If you are making a lot of SSH connections, starting each connection can add noticeable overhead. Even worse, a firewall might start blocking the connections as many SSH connections from the same source looks a lot like an attacker trying to guess a password, as one of my officemates discovered recently.
SSH has a feature called multiplexing, which is described in this blog post, along with a few other useful SSH tips. Here's the relevant excerpt:
In a shell:$ mkdir -p ~/.ssh/connections $ chmod 700 ~/.ssh/connections
Add this to your
~/.ssh/configfile:Host * ControlMaster auto ControlPath ~/.ssh/connections/%r_%h_%p
ssh is often used as just a secure version of
telnet, it's actually closer to being a VPN
system, supporting many channels of communication over the same encrypted
link, which is how port forwarding over SSH is implemented.
Normally SSH makes a connection and opens a single channel for the
terminal. Multiplexing merely means keeping that connection open for
additional terminal channels. The settings described tell SSH to keep
track of open connections in
~/.ssh/connections/ and automatically
reuse an open connection whenever possible.
The firewall which caused this post to get written was keeping track of how many new SSH connections were made to a host and only allow a maximum of 3 new connections each minute. As the firewall was not paying attention to whether the connections were accepted, my officemate's script which performed multiple copies and remote commands was getting blocked.