Книга: Fedora™ Unleashed, 2008 edition

Using ssh-keygen to Enable Key-Based Logins

Using ssh-keygen to Enable Key-Based Logins

There is a weak link in the SSH system, and, inevitably, it lies with users. No matter what lengths system administrators go to in training users to be careful with their passwords, Post-it notes with "pAssw0rd" written on them are attached to monitors around the world. Sure, it has a mix of letters and numbers, but it can be cracked in less than a second by any brute-force method. Brute-forcing is the method of trying every password possibility, starting with likely words (such as password and variants, or god) and then just trying random letters (for example, a, aa, ab, ac, and so on).

Even very strong passwords are no more than about 16 characters; such passwords take a long time to brute-force but can still be cracked. The solution is to use key-based logins, which generate a unique, 1024-bit private and public key pair for your machine. These keys take even the fastest computers a lifetime to crack, and you can back them up with a password to stop others from using them.

Creating an SSH key is done through the ssh-keygen command, like this:

ssh-keygen -t dsa

Press Enter when it prompts you where to save your key, and enter a passphrase when it asks you to. This passphrase is just a password used to protect the key — you can leave it blank if you want to, but doing so would allow other people to use your account to connect to remote machines if they manage to log in as you.

After the key is generated (it might take up to 30 seconds depending on the speed of your machine), change the directory to .ssh(cd ~/.ssh), which is a hidden directory where your key is stored and also where a list of safe SSH hosts is kept. There you will see the files id_dsa and id_dsa.pub. The first is your private key and should never be given out. The second is your public key, which is safe for distribution. You need to copy the public key to each server you want to connect to via key-based SSH.

Using scp, you can copy the public key over to your server, like this:

scp id_dsa.pub 10.0.0.1:

This places id_dsa.pub in your home directory on 10.0.0.1. The next step is to SSH into 10.0.0.1 normally and set up that key as an authorized key. So you can SSH in as yourself and then type the following:

touch .ssh/authorized_keys
cat id_dsa.pub >> .ssh/authorized_keys
chmod 400 .ssh/authorized_keys

The touch command creates the authorized_keys file (if it does not exist already); then you use cat to append the contents of id_dsa.pub to the list of already authorized keys. Finally, chmod is used to make authorized_keys read only.

With that done, you can type exit to disconnect from the remote machine and return to your local machine. Then you can try running ssh again. If you are prompted for your passphrase, you have successfully configured key-based authentication.

Now the current machine is secured, but what about every other machine? It is still possible to log in from another machine using only a password, which means your remote machine is still vulnerable.

The solution to this is to switch to root and edit the /etc/ssh/sshd_config file. Look for the PasswordAuthentication line and make sure it reads no (and that it is not commented out with a #). Save the file, and run kill -HUP `cat /var/run/sshd.pid` to have sshd reread its configuration files. With that done, sshd accepts connections only from clients with authorized keys, which stops crackers from brute-forcing their way in.

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