SSH Service Guide

SSH (Secure Shell) is a secure protocol for remote login and other network services. It provides an encrypted communication channel.

Note

SSH has become the standard tool for Linux/Unix system administration, replacing insecure protocols such as Telnet and rlogin. Beyond remote login, it also supports file transfer, port forwarding and more.

Installing SSH

On Debian/Ubuntu

sudo apt update
sudo apt install openssh-server

On CentOS/RHEL

sudo yum install openssh-server

Configuring SSH

The SSH configuration file is located at /etc/ssh/sshd_config.

Sample configuration

# Port number
Port 22

# Listen addresses
ListenAddress 0.0.0.0
ListenAddress ::

# Allowed users
AllowUsers user1 user2

# Disable password authentication, key authentication only
PasswordAuthentication no
PubkeyAuthentication yes

# Enable X11 forwarding
X11Forwarding yes

# Log level
LogLevel INFO

Explanation

• Port: the port the SSH service listens on.
• ListenAddress: the addresses the SSH service listens on.
• AllowUsers: the list of users allowed to log in.
• PasswordAuthentication: whether password authentication is allowed.
• PubkeyAuthentication: whether public key authentication is allowed.
• X11Forwarding: whether X11 forwarding is enabled.
• LogLevel: the log level.

Starting and Managing the SSH Service

sudo systemctl start ssh
sudo systemctl enable ssh
sudo systemctl status ssh

Using SSH

Connecting to a remote server

ssh username@hostname

Using SSH key authentication

Generate an SSH key pair:

ssh-keygen -t rsa -b 4096 -C "your_email@example.com"

Copy the public key to the remote server:

ssh-copy-id username@hostname

Port forwarding

Forward a local port to a remote port:

ssh -L local_port:localhost:remote_port username@hostname