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SSH power unleashed – Part 1: use private keys

I’m going to talk about some advanced tips regarding SSH in this series. Many people use it on a daily basisi, yet still only use it’s very basic functions: log into a server, do some work, log out. But SSH actually is very feature rich and flexible. It can even do something many people don’t heard before. Let’s first start with the well known public and private key authentication function.

Tip 1: never login use password. Use public key authentication instead.

The first rule of running a production server is , disable root remote login and password login for normal users. Make sure “PermitRootLogin=no” and “PasswordAuthentication=no” are in your SSHd configuration file, usually /etc/ssh/sshd_config.

You may also want to lockout your root passwd. This adds extra protection to your root power. it can be done by below command:

passwd -l root

Once root password is locked, no one can login as root with password. The only way to become root is to either login use some other authentication method, such as SSH public key, or use su/sudo to elevate from a normal user.

Now it is time to get a pair of public and private key for yourself to login. On a Linux machine, this can be done with below command.

Note: these should be done on your local machine, not your server. NEVER put your private key on remtoe server!

ssh-keygen -t rsa -b 2048

Then follow the on screen message to provide file name to save the keys(the default is OK), and the passphrase.

If you want ultra-secure keys, just raise the key bits given for -b option. 2048 is sufficient for nowadays, 4096 may give you longer confidence.

Now look at your ~/.ssh/ directory, your new keys are there. The public key file is named id_rsa.pub, and the private key file is named id_rsa. The public key file can be disclosed to the world, while the private key file should be kept safe, as safe as how you keep the key of your home, may be even more 😉 .

Now upload the public key file id_rsa.pub to your remote server, and make it useable for SSH. It can be done as below.

cat id_rsa.pub >> ~/.ssh/authorized_keys

This puts the content of your newly generated public key into the authorized_keys file, which will be check by SSH upon your login.

Now login from your local machine to the server with key authentication, you need to provide the passphrase of the private key instead of your system account password. And you can safely disable password login in SSHd configuration now.

Tip 2: use ssh-agent to cache your keys

Imagine that you manage a few servers, and you need to frequently login to perform some tasks. It’s going to be a pain to enter the password of the keys every time. It is quite boring for myself – my password is quite long and made of non-sense characters, everytime I enter the password it feels like a finger dance!

The ssh-agent command is made for such purpose. It can be used to invoke a shell and cache yoru private key in memory. Next time when a SSH key is requested, it will provide the data directly without requiring your to enter the password again. Let’s see how to do that.

First start a shell by ssh-agent:

ssh-agent /bin/bash

It seems nothing happend, you are just dropped back to shell prompt. But actually you are in a newly invoked shell now. In this shell, ssh-agent caches the private key password for you.

Now load your private key(s).

ssh-add

It will prompt you the file name of the current private key it going to load, ask you to input the password. Once you are done, the priviate key and its password is cached in memory. Now try to log into your remote server which has the public key installed. You will notice that no password is required during SSH login!

Tips 3: start ssh-agent upon login

Above tip makes life easier, let’s move on to see how we can make it even more easier.

Instead of start ssh-agent every time after you logged in your local machine, it is possible to start it automatically. If you are using Bash as your login shell, put below at the end of your ~/.bashrc file.

eval $(/usr/bin/ssh-agent -s)

/usr/bin/ssh-add

And below at the end of your ~/.bash_logout file.

if [ -n "$SSH_AGENT_PID" ] ; then
    /usr/bin/ssh-agent -k
fi

During your login, you will notice the message of “Agent PID xxxxx”, which means your newly added code in ~/.bashrc just ran and started ssh-agent and setup the environment for you. Then the ssh-add command asks you to load your private keys. Type in your password and your ssh-agent is up running just like described in Tip 2. You can SSH login to remote servers without typing in password.

When you logout, the code in ~/.bash_logout will make sure the agent is killed so no key data is left in memory.

Tip 4: use SSH agent forwarding

Still remember that I mentioned above the private key should be kept safe and never uploaded to any remote servers? What if you need to log into another server(let’s call it server B) from your remote server(let’s call it server A)? You don’t have the private key on server A. To log into server B, you have to either use password authentication, or from your local machine that have the private key. But what if you do need to connect from server A to B and don’t want to use the less secure password authentication way?

This is how SSH agent forwarding comes to save you. With this technology, you an “forward” the encrypted information of your private key located on your local machine, via server A, to server B, without actually copying the key file to server A. Let’s see how we can do this.

When you start the connection to server A, first make sure you followed Tip 2 & 3 and in the ssh-agent shell with private key loaded, then use the -A option like below:


ssh -A -p <remote_ssh_port> [email protected]_A

The -A option tells SSH program to forward local ssh-agent to server A. When you use SSH there, it can acccess the private key via the secure SSH connection between your local machine and server A.

Now if on server A you need to connect to server B, just run SSH and you will notice that no password is required, you just log into server B with your private key on your local machine!

If you are using PuTTY on Windows as your SSH client, the agent forwarding option is under Connection -> SSH -> Auth -> Authentication Parameters -> Allow agent forwarding, as shown in below screenshot.

Caution: always make sure server A is trusted before you forward your agent to it.

Published inTips