Reusing an existing SSH connection for subsequent connects

From BNL Physics Computing

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Reusing an established SSH connection

Recent OpenSSH provides a "master" mode which allows a connection to be shared by subsequent ones. It can be accessed via the command line or for commonly used servers it can be set up more conveniently in the .ssh/config file. This can be handy for users of CryptoCard as it lessens the fumbling about with the hardware token.

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Command line

Start the master: 

 user@home> ssh -M -S ~/.ssh/gateway.socket user@gateway

This will give you an otherwise normal connection. Subsequent connections can make use of this established one by doing the same command w/out the -M: 

 user@home> ssh -S ~/.ssh/gateway.socket gateway

Note when the SSH client subsequently uses the socket it doesn't care what the target user@host is. You can send commands to the master like: 

 user@home> ssh -S ~/.ssh/gateway.socket -O check foo 
 Master running (pid=18247)
 user@home> ssh -S ~/.ssh/gateway.socket -O exit foo 
 Exit request sent.

The exit command will terminate the master and any subsequent connections.

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Automating mastermode

This requires OpenSSH 4.2 or later. You can automate the use of sockets by adding the following to ~/.ssh/config. 

 Host *
   ControlMaster auto
   ControlPath ~/.ssh/sockets/%r@%h:%p

You will need to first make the directory: 

 user@home> mkdir ~/.ssh/sockets

Then, just log in everywhere as normal. The sockets will be used automatically.

If you do not have OpenSSH 4.2, you can still specify a ControlPath but it must be explicit for each host and ControlMaster must be yes

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