How to Configure a Firewall with UFW

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UFW, or uncomplicated firewall, is a frontend for managing firewall rules Arch Linux, Debian or Ubuntu. UFW is used through the command line (although it has GUIs available), and aims to make firewall configuration easy (or, uncomplicated).

How to Configure a Firewall with UFW

Before You Begin

  1. Familiarize yourself with our Getting Started guide and complete the steps for setting your Linode’s hostname and timezone.

  2. This guide will use sudo wherever possible. Complete the sections of our Securing Your Server guide to create a standard user account, harden SSH access and remove unnecessary network services. Do not follow the Creating a Firewall section–this guide is an introduction to using UFW, which is a separate method of controlling a firewall than iptables commands.

  3. Update your system.

    Arch Linux

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    sudo pacman -Syu
    

    Debian / Ubuntu

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    sudo apt-get update && sudo apt-get upgrade
    

Install UFW

UFW is included in Ubuntu by default but must be installed in Arch and Debian. Debian will start UFW’s systemd unit automatically and enable it to start on reboots, but Arch will not. This is not the same as telling UFW to enable the firewall rules, as enabling UFW with systemd or upstart only tells the init system to switch on the UFW daemon.

By default, UFW’s rulesets are blank so it is not enforcing any firewall rules–even when the daemon is running. Enforcing your firewall ruleset is covered further down the page.

Arch Linux

  1. Install UFW:

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    sudo pacman -S ufw
    
  2. Start and enable UFW’s systemd unit:

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    sudo systemctl start ufw
    sudo systemctl enable ufw
    

Debian / Ubuntu

  1. Install UFW

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    sudo apt-get install ufw
    

Use UFW to Manage Firewall Rules

Set Default Rules

Most systems will need a only a small number of ports open for incoming connections, and all remaining ports closed. To start with an easy basis of rules, the ufw default command can be used to set the default response to incoming and outgoing connections. To deny all incoming and allow all outgoing connections, run:

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sudo ufw default allow outgoing
sudo ufw default deny incoming

The ufw default command also allows for the use of the reject parameter.

Configuring a default reject or deny rule can lock you out of your Linode unless explicit allow rules are in place. Ensure that you have configured allow rules for SSH and other critical services as per the section below before applying default deny or reject rules.

Add Rules

Rules can be added in two ways: By denoting the port number or by using the service name.

For example, to allow both incoming and outgoing connections on port 22 for SSH, you can run:

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sudo ufw allow ssh

You can also run:

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sudo ufw allow 22

Similarly, to deny traffic on a certain port (in this example, 111) you would only have to run:

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sudo ufw deny 111

To farther fine-tune your rules, you can also allow packets based on TCP or UDP. The following will allow TCP packets on port 80:

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sudo ufw allow 80/tcp
sudo ufw allow http/tcp

Whereas this will allow UDP packets on 1725:

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sudo ufw allow 1725/udp

Advanced Rules

Along with allowing or denying based solely on port, UFW also allows you to allow/block by IP addresses, subnets, and a IP address/subnet/port combinations.

To allow connections from an IP address:

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sudo ufw allow from 123.45.67.89

To allow connections from a specific subnet:

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sudo ufw allow from 123.45.67.89/24

To allow a specific IP address/port combination:

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sudo ufw allow from 123.45.67.89 to any port 22 proto tcp

proto tcp can be removed or switched to proto udp depending upon your needs, and all instances of allow can be changed to deny as needed.

Remove Rules

To remove a rule, add delete before the rule implementation. If you no longer wished to allow HTTP traffic, you could run:

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sudo ufw delete allow 80

Deleting also allows the use of service names.

Edit UFW’s Configuration Files

Although simple rules can be added through the command line, there may be a time when more advanced or specific rules need to be added or removed. Prior to running the rules input through the terminal, UFW will run a file, before.rules, that allows loopback, ping, and DHCP. To add to alter these rules edit the /etc/ufw/before.rules file. A before6.rules file is also located in the same directory for IPv6.

An after.rule and an after6.rule file also exists to add any rules that would need to be added after UFW runs your command-line-added rules.

An additional configuration file is located at /etc/default/ufw. From here IPv6 can be disabled or enabled, default rules can be set, and UFW can be set to manage built-in firewall chains.

UFW Status

You can check the status of UFW at any time with the command: sudo ufw status. This will show a list of all rules, and whether or not UFW is active:

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Status: active

To                         Action      From
--                         ------      ----
22                         ALLOW       Anywhere
80/tcp                     ALLOW       Anywhere
443                        ALLOW       Anywhere
22 (v6)                    ALLOW       Anywhere (v6)
80/tcp (v6)                ALLOW       Anywhere (v6)
443 (v6)                   ALLOW       Anywhere (v6)

Enable the Firewall

With your chosen rules in place, your initial run of ufw status will probably output Status: inactive. To enable UFW and enforce your firewall rules:

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sudo ufw enable

Similarly, to disable UFW’s rules:

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sudo ufw disable

This still leaves the UFW service running and enabled on reboots.

Logging

You can enable logging with the command:

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sudo ufw logging on

Log levels can be set by running sudo ufw logging low|medium|high, selecting either low, medium, or high from the list. The default setting is low.

A normal log entry will resemble the following, and will be located at /var/logs/ufw:

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Sep 16 15:08:14 <hostname> kernel: [UFW BLOCK] IN=eth0 OUT= MAC=00:00:00:00:00:00:00:00:00:00:00:00:00:00 SRC=123.45.67.89 DST=987.65.43.21 LEN=40 TOS=0x00 PREC=0x00 TTL=249 ID=8475 PROTO=TCP SPT=48247 DPT=22 WINDOW=1024 RES=0x00 SYN URGP=0

The initial values list the date, time, and hostname of your Linode. Additional important values include:

  • [UFW BLOCK]: This location is where the description of the logged event will be located. In this instance, it blocked a connection.

  • IN: If this contains a value, then the event was incoming

  • OUT: If this contain a value, then the event was outgoing

  • MAC: A combination of the destination and source MAC addresses

  • SRC: The IP of the packet source

  • DST: The IP of the packet destination

  • LEN: Packet length

  • TTL: The packet TTL, or time to live. How long it will bounce between routers until it expires, if no destination is found.

  • PROTO: The packet’s protocal

  • SPT: The source port of the package

  • DPT: The destination port of the package

  • WINDOW: The size of the packet the sender can receive

  • SYN URGP: Indicated if a three-way handshake is required. 0 means it is not.

This guide is published under a CC BY-ND 4.0 license.