Auto ssh over a set of machines

I just found out that every pc in the lab have sshd running and waiting at port 22.
Now I immediately thought some nice and nasty things like:

running the same process everywhere

do some funny jokes to the users connected

etc etc

Or simply get a shell with all the laboratory software you need automatically.

If you also have on those machines a listening xorg then you could also use X11 programs over the ssh, which works like a charm. (ssh -Yc “hostname” “command” for example)

The only two “problems” are:

the need to insert the password every time

the need to accept the fingerprint for each new pc

Let’s see how to solve them.

First we generate a new key:

ssh-keygen -t dsa -f $HOME/.ssh/unilab

Then we upload the public key to one of the pc’s, as long as the home directory is mounted over the network (of course) it doesn’t matter which one.

 scp $HOME/.ssh/unilab.pub utente@host:.ssh/ 

Last thing you need to add to the authorized keys your public key (on whatever lab pc you’re logged in).

 cat $HOME/.ssh/unilab.pub >> $HOME/.ssh/authorized_keys 

Now we can set a really nice configuration in our $HOME/.ssh/config.

Here it is:

Host a10?pc*
  User <youruser>
  IdentityFile <your private key>
  Port 22
  StrictHostKeyChecking no

And that’s it, if for example you now do

 ssh a105pc08 

ssh will find the matching on the regular expression and use those settings, letting you login automatically.

But we’re not over, how do we know where sshd is up and running?

We’ll we can do a scan of a list of hosts given in input until we find an ssh up and running

#!/usr/bin/env bash
# usage cluster.sh <command> 
# gives you the first possible shell launching the optional command

NC="/opt/local/bin/nc"
FPING="/opt/local/sbin/fping"

function get_first_shell {
	for host in $(lab5)
	do
    echo "analyzing $host"
    if $FPING -t50 $host > /dev/null
      then 
  		if $NC -z -w 1 $host 22 > /dev/null
  		then
  	    open_shell $host $1 && break
  		fi
		fi
	done
}

function open_shell {
  # -t useful lo launch remote interpreters
	ssh -t $1 $2; return
}

function lab5 {
	for ((i=1; $i < 40; i=$i+1))
	do
		echo "a105pc$(printf '%02d' $i)"
	done
}

get_first_shell $1

This is just an example, it’s enough to substitute the lab5 function with a static list or a generator of your hostnames/ip address you want to scan.
The function get_first_shell first checks that the host is up with fping and then checks that the port 22 is open with netcat , and that’s it!
This is an example of execution:

andreaMb:~ andrea$ get_first_shell 
analyzing a105pc01
analyzing a105pc02
analyzing a105pc03
analyzing a105pc04
analyzing a105pc05
analyzing a105pc06
analyzing a105pc07
analyzing a105pc08
Linux austudNONETWORK 2.6.24-23-generic #1 SMP Thu Nov 27 18:44:42 UTC 2008 i686

The programs included with the Ubuntu system are free software;
the exact distribution terms for each program are described in the
individual files in /usr/share/doc/*/copyright.

Ubuntu comes with ABSOLUTELY NO WARRANTY, to the extent permitted by
applicable law.

To access official Ubuntu documentation, please visit:
http://help.ubuntu.com/
Last login: Thu Mar 19 12:22:31 2009 from 172.31.121.180
andrea.crotti@a105pc08:~$

Now we have the automatic access, GREAT!

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