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Hack 94 Quick Logins with SSH Client Keys

Using SSH keys instead of password
authentication to speed up and automate logins.

When
you're an admin on more than a few machines, being
able to navigate quickly to a shell on any given server is critical.
Having to type ssh
my.server.com (followed by a password) is
not only tedious, but it also breaks your concentration. Suddenly
having to shift from "where's the
problem?" to "getting
there," and back to
"what's all this,
then?" has led more than one admin to premature
senility. It promotes the digital equivalent of "why
did I come into this room, anyway?" (and the problem
is only made worse by /usr/games/fortune!).

At any rate, more effort spent logging
into a machine means less effort spent solving problems. Recent
versions of SSH offer a secure alternative to endlessly entering a
password: public key exchange.

For these examples, I assume that you're using
OpenSSHv3.4p1 or later. To use public keys with an SSH server,
you'll first need to generate a public/private key
pair:

 $ ssh-keygen -t rsa

You can also use -t dsa for DSA keys, or
-t rsa1 if you're using Protocol
v1. And shame on you if you are using v1! Upgrade to v2 as soon as
you can!

After you enter the above command, you should see something like this:

 Generating public/private rsa key pair.
Enter file in which to save the key (/home/rob/.ssh/id_rsa):

Just press Enter there. It will then ask you for a passphrase; just
press Enter twice (but read the Security note below).
Here's what the results should look like:

 Enter passphrase (empty for no passphrase): 
Enter same passphrase again: 
Your identification has been saved in /home/rob/.ssh/id_rsa.
Your public key has been saved in /home/rob/.ssh/id_rsa.pub.
The key fingerprint is:
a6:5c:c3:eb:18:94:0b:06:a1:a6:29:58:fa:80:0a:bc rob@localhost

This created two files: ~/.ssh/id_rsa and
~/.ssh/id_rsa.pub. To use this keypair on a
server, try this:

 $ ssh server "mkdir .ssh; chmod 0700 .ssh"
$ scp .ssh/id_rsa.pub server:.ssh/authorized_keys2

Of course, substitute your server name for
server. It should ask for your password
both times. Now, simply ssh
server and it should log you in
automatically, without a password. And yes, it will use your shiny
new public key for scp, too.

If this didn't work for you, check your file
permissions on both ~/.ssh/* and server:~/.ssh/*.
Your private key (id_rsa) should be 0600 (and be
present only on your local machine), and everything else should be
0655 or better.

Terrific. So you can now SSH to any machine quickly and with a
minimum of fuss. Is it possible to make it even quicker to connect to
machines you frequently touch? You bet [Hack #95].

Security Concerns

Some consider the use of public keys
a potential security risk. After all, one has only to steal a copy of
your private key to obtain access to your servers. While this is
true, the same is certainly true of passwords.

Ask yourself, how many times a day do you enter a password to gain
shell access to a machine (or scp a file)? How
frequently is it the same password on many (or all) of those
machines? Have you ever used that password in a way that might be
questionable (on a web site, on a personal machine that
isn't quite up to date, or possibly with an SSH
client on a machine that you don't directly
control)? If any of these possibilities sound familiar, then consider
that an SSH key in the same setting would make it virtually
impossible for an attacker to later gain unauthorized access
(providing, of course, that you keep your private key safe).

Another way to balance ease of use with security is to use a
passphrase on your key, but use the SSH agent to manage your keys for
you. When you start the agent, it will ask you for your passphrase
once, and will cache it until you kill the agent. Some people even go
as far as to store their SSH keys on removable media (like a USB key
chain) and take their keys with them wherever they go. However you
choose to use SSH keys, you'll almost certainly find
that they're a very useful alternative to
traditional passwords.