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I have an already established ssh connection between two machines.
Is there a way to send commands to the remote machine from a shell script that is run on the local machine, using the already open connection, and without starting another ssh session?
It's very simple with recent enough versions of OpenSSH if you plan in advance.
Open a master connection the first time. For subsequent connections, route slave connections through the existing master connection. In your ~/.ssh/config, set up connection sharing to happen automatically:
ControlMaster auto
ControlPath ~/.ssh/control:%h:%p:%r
If you start an ssh session to the same (user, port, machine) as an existing connection, the second session will be tunneled over the first. Establishing the second connection requires no new authentication and is very fast.
/var/log/secure and /var/log/auth.log log SSH connections; a slave connection does not appear there because it piggybacks on an existing connection. If your ssh session allocates a terminal (i.e. ssh somehost with no command supplied, or ssh -t), that is (normally) logged in wtmp, regardless of how that terminal appeared (sshd whatever method was used to establish the connection, terminal emulator application, …).
Sep 6, 2013 at 15:51
ControlPersist 600 which is a delay in seconds of the socket being in idle before it gets automatically deleted. Otherwise it will close down automatically when the master connection ends. That's no good for executing a series of commands remotely (eg. a series of rsync commands to different folders)
-S (specify socket) and -M (create master connection) of the SSH client.
That's pretty easy to achieve using the nc tool and ssh tunnels.
In your ssh session, type ~C on a new line. You will get the ssh "service console" prompt which looks like this:
ssh>
Type in the local forward command to open an ssh tunnel:
ssh> -L22000:targethost:22001
Forwarding port.
Where targethost is the hostname or IP address of the machine you are connected to.
Now, assuming the ssh server on the target machine wasn't configured to forbid tunnels, you have the desired connection forwarding: ssh client on your machine listens to port 22000, and it will forward any traffic sent to it to the 22001 port on targethost.
This is as simple as entering into your already open ssh session the following command:
remote$ nc -l localhost 22001 | sh
This will start a TCP server listening on port 22001 – which is the target port of our ssh tunnel – and route the received data (presumably, shell commands) to a targethost shell instance.
local$ cat yourscript.sh | nc localhost 22000
This will send the script's body to your ssh tunnel and will end up being executed in a shell on the targethost. You will see script's output in your terminal with ssh session.
I'll also note that ssh tunnel (step 1.) in this scenario isn't strictly required; you could as well start the server open and connect to it directly over the internet. However, you will need to use the tunnel if the target host can't be reached directly (e.g. is behind a NAT), or ssh encryption is desired.
~ character must come after a newline, so LF ~ C is probably a better sequence.
less, the common default pager, supports search which can save you some scrolling if you know your keywords: just type man ssh /ESCAPE and you're there.
It took a while to get this to work, see here for a full example and explanations.
In short you need to:
create an ssh config file that hold information about the ssh session
remote=your_ip_address
echo "HostName $remote" > ssh_config
echo "User root" >> ssh_config
echo "ControlMaster auto" >> ssh_config
echo "ControlPath ~/.ssh/%C" >> ssh_config
master_ssh='ssh -F ssh_config'
start the master session
$master_ssh -MNf $remote
do the rsyncs
rsync -e "$master_ssh" $opts $remote:/etc/some_file1 local_directory/
rsync -e "$master_ssh" $opts $remote:/etc/some_file2 local_directory/
rsync -e "$master_ssh" $opts $remote:/etc/some_file3 local_directory/
close the master session
$master_ssh -O exit $remote
