finally connects to the host. I'm sure it can be improved, so I'm open for feedback.

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