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Currently i invoke the following:

$ ssh [email protected] my_cmd

This is slow and not easy to automate safely. I would like to establish ssh connection once and have some script that will forward my commands to host.com and print output.

Is that possible ?

Adding my machine to authorized_keys is not an option for me and it wouldn't solve slowness issue.

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2 Answers 2

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The feature is called ControlMaster which does multiplexing over one existing channel. It causes ssh to do all of the key exchanges and logging in only once; thus, the later commands will go through much faster. You activate it using these three lines in your .ssh/config:

Host host.com
  ControlMaster auto
  ControlPath ~/.ssh/master-%C
  # for openssh < 6.7 you need to use this one:
  # ControlPath ~/.ssh/master-%r@%h-%p
  ControlPersist 5m

You can adjust it to your needs; one alternative is that you could open one master connection that stays open during your other commands; then you would not need ControlPersist.

There are many possibilities with this feature to tweak, but make sure you store your ControlPath socket in a safe place, not readable by other users, otherwise it could be misused.

More info can be found in the ssh_config(5) manual page.

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  • percent_expand: unknown key %C Commented Jul 26, 2015 at 1:32
  • 1
    It looks like the %C substitution was added in openssh-6.7 so it doesn't work in all openssh versions. If it doesn't work, you can always use %r@%h-%p as proposed in the other answer. I will update mine accordingly. Commented Jul 26, 2015 at 7:59
  • do note that multiplexed connections do not have access to the following escape options: ~C - open a command line. ~V/v - decrease/increase verbosity (LogLevel). ~^Z - suspend ssh. ~& - background ssh (when waiting for connections to terminate). you still have access to them in the "primary" connection though (the one that originally created the multiplexed socket) Commented Jun 23, 2024 at 6:52
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If you have control of the machine to the point that you are automating tasks on it, then why is adding your key to authorized_keys not an option? 

ssh-copy-id -i ~/.ssh/foo [email protected]

Then you don't have to enter a password every time you connect.

If the biggest problem is that connections take a long time to connect, you could reuse a single connection by adding control master to your ssh config. Leave that one connection running, an any subsequent connections will be nearly instantaneous. 

Host *
ControlMaster auto
ControlPath ~/.ssh/sockets/%r@%h-%p
ControlPersist 600

https://puppetlabs.com/blog/speed-up-ssh-by-reusing-connections

In the long run, if you are automating tasks, you are proabably better off using an automation framework that handles establishing the connection for you, like :

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1
  • Server owner policy might prevent use of authorized_keys Commented Jul 25, 2015 at 23:01

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