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Running a Program on a Remote Server Using SSH

By: C.S. Deam

How do you run a program on a remote server using ssh?

For this example we'll have two servers, one named Johnny and another named Cash. Both are running openssh. Our goal is to have a program on Johnny login to Cash and run a program on Cash. To make the task a little more complex we'll be using different users on each machine.

The first thing we'll need to do is generate public and private keys on Johnny. So, logged into Johnny as user 'boy' we create public and private keys by creating them in the .ssh directory as follows:


Johnny$> pwd


/home/boy/.ssh


Johnny$> ssh-keygen -t rsa -f sue


Generating public/private rsa key pair.


Enter passphrase (empty for no passphrase):


Enter same passphrase again:


Your identification has been saved in sue.


Your public key has been saved in sue.pub.


The key fingerprint is:


8d:e9:c0:g1:c7:1f:e3:b3:2f:38:12:aa:b5:3b:2e:b3 boy@Johnny


Johnny$>

In the example above we picked an arbitrary name, sue, to identify the files that hold the generated keys. When prompted to enter a passphrase (twice) we simply hit enter twice.

As the output of ssh-keygen indicates, the public key has been saved in a file named sue.pub.

The output of ssh-keygen implies, but doesn't indicate directly, that the private key is in a file named sue (Yes, a user named boy created a file named sue.)


Johnny$>ls -l


-rwx------ 1 sue suegrp 887 Oct 17 14:27 sue


-rwx------ 1 sue suegrp 223 Oct 17 14:27 sue.pub

The private key file, sue, will remain on Johnny for the reaminder of this exercise, but the public key must be moved to the remote server, Cash.

Note that the .ssh directory itself, as well as the sue and sue.pub files should have permissions of 700.

Now you'll need to ftp the sue.pub file from Johnny to Cash. The user on Cash that we'll login as is user 'named'.


Johnny$> ftp Cash


Connected to Cash


220 Cash - Propery of Xyz. - Authorized users only


Name (Cash:boy): named


331 Password required for named.


Password:


230-Last unsuccessful login: Fri Oct 17 13:12:55 2003 on ftp from Johnny 230-Last login: Fri Oct 17 16:02:11 2003 on /dev/pts/1 from Johnny


230 User named logged in.


ftp> cd .ssh


ftp> mput sue.pub


mput sue.pub? y


200 PORT command successful.


150 Opening data connection for sue.pub.


226 Transfer complete.


224 bytes sent in 0.000781 seconds (280.1 Kbytes/s)


local: sue.pub remote: sue.pub


ftp>bye


221 Goodbye.


Johnny$>

Now we'll telnet into Cash and concatenate the sue.pub file into /home/named/.ssh/authorized_keys file.


Cash$> pwd


/home/named/.ssh


Cash$> cat sue.pub >> authorized_keys


Cash$> exit


Connection closed.


Johnny$>

Let's recap what we've done so far.

1) We've created public and private keys on Johnny.

2) We've ftp'd the public key file, sue.pub, from Johnny to Cash.

3) We've telnetted to Cash, and concatenated the contents of sue.pub into authorized_keys

We're now ready to manually login from Johnny to Cash using ssh.


Johnny$> ssh -i /home/boy/.ssh/sue named@Cash


The authenticity of host 'Cash (xxx.yyy.zzz.aaa)' can't be established.


RSA key fingerprint is 65:11:7d:ef:ed:a3:cc:34:d1:b5:ba:c9:16:22:31:23.


Are you sure you want to continue connecting (yes/no)? yes


================================================================


*** NOTICE TO ALL USERS ***


================================================================


Cash$>exit


Connection to Cash closed.


Johnny$>

Now on Johnny, create a shell script called 'boynamedsue.sh' with the following one line of contents and chmod the script to 777.

ssh -i /home/boy/.ssh/sue named@Cash /usr/bin/ls -l

Next, execute the script on Johnny.


Johnny$> boynamedsue.sh


================================================================


*** NOTICE TO ALL USERS ***


================================================================


-rwxrwxr-x 1 named namedgrp 10020 Oct 17 14:35 namedfile1.txt


-rw-r--r-- 1 named namedgrp 680 Aug 14 16:18 namedfile.html


-rw------- 1 named namedgrp 1148 Aug 18 09:51 mbox


drwxr-xr-x 2 named namedgrp 512 Jun 17 13:38 old


Johnny$>

You just executed a program on Johnny, that logged into Cash and ran a program (unix 'ls -l').

The next step you'll want to take is to replace the '/usr/bin/ls -l' command in the boynamedsue.sh program with the path and name of the program that you want to run.

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