How to Secure an SSH Server in Ubuntu

SSH (Secure Socket Shell) is a command line interface and protocol for securely getting access to a remote Linux server. It provides a secure and encrypted communication over a network and allows data to be exchanged over a secure channel between two servers. It is widely used by system admins to control and manage their web servers remotely. This tutorial shows you how to secure your SSH server.

Note: while the tutorial here is done on Ubuntu, the steps will apply for most Linux distributions.

Install and Update SSH

First, update your system and install necessary packages to your system.

To update the system and install the SSH server on the server and client machines, run the following command:

sudo apt update && sudo apt upgrade
sudo apt install ssh

Configure SSH for Passwordless Login

There are two different methods of logging in to an SSH server: password authentication and public key authentication. Password authentication is a very basic method that is easy to use and crack. Using password authentication is very insecure, especially if a weak password is used. On the other hand, SSH keys provide an easy and secure way of logging into a remote server, and this method is recommend for all users.

On your client machine, generate SSH keys with the following command:

ssh-keygen

Press the Enter key at every prompt. This will produce two files: "id_rsa.pub" (public key) and "id_rsa" (private key).

On your server, create the following folder (if it doesn't exist):

mkdir -p ~/.ssh/

Back to your client machine, copy the SSH key to your server using the following command:

ssh-copy-id username@your.server.ip.address

On your server machine, make sure that the ".ssh" folder has the right file permissions.

chmod 700 ~/.ssh
chmod 600 ~/.ssh/authorized_keys

To test whether the public key authentication method works, try connecting to your SSH server from the client machine:

ssh username@your.server.ip.address

If you are able to connect without entering a password, then the public key authentication method works.

Note: if you are using Windows, use the instructions here to generate SSH public/private keys.

Secure SSH Configuration File

The "/etc/ssh/sshd_config" file is the system-wide configuration file for SSH that allows you to set different options to improve the security of an SSH server. The default configuration in the config file is very insecure, so you need to edit it first and set proper options to improve the security.

To edit the "/etc/ssh/sshd_config" file, run:

sudo nano /etc/ssh/sshd_config

Change the SSH Listening Port

By default, SSH listens on port 22. Attackers use port scanners to see whether an SSH service is running or not. It is good practice to change the default port.

To change the default port to 2200, change:

Port 22

to

Port 2200

Only Use Protocol 2

Version 1 of the protocol contains security vulnerabilities. Protocol 2 is the default entry on Ubuntu.

Change the line shown below:

Protocol 2

Limit User Access

It is necessary to allow only specific users to log in to SSH. It can improve your security. By default, this option is not available in the SSH configuration file.

To allow "user1" and "user2," add the following line:

AllowUsers user1 user2

To deny "baduser1" and "baduser2," add the following line:

DenyUsers baduser1 baduser2

Disable Root Login

It is not necessary to log in as root via ssh over a network. Normal users can also use su or sudo to gain root level access. Most attackers will try to use root user to log in. It's a big security risk, so it is good practice to deny the root login.

To disable root login, change the line

PermitRootLogin prohibit-password

to

PermitRootLogin no

Hide Last Login

You can hide who logged in last when a user logs in.

Change the line

PrintLastLog yes

to

PrintLastLog no

You can still view all active SSH connections with the methods in this list.

Restrict the Interface to Log In

By default, SSH will listen on all network interfaces. If you want to allow an SSH connection from a specific IP address, you can change the line:

#ListenAddress ::

to

ListenAddress 192.168.68.175

Disable Password Authentication

Using password authentication is a big security risk if a weak password is used. Using "ssh keys" is good practice. An "ssh key" can contain over 600 random characters and be difficult to break.

For this, change the line

# PasswordAuthentication yes

to

PasswordAuthentication no

Disable .rhosts Files

The .rhosts files specify which users can access the r-commands (rsh, rcp, rlogin, etc.) on the local machine without a password. By default, an .rhosts file is disabled; if not, change the lines as shown below.

IgnoreRhosts yes
RhostsAuthentication no
RSAAuthentication yes

Disable Host-Based Authentication

SSH's host-based authentication is more secure than .rhosts authentication. However, it is not recommended that hosts trust one another. By default, this option is disabled.

If not, change the line as shown below:

HostbasedAuthentication no

Set a Login Grace Timeout

The "LoginGraceTime" specifies how long after a connection request the server will wait before disconnecting. It is good practice to reduce it to 60 seconds.

For this, change the line

LoginGraceTime 2m

to

LoginGraceTime 1m

Set Maximum Startup Connections

Setting up a proper maximum number of concurrent connections to the SSH daemon can be helpful against a brute-force attack.

For this, change the line

#MaxStartups 10:30:100

to

MaxStartups 2

Disable Forwarding

The port forwarding technique is used by attackers to tunnel network connections through an SSH session to log into systems. Because of that, it is good practice to disable this option.

For this, change the line

X11Forwarding yes

to

X11Forwarding no

Log More Information

By default, SSH logs everything. If you want to log more information like failed login attempts. change the value of this to "VERBOSE."

For this, change the line

LogLevel INFO

to

LogLevel VERBOSE

Disable Empty Passwords

It is necessary to deny users with empty passwords on your server. By default, PermitEmptyPasswords is disabled in Ubuntu.

If not, change the line as shown below.

PermitEmptyPasswords no

Set Idle Timeout Interval

By default, this option is not available in the default SSH configuration file, so it is good practice to set a proper idle timeout to avoid an unattended session.

For this, add the following lines.

ClientAliveInterval 300
ClientAliveCountMax 0

Strict Mode

This will prevent the use of insecure home directory and key file permissions. By default, this option is enabled.

If not, change the following line:

StrictModes yes

Save and exit the "/etc/ssh/sshd_config" file and restart the SSH server.

sudo systemctl restart ssh

Secure SSH Using TCP Wrappers

A TCP wrapper provides host-based access control to network services used to filter network access to the Internet. Edit your "/etc/hosts.allow" file to allow SSH only from "192.168.68.1" and "192.168.68.175."

sudo nano /etc/hosts.allow

Add the following line:

sshd : 192.168.68.1 192.168.68.175

Secure SSH Using iptables

By default, an SSH server must only accept connections from your LAN or other remote sites. It is good practice to allow only specific IP addresses to access SSH and block access to SSH to unauthorized IP addresses.

To only allow SSH connections from "192.168.68.1," set the rules in iptables:

sudo iptables -A INPUT -p tcp -m state --state NEW --source 192.168.68.1 --dport 2200 -j ACCEPT

Disable SSH connections from all other hosts by running the following command:

sudo iptables -A INPUT -p tcp --dport 2200 -j DROP

Customize SSH Connections on Local Machine

On your local machine (the machine you used to connect to the remote server), add a config file to simplify the command line option.

1. Create a "config" (no file extension required) file in your "/home/user/.ssh" folder.
2. Open the file and add the following lines:

Host sshserver
    HostName remote.server.ip.address.com
    Port 2200
    User foobar
    IdentityFile ~/.ssh/remote.server.key

Change the details of your own remote server's configuration.

3. Save and exit the text editor. To connect to your remote server, type the following command:

ssh sshserver

In addition to securing your SSH server, you should also make use of these tools to secure other parts of your server, too.

Frequently Asked Questions

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Ramces Red - Staff Writer

Ramces is a technology writer that lived with computers all his life. A prolific reader and a student of Anthropology, he is an eccentric character that writes articles about Linux and anything *nix.