SSH Jump

SSH Jump

A while ago while taking CS2106 Operating Systems, I had to test my programs in the designated server for students. However, this server is even inaccessible from the NUS-wide WLAN, so we had to SSH into the course-specific server, let’s call it 2106-server, first through a more general server called sunfire.

SSH Keys

First things first, if you are still typing your passwords every time you are ssh-ing into your servers, you are doing it Wrong. There is this thing called ssh-keygen, which allows you to generate a public-private key pair, and once you keep the private key and put the public key onto the server, you will never have to type your password again, as long as you have the private key.

I always created my keys using RSA, since this encryption algorithm is widely supported. If you have more time, you can take a look at ECDSA as well. Typing either of the below lines should help you create your SSH key securely.

ssh-keygen -t rsa -b 4096

or

ssh-keygen -t ecdsa -b 521

The above process will tell you where your public and private keys are stored.

Now you can copy your public SSH key to the target machine:

ssh-copy-id -i ~/.ssh/id_rsa user@host

Where the script will actually infer the public key and push it onto the target machine, dealing with duplication, creation of ~/.ssh folder and whatnot. So the next time, you can simply type

ssh [email protected]

and SSH into the target machine.

What, that is not enough?

SSH Config File

The aim of this section is to teach you to do a shorter command with SSH, as well as ridding you of the troubles of having to

  1. Remember your username. Which one is it?
  2. Remember your target machine’s address. This is especially useful if you are dealing with IP addresses, which are a pain
  3. Add a bunch of configs, which you can check at man ssh_config

Credits to Hao Wei (@angelsl) for posting this awesome config for NUS computing users:

# ~/.ssh/config
Host xcn?? xcn??? xgp?? xgp??? sunfire sunfire0
    HostName %h.comp.nus.edu.sg
    User your_username
    IdentityFile ~/.ssh/id_rsa

which included a bunch of wildcards to access many of the computing clusters with just one config entry. Save this to ~/.ssh/config and thank Hao Wei, not me.

Now, SSH-ing to hosts other than sunfire and sunfire0 is just:

ssh -J sunfire xcnd0

, instead of the tedious

ssh -J [email protected] [email protected]

Some other Quality of Life hints

  1. Since sunfire is running SunOS, setting your TERM=xterm could give you some convenience such as clearing your prompt by pressing C-l. You can do that in bash by

    TERM=xterm ssh sunfire

    Or even aliasing your ssh to TERM=xterm ssh in your ~/.bashrc

  2. Have fun talking to (annoying) your friends by finding out their username through the command w, then writing to them by the command write other_username then typing your message. When you are done, you can just press C-c or C-d.