Uncomplicated Fire Wall

The Linux kernel includes the Netfilter subsystem, which is used to manipulate or decide the fate of network traffic headed into or through your server. All modern Linux firewall solutions use this system for packet filtering.

The kernel’s packet filtering system would be of little use to administrators without a userspace interface to manage it. This is the purpose of the **iptables** utility: when a packet reaches your server, it will be handed off to the Netfilter subsystem for acceptance, manipulation, or rejection based on the rules supplied to it from the userspace (via iptables). Thus, iptables is all you need to manage your firewall, if you’re familiar with it, but many are available to simplify the task. We’ll take a look at the default frontend used in Ubuntu here.

ufw - Uncomplicated Firewall

The default firewall configuration tool for Ubuntu is ufw. Developed to ease iptables firewall configuration, ufw provides a user-friendly way to create an IPv4 or IPv6 host-based firewall.

ufw by default is initially disabled. From the ufw man page:

ufw is not intended to provide complete firewall functionality via its command interface, but instead provides an easy way to add or remove simple rules. It is currently mainly used for host-based firewalls.

Enable or disable ufw

To enable ufw, run the following command from a terminal prompt:

sudo ufw enable

To disable it, you can use the following command:

sudo ufw disable

Open or close a port

To open a port (SSH in this case):

sudo ufw allow 22

Similarly, to close an opened port:

sudo ufw deny 22

Add or remove a rule

Rules can also be added using a numbered format:

sudo ufw insert 1 allow 80

To view the numbered format:

sudo ufw status numbered

To remove a rule, use delete followed by the rule:

sudo ufw delete deny 22

Allow access from specific hosts

It is possible to allow access from specific hosts or networks to a port. The following example allows SSH access from host 192.168.0.2 to any IP address on this host:

sudo ufw allow proto tcp from 192.168.0.2 to any port 22

Replace 192.168.0.2 with 192.168.0.0/24 to allow SSH access from the entire subnet.

The --dry-run option

Adding the --dry-run option to a ufw command will output the resulting rules, but not apply them. For example, the following is what would be applied if opening the HTTP port:

sudo ufw –dry-run allow http

*filter
:ufw-user-input - **[****0**:0**]**
:ufw-user-output - **[****0**:0**]**
:ufw-user-forward - **[****0**:0**]**
:ufw-user-limit - **[****0**:0**]**
:ufw-user-limit-accept - **[****0**:0**]**
_### RULES ###_

_### tuple ### allow tcp 80 0.0.0.0/0 any 0.0.0.0/0_
-A ufw-user-input -p tcp --dport **80** -j ACCEPT

_### END RULES ###_
-A ufw-user-input -j RETURN
-A ufw-user-output -j RETURN
-A ufw-user-forward -j RETURN
-A ufw-user-limit -m limit --limit **3**/minute -j LOG --log-prefix "[UFW LIMIT]: "
-A ufw-user-limit -j REJECT
-A ufw-user-limit-accept -j ACCEPT
COMMIT
Rules updated

Check the status

To see the firewall status, enter:

sudo ufw status

sudo ufw show added

And for more verbose status information use:

sudo ufw status verbose

Note

If the port you want to open or close is defined in /etc/services, you can use the port name instead of the number. In the above examples, replace 22 with ssh.

This is a quick introduction to using ufw. Please refer to the ufw man page for more information.

ufw application integration

Applications that open ports can include a ufw profile, which details the ports needed for the application to function properly. The profiles are kept in /etc/ufw/applications.d, and can be edited if the default ports have been changed.

To view which applications have installed a profile, run the following in a terminal:

sudo ufw app list

Similar to allowing traffic to a port, using an application profile is accomplished by entering:

sudo ufw allow Samba

An extended syntax is available as well:

ufw allow from 192.168.0.0/24 to any app Samba

Replace Samba and 192.168.0.0/24 with the application profile you are using and the IP range for your network.

Note

There is no need to specify the protocol for the application, because that information is detailed in the profile. Also, note that the app name replaces the port number.

To view details about which ports and protocols, and so on, are defined for an application, enter:

sudo ufw app info Samba

Not all applications that require opening a network port come with ufw profiles, but if you have profiled an application and want the file to be included with the package, please file a bug against the package in Launchpad.

ubuntu-bug nameofpackage