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Setting up a firewall on Linux Ubuntu 14.04

Uses

Firewalls allow only particular access to particular network ports and paths. You can remove access to a port or a specific IP address or a range of IP addresses.

Admins can apply rules according to what access they would like - these rules include both outgoing and incoming network traffic. This allows admins to build barriers of trust across machines.

Modern firewalls are based on Application Layer Firewalls and are aware of the kinds of applications and protocols that the connection is using. They can block anything which is not part of the normal flow of traffic.

Set up

The firewalld package needs to be installed:

$ sudo aptitude install firewalld 

You should not run the program iptables alongside firewalld as this is the older version and will create a conflict.

firewalld is a service which needs to be running to use and configure the firewall, and is started in the usual way.

$ sudo /etc/init.d/firewalld start 

You can test if the service is running with:

$ sudo firewall-cmd --state

If you have more than one network interface on IPV4, you will need to turn on ip forwarding. This is achieved with:

$ sudo sysctl net.ipv4.ip_forward=1

However, this is not persistent across boots. To achieve this, you will need to add the following line to /etc/sysctl.conf:

net.ipv4.ip_forward=1 

and then reboot or type:

$ sudo sysctl -p 

to read in the new setting.

Zones

Firewalls work with zones each which has a level of trust defined. The zones are

  • drop - only particular incoming connections are allowed
  • block - all incoming connections are rejected
  • public - do not trust any computers on the network; only defined incoming connections are permitted
  • external - used when masquerading is being used, usually in routers.
  • dmz - used when access is allowed only to some services to the public.
  • work - trust connected nodes to be not harmful
  • home - mostly trust other network nodes, but still select which incoming connections are allowed
  • internal - similar to work zone
  • trusted - everything is allowed.

On installation, nearly all Linux distributions will select the public zone as the default for all interfaces.

Get the default zone:

$ sudo firewall-cmd --get-default-zone

Obtain a list of zones being used:

$ sudo firewall-cmd --get-active-zones 

List all available zones:

$ sudo firewall-cmd --get-zones 

To change the default zone to trusted and then change it back.

$ sudo firewall-cmd --set-default-zone=trusted

$ sudo firewall-cmd --set-default-zone=public

To assign an interface temporarily to a particular zone:

$ sudo firewall-cmd --zone=internal --change-interface=eth0

This will only change the interface until the next boot. To make it permanent, we add the --permanent flag.

$ sudo firewall-cmd --permanent --zone=internal --change-interface=eth0

which will create a file in /etc/firewalld/zones/internal.xml.

To ascertain the zone associated with a particular interface:

$ sudo firewall-cmd --get-zone-of-interface=eth0

To get the details of a particular zone:

$ sudo firewall-cmd --zone=public --list-all 

An admin can also bind a zone to a network address (as well as an interface).

To bind a source to a zone:

$ sudo firewall-cmd --permanent --zone=trusted --add-source=192.168.1.0/24

With this command, anyone from address 192.168.1.x will be added to the trusted zone.

Services and ports within a zone

Setting zones is now possible, but for these to become effective, we need to set particular services within each zone.

We can see what services are associated with a particular zone:

$ sudo firewall-cmd --list-services --zone=public

To add a service to a zone:

$ sudo firewall-cmd --permanent --zone=work --add-service=http
$ sudo firewall-cmd --reload 

--reload is required to make the change effective and --permanent is required to add a service to a zone.

Port management is very similar:

$ sudo firewall-cmd --zone=work --add-port=21/tcp
$ sudo firewall-cmd --reload 

You can also remove these with the commands --remove-service and --remove-port.