getting started iptables

8/3/2019 Getting Started Iptables

1/17

Getting Started: Using FirewallBuilder to Configure Linux iptables


2/17

Getting Started: Using Firewall Builder to Configure Linux iptables

$Id$

Copyright 2003,2011 NetCitadel, LLC

The information in this manual is subject to change without notice and should not be construed as a commitment by NetCitadel LLC. NetCitadel

LLC assumes no responsibility or liability for any errors or inaccuracies that may appear in this manual.


3/17

iii

Getting Started: Configuring Linux iptables Firewalls ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... 1

1. How Firewall Builder works with Linux iptables firewalls ... ... ... ... ... ... ... ... ... ... ... ... ... ... ... . 1

2. Firewall Builder GUI Layout ..... ...... ..... ..... ...... ..... ...... ..... ...... ...... ..... ...... ..... ...... ...... . 2

3. Example Linux iptables Deployment ...... ..... ...... ..... ..... ...... ..... ...... ..... ...... ...... ..... ...... .. 2

4. Creating a Linux iptables Firewall ..... ...... ..... ...... ...... ..... ...... ..... ...... ...... ..... ...... ..... ..... 3

5. Working With Objects ...... ...... ..... ...... ..... ...... ...... ..... ...... ..... ..... ...... ..... ...... ..... ...... .... 5

6. Configuring iptables Policy Rules ..... ...... ..... ..... ...... ..... ...... ..... ...... ...... ..... ...... ..... ...... . 66.1. Additional Tips For Working with Rules ......................................................... 10

7. Configuring NAT Rules ..... ...... ..... ..... ...... ..... ...... ..... ...... ...... ..... ...... ..... ...... ...... ..... . 11

8. Compile and Install ..... ...... ..... ...... ..... ..... ...... ..... ...... ..... ...... ...... ..... ...... ..... ...... ...... . 12


4/17

1

Getting Started: Configuring Linuxiptables Firewalls

Firewall Builder is a configuration management application that can be used to configure and managefirewall rules for multiple types of firewalls. This guide will go through the steps of creating a Linux

iptables firewall in Firewall Builder and then creating and installing rules created in Firewall Builder onto

the firewall.

1. How Firewall Builder works with Linuxiptables firewalls

Conceptually Firewall Builder works the same way no matter what type of firewall is being configured.

Firewall Builder is based on the idea of using objects to represent rule elements, such as IP networks Host

addresses, and then using these objects to define firewalll rules.

For Linux iptables firewalls after the firewall rules have been created Firewall Builder will generate a

shell script file that includes all the iptables commands required to implement the user defined security

policy. This script file can also include information required to configure interfaces, static routes and other

configuration options.

Once the firewall script file has been created Firewall Builder can use the secure SSH and SCP protocols

to transfer the script file to the Linux firewall server and to activate it, or user's can copy the the script file

themselves to the server and run it to activate the firewall rules.

Figure 1. Basic Workflow for Configuring Linux iptables with Firewall Builder


5/17

Getting Started: Configuring

Linux iptables Firewalls

2

2. Firewall Builder GUI Layout

Before we get started configuring our example firewall, let's take a minute to orient ourselves with the

Firewall Builder application. The GUI is comprised of three primary panels shown in the screenshot below.

Figure 2. Firewall Builder GUI Layout

Object Panel. Objects in the active Library are displayed in an object tree in the Object Panel. Empty

folders are defined for all possible object types that a user can create in the User library.

Rules Panel. When a Policy object is opened for editing it is displayed in the Rules Panel. Clicking on

the '+' button at the top of the panel creates a new rule.

Editor Panel. Double-clicking on objects opens them for editing in the Editor Panel. Changes to object

attribute fields take effect immediately.

Panels open dynamically based on what activity the user is performing. For example, double-clicking

an object to edit it will open the Editor Panel if it is not already open. Now we are ready to get started

configuring our firewall example.

3. Example Linux iptables DeploymentThis Getting Started Guide is based on configuring a Linux iptables firewall to match the network

deployment shown in the diagram below.


