SSH is the Secure Shell, a powerful encrypted communication tool. This page contains some basic information about the use of SSH.

SSH client and server programs are included with almost all Linux distributions, Mac OS, and recent versions of Windows (later updates of 10 as well as 11). You can obtain SSH client and server software for most other platforms, including Android.

Here are some SSH clients (there are many others available):

The most basic use of SSH is for a text-based connection used to enter commands and view command output, or execute a text-based application such a menu-driven system.

Some SSH clients have a graphical interface. For OpenSSH and others used from the command line, the most basic form of invocation is:

 ssh [[OPTIONS]] [user@]host command

Note that the ssh command name may vary, depending on the software used.

Where:

• user is the name of the user account on the remote computer (optional, if the user account name is the same on both systems)
• host is the name or IP address of the remote computer
• command is the command to be executed on the remote computer (optional - the default is to open whatever shell or application is the default for the specified user)

Useful options include:

• Connect to a shell for account 'kim' on the computer 'winter': ssh kim@winter
• Like above, with compression: ssh -C kim@winter
• Run df -h on the remote system, and display the graphics on the local computer: ssh -XC kim@winter firefox

If you're using OpenSSH (the ssh client used on most Linux systems and other platforms), you can simplify ssh command lines by placing host connection details in the file ~/.ssh/config (or on Windows, %HOME%\.ssh\config), like this:

 Host "matrix"
         hostname "matrix.senecapolytechnic.ca"
         user "YourUserID"

Once you have added these lines (inserting your user ID where appropriate) and set the permission on that file (on Linux, chmod 0600 ~/.ssh/config) you can use this simplified command to access the server:

 ssh matrix

You can similarly configure simplified access in most other SSH client programs.

SSH provides two mechanisms for file transfer: scp and sftp. You can use these from the command line using the OpenSSH software (installed by default in most MacOS, Windows, and Linux systems) or you can use a graphical tool that uses these protocols.

The scp command, which is included as part of the OpenSSH software, functions like the normal Unix/Linux copy (cp) command, but accepts a hostname (and optional user ID) prepended to a filename for copying to/from remote systems:

  scp user@host:sourcefilename destinationfilename

or

  scp sourcefilename user@host:destinationfilename

• Copy the file /etc/hosts from this computer to the /tmp directory on the computer ireland: scp /etc/hosts ireland:/tmp
• Copy the file test.tar from the home directory of the user chris on the computer ireland to the current directory: scp chris@ireland:~/test.tar .

Many graphical file managers, such as the GNOME Nautilus and KDE Konqueror programs (both on Linux) or graphical file managers (on many operating system platforms), know how to manage files remotely using the ssh/scp/sftp protocols.

For example, on GNOME, you can select the menu option Places>Connect to Server, enter the connection details, and view a file management window on a remote server. You can then rename, copy, move, and delete files using drag-and-drop operations (for example, you can copy a file by dragging it from your desktop to the remote window). You can also create bookmarks for rapid access to remote filesystems at a later date.

Although SSH can be used with passwords, a sysadmin may need to initiate hundreds of SSH connections a day, and typing passwords that often is tedious. SSH therefore permits authentication using public and private keys.

See Public Key Cryptography for an overview of how this technology works.

These instructions assume a Linux or Mac OS/X system running OpenSSH. Using OpenSSH on Windows will be similar except for file paths. Other SSH clients will have different procedures, and you'll need to check that the key produced is in OpenSSH format (you can convert key types using the ssh-keygen command on a Linux system such as Matrix if needed).

1. Enter this command: ssh-keygen -ted25519
   • Answer the questions asked by ssh-keygen. Use default values for most questions. The use of a passphrase is strongly recommended. Avoid changing the filename from the default (unless there is a really good reason), because the location, name, and permission of keys is critical.
   • ssh-keygen will produce two files:
     1. ~/.ssh/id_ed25519 - your private key
     2. ~/.ssh/id_ed25519.pub - your public key

A note on key type and length: ssh-keygen can generate multiple types of keys, including rsa, dsa, ecdsa, and ed25519. Any of these types serves the same purpose, and rsa/dsa can be generated with varying key lengths. The longer the key, the more difficult it is to break the key by guessing it (trying successive values until the right key is found) - each bit added to the key doubles the number of possible key values. Key lengths of 1024-4096 bits are considered reasonably secure; as computers become faster, key lengths should be increased. Better yet, use the ed25519 eliptic curve option, which is considered the most secure format supported by the current OpenSSH implementation (8.4 as of the time of writing).

1. Upload or email (or otherwise send) the ~/.ssh/id_rsa.pub file to the remote system administrator. Do NOT send your private key!

Automatically (preferred approach):

1. Run the command: ssh-copy-id user@host

Manually:

1. Copy the public key to the remote system you wish to access.
2. Create the ~/.ssh directory if it does not exist.
   • Set the permission on ~/.ssh to 0700
3. Append the public key to the file ~/authorized_keys
   • Set the permission on ~/authorized_keys to 0600

The GNU screen utility permits you to start a session, disconnect from it (accidentally or intentionally), and then reconnect from the same or another network connection. This is useful when you're using an unstable WiFi connection (e.g., in a coffee shop or mobile link) or want to start a long operation (such as a build or a test suite run) on campus, commute home while it's running, and then check on its status. Other tools such as tmux provide similar capabilities.

Note that this approach does not work with graphical applications (ssh -X), although software such as VNC can be used to provide disconnect/reconnect ability for graphical sessions.