openssh-client vs openssh-server on Ubuntu

Secure Shell (SSH) is the standard protocol for securing remote connections and administrative tasks on Linux systems. In Ubuntu, SSH capabilities are divided into two distinct software packages: openssh-client and openssh-server. This article explains the fundamental differences between these two packages, their key components, and how to determine which one you need to install on your system.

The Core Difference

The primary difference lies in the direction of the connection.

openssh-client is used to initiate connections outbound from your local machine to a remote computer.
openssh-server is used to accept inbound connection requests from remote machines, allowing them to control your local system.

What is openssh-client?

The openssh-client package contains the tools required to connect to other systems. If you want to log into a remote server, transfer files securely to another machine, or run commands on a remote VPS, you only need the client package.

Key Tools Included:

ssh: The main command-line tool used to log into a remote machine (e.g., ssh user@remote-host).
scp: Secure Copy, used to copy files between hosts over an encrypted SSH connection.
sftp: Secure File Transfer Protocol, an interactive file transfer program similar to FTP but fully encrypted.
ssh-keygen: A utility to generate, manage, and convert authentication keys for passwordless logins.

Installation Status on Ubuntu:

The openssh-client package is installed by default on almost all Ubuntu installations, including Ubuntu Desktop and Ubuntu Server.

What is openssh-server?

The openssh-server package allows your Ubuntu machine to act as an SSH host. Once installed and configured, it listens for incoming connection requests. If you want to access your Ubuntu machine from another computer, you must install this package.

Key Components Included:

sshd: The SSH daemon. This is a background service that continuously listens for incoming connections (by default on TCP port 22).
Configuration files: Located in /etc/ssh/sshd_config, which allow you to configure port numbers, disable password logins, and set up firewall rules.

Installation Status on Ubuntu:

Ubuntu Server: Usually installed by default during the operating system installation.
Ubuntu Desktop: Not installed by default for security reasons. If you want to access your desktop remotely, you must install it manually.

Comparison Summary

Feature	openssh-client	openssh-server
Primary Purpose	Connect to remote systems	Allow remote systems to connect to you
Background Service	None (runs on demand)	Runs continuously in the background (`sshd`)
Default Port	Not applicable	Port 22 (configurable)
Ubuntu Desktop Default	Pre-installed	Not installed
Typical Use Case	Developers, system administrators, daily desktop users	Web servers, cloud instances, remote home servers

How to Install and Manage the Packages

You can manage both packages using Ubuntu’s default package manager, apt.

Installing openssh-client

If for some reason the client is missing from your system, install it with:

sudo apt update
sudo apt install openssh-client

Installing openssh-server

To allow other machines to connect to your Ubuntu system, install the server package:

sudo apt update
sudo apt install openssh-server

Managing the SSH Server Service

Unlike the client, the server runs as a system service. You can control its status using the following commands:

Check if the server is running:
```
sudo systemctl status ssh
```
Start the SSH server:
```
sudo systemctl start ssh
```
Enable the SSH server to start automatically on boot:
```
sudo systemctl enable ssh
```