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SSH tutorial (UNDER DEVELOPMENT)

This work by Konrad Förstner is licensed under a Creative Commons Attribution 4.0 International License.

Version: 0.1dev

Introduction

The tool SSH (Secure Shell) can be used to connect to another computer and execute programs there. The remote machine is often a server that for example offer more computational power than you laptop. The most widely used implemenation of SSH is OpenSSH.

Basic usage

In order to connect to the remote machine you need to have an account on it. In the following examples we assume that you have an account called alice and a remote machine called bobs_server (Your name is not Alice and you don’t know any Bob? Read this to understand the choice of names).

You can connect to a remote machine and have an interactive session by calling ssh like this (don’t copy the $ sign in any of the following commands!):

$ ssh <your_username>@<server_name_or_ip>

In our example this would be

$ ssh alice@bobs_server

SSH will presend you the host finger print

Host key fingerprint is SHA256:ZwDBE5pV7SCqTXrHYzBChkhn3Nkx6cvvF9StX+W25No
+---[ECDSA 256]---+
|oo.o..B*+.       |
|o =. *++o .      |
| o  o..o.o  . .  |
|  . =  . ... . ..|
|   * +. S +   ...|
|  o o =o o . . .+|
|   . o ..   . +.o|
|         . .  .+ |
|        ...  ..E |
+----[SHA256]-----+

The first time your are connting to a remove machine you will be asked to continue. Please type yes and press Enter.

Are you sure you want to continue connecting (yes/no)?
Warning: Permanently added 'hawaii' (ECDSA) to the list of known hosts

Then you will be asked for your password.

alice@bobs_server's password:

Please enter your password and press Enter. Now you should have an interactive shell session on the remote machine. You prompt might indicate this but you can use hostname to make that you are on the remote machine.

$ hostname
bobs_server

You can disconnect by typing

$ exit

You will be informed that hte connection is closed

Connection to bobs_server closed.

Now you can run commandline tools like on your local machine. If the server is adequately configured you can even run tools with a graphical interface (GUI) using so called X11-forwarding. For this connect with ssh using the -X flag (disconnect first if you are connected).

$ ssh -X alice@bobs_server

You can also run SSH non-interactively by calling it with a command to execute:

$ ssh <your_username>@<server_name_or_ip> <command>

Example:

$ ssh alice@bobs_server ls

This will list the file in the home directory of the remote machine. You will not have an interactive session on that machine but will stay in the session of you local machine after the file list is returned.

Copying files with scp

Basic

$ scp <source> <target>

Folder

$ scp -r <source> <target>

Using compression

$ scp -C <source> <target>

Using compression when sending

$ scp -rC <source> <target>

Examples. Be aware that ~/ represent your home directory on GNU/Linux and the different BSD this is usually /home/your_username. On OS X this is /Users/your_username.

This will copy the file my_local_big_file.txt from your local machine into your home directory of the remote machine.

$ scp my_local_big_file.txt alice@bobs_server:~/

The following command will send the file to the server and rename it to my_remote_big_file.txt.

$ scp my_local_big_file.txt alice@bobs_server:~/my_remote_big_file.txt

Sure you can use scp to copy from the server to you local machine:

$ scp alice@bobs_server:/data/my_local_big_file.txt .

Note: scp is not the ideal tool. The developer of SSH state: “The scp protocol is outdated, inflexible and not readily fixed. We recommend the use of more modern protocols like sftp and rsync for file transfer instead.” So rather use rsync over ssh.

Configure SSH

While you are now capable of working on the remote machine you have to type a lot to connect to it. Laziness is virtue - in the following we will make working with SSH more convinient and reduce the need of typing.

Create in the home directory of you local machine a folder called .ssh.

$ mkdir -p ~/.ssh

Make it unaccessible (read/write/executable) by other users:

$ chmod -R go-rwx ~/.ssh

Create a configuration file

$ touch ~/.ssh/config

Open this configuration file in you favorite text editor (e.g. Emacs, vi, nano, gedit) and add

Host bobs
  User alice
  HostName bobs_server

Now you can connect to the server simply by calling

$ ssh bobs

or copy files without by using the short alias

$ scp my_file.txt bobs:~/

Using SSH keys

Currently you have to type your password everytime you connect to the remote machine. In order to avoid that we will generate once a so called SSH key pair. A private key will be stored at your local machine and should be keep private! The public key has to be put on the server and can be liberately given away (e.g. a system administrators might ask you for the public key in order to give you access to a remote machine without any password). More details regarding this public/private key approach can be found here.

Generate a key pair on your local machine. We use -b 4096 in order to generate a stronger key than with the default settings and -N "" to set no passphrase for the key. Generating the key can take a while.

$ ssh-keygen -b 4096 -N ""

During the procedure you will be asked for the name of the key file. Use the default by pressing Enter:

Generating public/private rsa key pair.
Enter file in which to save the key (/home/your_local_user_name/.ssh/id_rsa):
Your identification has been saved in /home/your_local_user_name/.ssh/id_rsa.
Your public key has been saved in /home/your_local_user_name/.ssh/id_rsa.pub.
The key fingerprint is:
0b:5c:b3:e8:51:d2:0e:3b:b8:cc:6d:97:4d:1d:e7:f0 your_local_user_name@your_local_machine
The key's randomart image is:
+--[ RSA 4096]----+
|                 |
|       .         |
|      o =   o .  |
|     o O o . *   |
|    . B S . . E  |
|   o + + =       |
|    + + + .      |
|     . .         |
|                 |
+-----------------+

As ssh-keygen reported we have now two files:

  • Your private key file /home/your_local_user_name/.ssh/id_rsa
  • Your public key file /home/your_local_user_name/.ssh/id_rsa.pub

Those are just plain test files and you can have a look at them using cat or less:

$ cat ~/.ssh/id_rsa
$ cat ~/.ssh/id_rsa.pub

We now want to copy the public key to the server. To set up a .ssh folder on the server run the following command on your local machine:

$ ssh alice@bobs_server "mkdir -p ~/.ssh"

Add the public key to the file (as you can see you can even pipe commands into SSH) by running the following command on your local machine:

$ cat ~/.ssh/*.pub | ssh alice@bobs_server 'cat >> ~/.ssh/authorized_keys'
$ cat ~/.ssh/*.pub | ssh alice@bobs_server 'chmod 700 ~/.ssh'
$ cat ~/.ssh/*.pub | ssh alice@bobs_server 'chmod 600 ~/.ssh/authorized_keys'

Add the private key to local SSH key-chain:

$ ssh-add ~/.ssh/id_rsa

Persistent session with tmux

Alternatives:

  • nohub
  • screen

The terminal multiplexer tmux

Start tmux for the first time

$ tmux

List sessions:

$ tmux list-sessions

Reconnect to running tmux sessions:

$ tmux attach

or simply

$ tmux a

Important keys:

  • Ctr-b c - new window
  • Ctr-b , - rename window
  • Ctr-b x - close window
  • Ctr-b <NUMBER> - go to window with the given number
  • Ctr-b [ - switch scrolling mode; use q to quit

List keys

$ tmux list-keys

List commands

$ tmux list-commands

Configure ~/.tmux.conf:

# Change prefix to Control + o
set -g prefix C-o

# Increase history length
set -g history-limit 1000

Further topics

mkdir -p server_folder sshf alice@bobs_server:/home/alice/
umount server_folder