I’m no guru – I simply “borrowed” this from a website.
So now you’re sshing and scping your brains out. Sooner or later you’ll come across one or both of these situations:
1. You want to automate some ssh/scp process to be done after hours, but can’t because no one will be around to type the passphrase.
2. You want to allow an account to do some sort of ssh/scp operation on another machine, but are hesitant to append a key to your authorized_keys2 file because that essentially “opens the barn door” to anything that other account wants to do, not just the one operation you want to let it do. (This is the situation if you use a .shosts file.)
Single-purpose keys to the rescue!
1. Make yourself another key:
ssh-keygen -t dsa -f ~/.ssh/whoisit
Just press return when it asks you to assign it a passphrase- this will make a key with no passphrase required. If this works right you will get two files called whoisit and whoisit.pub in your .ssh dir.
2. cp ~/.ssh/whoisit.pub tempfile
We want to work on it a little. tempfile should consist of one really long line that looks kind of like this:
ssh-dss AAAAB3NzaC1k[…]9qE9BTfw== email@example.com
3. Edit tempfile and prepend some things to that line so that it looks like this:
command=”echo I\’m `/usr/ucb/whoami` on `/usr/bin/hostname`”,no-port-forwarding,no-X11-forwarding,no-agent-forwarding ssh-dss AAAAB3NzaC1k[…]9qE9BTfw== whoisitnow
That will do what we want on Solaris; to try this example on Linux use this:
command=”echo I\’m `/usr/bin/whoami` on `/bin/hostname`”,no-port-forwarding,no-X11-forwarding,no-agent-forwarding ssh-dss AAAAB3NzaC1k[…]9qE9BTfw== whoisitnow
The stuff to prepend is your command that will be run when this key is activated, and some options to keep it from being abused (hopefully). The last thing on the line is just a comment, but you probably want to set it to something meaningful.
Also, most examples I see use no-pty as an additional option, but this messes up the carriage-return/linefeediness of the output of the above example. (Try it.) I haven’t looked into it enough to see why you would want it, but there you go.
4. cat tempfile |ssh burly ‘sh -c “cat – >>~/.ssh/authorized_keys2″‘
Append tempfile to your authorized_keys2 file on burly.
5. To “activate” (or perhaps “detonate”) the key from hurly (or anywhere that has the secret key), do this (maybe there is a better way?):
ssh -i ~/.ssh/whoisit burly
The following also works but is cumbersome:
ssh-agent sh -c ‘ssh-add ~/.ssh/whoisit < /dev/null && ssh burly'
You can also append this "command key" to a different account's authorized_keys2 file and trigger it from a different username. You just need the secret key. Like so:
ssh -i ~/.ssh/whoisit -l paulkeck burly'
The next leap in the pattern is something like this:
ssh -i /home/pkeck/.ssh/whoisit -l paulkeck burly'
This could be run by any user on the box if they could read your secret key, so always keep your .ssh dir and all your keys chmodded to 700 and 600 respectively.
6. You could make single-purpose keys with commands to (haven't tested all these):
mt -f /dev/nst0 rewind
Rewind a tape on a remote machine
nice -n 19 dd of=/dev/nst0
Send STDIN to that tape drive. Maybe STDIN is a tar stream from tar -cf -.
nice -n 19 dd if=/dev/nst0
Read stuff from there to my STDIN
cat claxon.au > /dev/audio
Play an alarm noise on a remote machine
cat – > /dev/audio
Play any sound you send on STDIN
cat – > /etc/dhcpd.conf
Replace /etc/dhcpd.conf with some stuff from STDIN on the triggering machine (sounds like a temp file would be better)
You can send the stuff on STDIN with something like this on the triggering machine:
ssh-agent sh -c ‘ssh-add ~/.ssh/whoisit < /dev/null && cat alarm.au | ssh burly'
ssh-agent sh -c 'ssh-add ~/.ssh/whoisit < /dev/null && tar cf - /home/pkeck | ssh burly'
Maybe for that one the corresponding command to "catch" that stream would be:
cat - > ~/backups/pkeck.tar.`date +%Y%m%d.%H-%M-%S`
You get the idea! Go crazy!
Tape examples from Ed Cashin’s Gettin’ Fancy with SSH Keys, my inspiration for getting into this whole thing!