ssh

Connecting to a Remote Server

The most basic use of ssh is connecting to a remote server. The syntax is straightforward:

ssh username@hostname_or_ip_address

Replace username with your username on the remote server and hostname_or_ip_address with the server’s hostname or IP address. For example, to connect to a server named example.com with the username john, you would use:

ssh john@example.com

If the server uses a non-standard port (other than the default port 22), you specify it using the -p option:

ssh -p 2222 john@example.com

This connects to example.com on port 2222.

Securely Transferring Files

ssh isn’t just for remote login; it also facilitates secure file transfer using scp (secure copy). To copy a file from your local machine to a remote server:

scp local_file username@hostname_or_ip_address:/remote/path/

For instance, to copy mydocument.txt to the /home/john/documents directory on example.com:

scp mydocument.txt john@example.com:/home/john/documents/

Copying a file from the remote server to your local machine is equally simple:

scp username@hostname_or_ip_address:/remote/path/local_file

To copy remote_file.log from /var/log on example.com to your current directory:

scp john@example.com:/var/log/remote_file.log .

Executing Remote Commands

The ssh command can execute commands on the remote server without requiring a full login session using the following syntax:

ssh username@hostname_or_ip_address 'command'

For example, to check the disk space on example.com:

ssh john@example.com 'df -h'

Note the single quotes around the command; this prevents local shell interpretation of special characters. For more complex commands, it’s often safer to use a script:

ssh john@example.com "bash -s" < my_remote_script.sh

This executes my_remote_script.sh on the remote server using bash.

Using SSH Keys for Passwordless Authentication

Typing your password every time you connect is cumbersome. SSH keys provide passwordless authentication. This involves generating a key pair (public and private), placing the public key on the remote server, and keeping the private key secure on your local machine.

Generating a key pair:

ssh-keygen

Follow the prompts; you can accept the defaults for most options. Then copy the public key (~/.ssh/id_rsa.pub) to the remote server’s ~/.ssh/authorized_keys file (you might need to create the .ssh directory first). After this, you should be able to connect without a password.

SSH Tunneling

SSH tunneling creates a secure connection through a remote server, allowing you to access services on other networks. This is useful for accessing servers behind firewalls.

For example, to create a local port forwarding tunnel to access a web server on a private network:

ssh -L 8080:internal_server_ip:80 username@gateway_server

This forwards traffic from port 8080 on your local machine to port 80 on internal_server_ip through gateway_server.

These examples illustrate the core functionality of the ssh command. Exploring its many other options and capabilities will improve your Linux system administration skills.