Connection through SSH

The ssh command is a very useful tool to connect to other computers when working in the terminal in Ubuntu. We summarise a few useful aspects here.

The main command

The normal use of the ssh command is simple and looks as follows:

# SYNTAX: ssh [-p <port>] [<username>@]host [-v]

# connect to computer 'tn1.uhn' as user 'test_user'
ssh test_user@tn1.uhn

The ssh configuration file

In order to shorten the ssh command, it is advised to create a configuration file in which all options of the call to ssh are defined per connection. The file must be stored in ${HOME}/.ssh/config and can look like this:

# Configuration for hosts 'tn3.uhn' and 'tn4.uhn'
Host tn3.uhn tn4.uhn
    User    test_user
    Port    22
    IdentityFile    ~/.ssh/id_rsa

# Configuration for host tn1.uhn
Host tn1.uhn
    User    test_user_2
    Port    8022
    IdentityFile    ~/.ssh/id_rsa_test_user_2

# ...

The ssh public / private key pair

When connecting to another computer via ssh, the user needs to authenticate themselves. While it is possible to define a password that is send to the host computer for authentication, it is advised to use a pair of personal identification keys. These keys come in pairs whereby one is private and remains locally on the computer where the user created it, while the other is public and needs to be distributed to other computers (or webservices like github) for authentication purposes. The central idea is that encryption with one of the two keys can only be decrypted with the respective other. This relationship allows for digital authentication in many ways.

In order to create an ssh-key pair, execute the following command. Note, that it is strongly recommended to use a password for the key.

sshkeygen

# When prompted,
# 1. specify the path where the keys will be stored
# 2. provide a self-chosen password

The keys should be stored in ${HOME}/.ssh/. Be careful not to overwrite existing key pairs when re-executing sshkeygen. The default location suggested by sshkeygen (accept by pressing ‘enter’) works well and results in

• the public key at ${HOME}/.ssh/id_rsa.pub,
• the private key at ${HOME}/.ssh/id_rsa.

Remember to not share or move the private key file. Even the permissions are crucial and should not be modified.

The authorized keys file

When connecting to a host computer with ssh-key authentication, it needs to be assured that one’s own public key is available in the file ${HOME}/.ssh/authorized_keys on the host computer. This can be done by copying the content of the public key file to the authorized_keys in the home folder on the host system. Note that the file permissions need to be set in a particular way (google it or check for other users’ examples). If the public key is not listed in the authorized_keys file or the file permissions of the latter are not as expected, the ssh command will terminate and comment by permission denied.