Using OpenSSH for Linux administration

Here are some hints and tips for those who are new to using ssh/OpenSSH for Linux system administration. Most of these tips have come from my recent work with a large number of Linux servers hosted on a VMware ESXi 4.x server farm.

Password authentication VS ssh key authentication

  • If you are administering only a few systems on a closed network (i.e. accessible only locally or by a secure VPN connection), then password authentication is probably OK, but you should consider using ssh keys anyway.
  • If your network needs to allow ssh access directly from the Internet or you are administering a large number of systems, then you should definitely use ssh keys.

Ssh-agent, scripting and cron

  • ssh-agent can save you typing in the password to your ssh key every time you need it.
  • This site gives a good overview of ssh-agent and includes some code you can add to your .bash_profile script to ensure your keys get added upon login.
  • Although there are hack-ish ways to get ssh-agent and cron to work together, you are probably better off setting up special keys to use with scripts that must be called via cron. Just keep in mind that keys without passwords are a security risk.
  • If you cannot risk using keys without passwords, consider running those cron scripts locally on each system. Utilize shared file space or e-mail to collect the results.

Bash one-liners and ssh with ssh keys

  • I’ve become a fan of using bash “one-liner” scripts to keep abreast of server stats such as load averages, available patches and disk usage.
  • Keep an up-to-date list of hosts in a file called hostlist.
  • Run your one-liners while ssh-agent has your ssh keys cached.
  • Here’s a template one-liner which checks uptime on each host listed in the file hostlist:

for e in `cat hostlist`; do echo $e; ssh $e "uptime"; done

  • In the above example, you can replace uptime with just about any command which exists on the remote host.
  • You can also synchronize some of the configurations under /etc with the above by utilizing either scp or rsync instead of ssh in that one-liner.

Turn your one-liners into scripts

  • If you find yourself using the same one-liner over and over, it is time to save yourself some typing and turn it into a script.
  • I like to keep these sorts of scripts under ~/bin. I also like to add that to my $PATH and create a simlink ~/scripts.
  • Some one-liners are good candidates to be turned in to cron scripts. Just keep in mind the risks of using ssh keys without passwords, and include logic to detect conditions you want to monitor. For example, you can run /proc/loadavg through awk to isolate one of the three figures and send yourself an e-mail if that average is too high.