2013-12-11

RSH Pitfall: Remote Exit Code

While writing some test scripts that use rsh (see below about why) to run commands on a remote test server I noticed that rsh and ssh have a significant difference:

ssh reports the remote exit code and rsh does not

As a result all my tests did not test anything, the error condition was never triggered:

My solution is this rsh wrapper:

The reason for using rsh instead of ssh is very simple: In a fully automated environment it provides the same level of security as ssh without the added trouble of maintaining it:

I need to make sure that ssh continues to work after all the SSH host keys change (e.g. after I reinstall the target). Also, to allow unattended operation I must deploy the SSH private keys so that in the end others could also extract them and use them. In the end I would be using IP/hostname restrictions on the SSH server side to restrict the use of the private key.

With rsh I don't need to worry about deploying or maintaining keys and just configure the RSH server to allow connections from my build hosts. Reinstalling either the build hosts or the target host does not change the name so that the RSH connection continues to work.

A couple of months ago I also co-authored an article about SSH security in automated environments in the German Linux Magazin called "Automatische Schlüsselverifikation". The article goes deeper into thchallange of using SSH in a fully automated environment and suggests to explore Kerberized RSH as an easier to maintain alternative.