Thursday, November 22, 2007

A Quick Guide To Setting Up SSH Keys Network Wide

Today, in honor of the spirit of the day, I thought I'd write a little something for which you might, eventually, give many thanks :)

As we mentioned in yesterday's post, once you have SSH keys set up network-wide, so that you can passwordlessly login as yourself on all the machines you administrate, our "scmd" script can be a blessing; especially if you're ever stuck with the seemingly impossible task of retrieving some obscure bit of information off of every single box!

I won't lie to you; this is the worst part of the process. In order to get your keys initially set up, you'll have to put forth a good deal of effort (Your misery will be commensurate with the size of your network). But, once you're done, you'll be on easy street.

For our purposes, we're going to assume that you're using Solaris' (now) standard implementation of SSH (Basically OpenSSH) and craft our instructions using those assumptions. Depending on what kind of SSH client/server you're using, there may be differences in the names of certain files (e.g. authorized_keys2 might be authorized_keys -- The man pages are your friends :) We're also assuming that you actually already have user accounts on all the machines you're required to do work on.

So, to get the worst part over with, simply do the following (cut it up into chunks if it becomes frustrating. Depending on the size of your network, this is quite possible):

1. Create a file with the names or IP addresses of all of the machines you need to work on. For our purposes, format that file with one hostname/IP per line. We'll assume it's named "HostFile"

2. If you haven't done so already, generate your personal ssh-keys on the one machine that you're going to use as your central hub for administrating all the others, like so:

ssh-keygen -t dsa
(Answer yes to the default file locations -- /your/home/directory/.ssh/id_dsa and /your/home/directory/.ssh/
(Also, just hit return both times you're prompted to enter a password. While this is just slightly less secure than entering the double-confirm password, it will completely defeat the purpose of setting this easy-administration up)

2. Run the following from your home directory (This is the first really long and trying part, as you'll have to interact manually):

for x in `cat HostFile`;do ssh -n $x "mkdir -m 700 .ssh";done
(Actuall SSH command may vary depending on your distro. I'm using the "-n" flag to avoid having SSH break out of the "for loop")

Now, if we're starting from scratch, you'll need to answer "yes" for each machine's initial prompt (it will ask you if you trust the host and want to accept and save the key) and enter your password.

3. When that's finally over, cd into your home directory's .ssh directory and run the following on the command line ("HostFile" is still in your home directory, so we're using a relative path to it here):

for x in `cat ../HostFile`;do scp $x:/your/home/dir/.ssh/authorized_keys2
(Again, you'll need to enter your password for every machine. Note that if you run a variety of servers, or build standards have changed over time so that your home directory isn't always in the exact same place on every box, you can use "~.ssh/authorized_keys2" as a substitute for "/your/home/dir/.ssh/authorized_keys2)"

4. And, now, you're all done, except to make sure that it actually worked! My favorite way to do this is to just run the same command again. Overwriting your authorized_keys2 file on all the remote hosts won't hurt, and - this time - you should be just kicking back, watching scp's progress meters fly by as you try not to fall asleep ;) Again the "~.ssh/authorized_keys2" substitution is perfectly valid and useful:

for x in `cat ../HostFile`;do scp $x:/your/home/dir/.ssh/authorized_keys2

For all the machines that still ask you for your password, take note of those hosts and troubleshoot as necessary. For the most part, you shouldn't have to worry about running into too many of those.

And from here on out, you can use the "scmd" command we put in our pre-Thanksgiving post, or your own custom scripts, to effortlessly run any command (as yourself, of course) on all your machines, by typing a single line on your hub computer.

Happy Thanksgiving :)

, Mike