How to Secure an Ubuntu Server

Reading time ~3 minutes

Recently I purchased a virtual ubuntu server on rackspace. However I have to manage it myself, and this includes first of all taking care for its security. Securing it is very important and is the first thing to be done, since all the rackspace servers have public IP-s and so are open to all the possible attacks from the Internet. Here I will describe some of the steps that can be done to secure the server.

1 Login to the Server

Initially you can login as user root, through ssh:

ssh root@123.45.67.890

Then you can change the password of the root (from the default one, assigned by rackspace), with the command passwd.

2 Add an Admin User

It is a common practice in Linux (and specially in ubuntu) to lock down the root account and to use another account for administrative tasks. Let's see how we can create it.

  • First create another user account:
    adduser administrator
    
  • Then assign the administrative user sudo priviledges (by adding it to the the group sudo):
    adduser administrator sudo
    

3 Set Up Public and Private Keys

One effective way of securing SSH access to your Cloud Server is to use a public/private key. This means that a public key is placed on the server and the private key is on your local workstation. This makes it impossible for someone to log in using just a password - they must have the private key. This consists of 3 basic steps: create the key on your local workstation, copy the public key to the Cloud Server, and set the correct permissions for the key.

  1. Create the public and private keys on you personal (local) computer:
    mkdir ~/.ssh
    ssh-keygen -t rsa
    cd ~/.ssh/
    mv id_rsa rackspace_rsa
    

    If you do not want a passphrase then just press enter when prompted.

    The id_rsa and id_rsa.pub are created in the .ssh directory. The id_rsa.pub file holds the public key. You'll place this file on you server. The id_rsa file is your private key. Never show, give away, or keep this file on a public computer. We rename it to rackspace_rsa to make it obvious that this is the private key that is used to access the rackspace server.

  2. Copy the public key to the remote server:
    scp ~/.ssh/id_rsa.pub administrator@123.45.67.890:/home/administrator/
    
  3. Modify ssh permissions (on the remote server):
    ssh administrator@123.45.67.890
    
    mkdir /home/administrator/.ssh
    mv /home/administrator/id_rsa.pub /home/administrator/.ssh/authorized_keys
    
    chown -R administrator: /home/administrator/.ssh
    chmod 700 /home/administrator/.ssh
    chmod 600 /home/administrator/.ssh/authorized_keys
    

4 Modify the SSH Configuration

Keeping the SSH service on the default port of 22 makes it an easy target. It is recommended to change the default SSH configuration to make it more secure. There are also some other configuration options that are used to lock down the ssh access to the server.

Modify the file /etc/ssh/sshd_config by adding or editing these lines:

Port 1234                  # change ssh port to 1234
Protocol 2
PermitRootLogin no         # user root is not allowed to log in
PasswordAuthentication no  # disable password login, only the private key is accepted
UseDNS no
AllowUsers administrator   # only user administrator is allowed to log in

Not all of these options are required, they overlap each-other, and you can choose which ones to use depending on your case and your security/flexibility requirements.

We need to restart the sshd service in order to enable these changes:

service ssh restart

After applying the changes, login from a second terminal (without logging out from the first one), in order to make sure that you can still login and you didn't lock yourself out of the server. Now you can login like this:

ssh -i ~/.ssh/rackspace_rsa -p 1234 administrator@123.45.67.890

So, we use the key ~/.ssh/rackspace_rsa for authentication, access the ssh server on the port 1234, and login as user administrator.

By the way, in case that we need to copy something through scp, we can use a command like this:

scp -i ~/.ssh/rackspace_rsa -P 1234 source_file administrator@123.45.67.890:destination_file

5 Setup a Firewall

For simple firewalls, ufw is a great tool for building them easily. Let's say that we would like to allow only the ports 80, 443, and 1234 (don't forget to allow the ssh port, otherwise you can lock yourself out!). We can build the firewall like this:

ssh -i ~/.ssh/rackspace_rsa -p 1234 administrator@123.45.67.890
ufw allow 1234
ufw allow 80
ufw allow 443
ufw enable

If you are familiar with iptables, then you may want to check out the iptables' rules that ufw has built, by using iptables-save.

Building SSH Tunnels

Building SSH TunnelsBuilding SSH TunnelsTable of Contents1. Bypass a firewall blocking2. Connect to a database behind a firewall3. Access...… Continue reading

How to Build a Twitter-Bot

Published on March 24, 2017

Linux Guest Account

Published on January 14, 2017