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Configuring Firewalls for Instances

Reviewed on 10 May 2021Published on 18 July 2018
  • compute
  • networking
  • ethernet

A firewall controls incoming and outgoing network traffic based on predefined security rules. Typically it establishes a barrier between a trusted (internal) network and untrusted external network, like the Internet.

UFW, or Uncomplicated FireWall, is a frontend for IPTables to simplify the configuration of your firewall.

Requirements:

Installing UFW

UFW is available as a pre-built package in the apt repositories of Ubuntu. It can be easily installed via apt:

sudo apt-get install ufw

Configuring Security Policies

Security policies applied by the firewall on your server depend on your needs and the applications you use.

The most secure configuration is to block all traffic, inbound and outbound by default and to allow ports on a a case by case policy.

In this tutorial, we will configure a policy that blocks inbound packets and authorizes outbound traffic by default.

Note:

If you are using a Bare Metal Cloud Server to follow this tutorial, make sure you do not to block the nbd ports before applying the rules.

  1. Start by defining the policy, that refuses everything by default:

    sudo ufw default deny
  2. Enable outgoing traffic.

    sudo ufw default allow outgoing

Establishing rules

To define rules, you have to know which services are running on the server and which are their associtated ports.

In this example, a SSH server, HTTP(S) and a DNS server are running on the machine.

Every known protocol uses an associated port from the Well Known Ports list.

The services running on the machine used in this example need the following ports:

  • Port 22 / TCP for SSH
  • Port 80 / TCP for HTTP
  • Port 443 / TCP for HTTPS
  • Port 53 / TCP & UDP for DNS
  1. Authorize SSH.

    sudo ufw allow 22/tcp
  2. Authorize HTTP.

    sudo ufw allow 80/tcp
  3. Authorize HTTPS.

    sudo ufw allow 443/tcp
  4. Authorize DNS.

    sudo ufw allow 53
    Note:

    In this case TCP has not to be specified, as both, TCP and UDP are needed.

  5. Activate the new rules.

    sudo ufw enable
  6. Verify the configuration.

    sudo ufw status numbered

    A list of all configured rules displays:

    Status: active
        To                         Action      From    --                         ------      ----[ 1] 22/tcp                     ALLOW IN    Anywhere[ 2] 80/tcp                     ALLOW IN    Anywhere[ 3] 443/tcp                    ALLOW IN    Anywhere[ 4] 53                         ALLOW IN    Anywhere[ 5] 22/tcp (v6)                ALLOW IN    Anywhere (v6)[ 6] 80/tcp (v6)                ALLOW IN    Anywhere (v6)[ 7] 443/tcp (v6)               ALLOW IN    Anywhere (v6)[ 8] 53 (v6)                    ALLOW IN    Anywhere (v6)

Adding more Rules

As the firewall is running now, it is possible to add more rules to it:

Allow the connection to port 25 (SMTP) via TCP to the server:

sudo ufw allow 25/TCP

Deleting Rules

Over the time you may recognize, that some of the rules you defined previously don’t match your requirements anymore.

  1. Display the list of all defined rules:

    sudo ufw status numbered

    The numbers at the beginning of each row are the number of the rule in UFW.

  2. To delete a rule, find its number and type:

    sudo ufw delete NUMBER

Enabling ufw on a BareMetal Instance

Scaleway BareMetal instances are using the NBD protocol to connect the volumes. When enabling a firewall, you have to make sure not to block these ports.

  1. Change the default INPUT-policy to ACCEPT and no longer to DROP. Open the file /etc/default/ufw and change the entry DEFAULT_INPUT_POLICY as following:

    DEFAULT_INPUT_POLICY="ACCEPT"
  2. Add a DROP-ALL rule at the end of the file /etc/ufw/after.rules, just before the final COMMIT line:

    -A ufw-reject-input -j DROP
  3. Disable the UFW logging:

    sudo ufw logging off
  4. Enable SSH to be able to connect to the server once the firewall is activated:

    sudo ufw allow ssh
  5. Enable UFW:

    sudo ufw enable

Refer to the man ufw manpage and the official website for more information.