6/17



3

Figure 3. Example Linux iptablesConfiguration

The goal of the Getting Started Guide is to help users become familiar with the basic process of configuring

a Linux iptables firewall using Firewall Builder, so there are many advanced features that won't be covered

here. The Users Guide [http://www.fwbuilder.org/4.0/docs/users_guide/index.html] has a complete listing

of all Firewall Builder features.

4. Creating a Linux iptables FirewallTo create a firewall object to represent your Linux iptables firewall click on the Create new firewall icon

on the main window of Firewall Builder or right-click on the Firewalls system folder in the object tree

and select "New Firewall". Either of these methods will launch a wizard that walks you through creating

your firewall object.

Enter a name for the firewall object, in this example we will use linux-1. Change the drop down menu for

software that is running on the firewall to be iptables.

Figure 4. New Firewall Dialog

Click the "Next >" button to continue to the next step in the wizard.

When creating a firewall in Firewall Builder you have a choice of configuring interfaces manually, or youcan use SNMP discovery if you have SNMP enabled on your firewall and you have know the Read-Only or

Read-Write community string. For this example we are going to configure the firewall interfaces manually.

Figure 5. Select Interface Configuration Method

Click the "Next >" button to continue to the next step.
http://www.fwbuilder.org/4.0/docs/users_guide/index.htmlhttp://www.fwbuilder.org/4.0/docs/users_guide/index.html


7/17



4

The firewall that you create in Firewall Builder needs to match the Linux server that you want to deploy

the iptables rules on. This means that the interface names andIP addresses in the firewall object that you

are creating must match exactly to what is configured on the Linux server.

Click the green icon to add a new interface to the firewall. Enter the name of the interface exactly

as it is shown on the Linux server when you run the ifconfig" command. In our example the interfaces

are eth0, eth1 and lo.

Set the interface name as eth0 and set the label to outside. Click on the Add address button and set the IP

address to 192.0.2.1 with a netmask of 255.255.255.240.

Figure 6. Set Interface IP Address

Click the green icon to add another interface to the firewall. Enter the information in to the wizard

to match the second interface as follows:

Figure 7. Interface Tabs

Click the green icon to add another interface to the firewall. Enter the information for the "lo" interface

with label "loopback", IP address 127.0.0.1 and netmask 255.0.0.0.

Click the "Finish >" button to create the firewall object.

After you create the firewall object representing the Linux server the firewall object will be displayed

in the object tree on the left side. The Policy object, which is where the iptables rules are configured, is

automatically opened in the main window.

Figure 8. Firewall Displayed in Object Tree


8/17



5

Before moving on you should save our data file that contains the new firewall object that you just created.

Do this by going to the File -> Save As menu item. Choose a name and location to save the file.

5. Working With Objects

Firewall Builder is based on the concept of objects. There are a variety of different object types that canbe used to define IP objects that can be used as the Source and Destination in your firewall rules. Two of

the most common IP objects used in firewall rules areNetworks andAddresses.

Network Objects

To create a Network object, for example a network to represent the internal 10.10.10.0/24 network shown

in the diagram on the previous page, go to the object tree on the left side of the screen and double-click the

folder labeled Objects to expand it. Right click on the folder calledNetworks and select New Network.

This creates a new network object. In the lower portion of your screen, called the Editor Panel, you can

modify the properties of this object.

Change the object name to something that matches the function, in this example we are going to call itInternal Network to represent the network connected to our "inside" interface. The address is set to

10.10.10.0 and the netmask is 255.255.255.0.

Figure 9. New Network Object

Note

When editing the attributes of an object there is no Apply or Submit button. Once you edit an

attribute as soon as you move away from the field you were editing the change will take effect

immediately.

Address ObjectsTo create an object that represents a single IP address, similar to the host parameter in a Cisco access

list, go to the object tree and right-click on theAddresses folder and select "New Address". In the Editor

Panel change the name of the object to something that reflects its function, for example POP3 Server,

and set the IP address.


9/17



6

Figure 10. New Address Object

You may have noticed that we did not create any objects for the TCP services, like HTTP and SSH, that

will be needed for the firewall rules shown in the example. This is because Firewall Builder comes with

hundreds of predefined objects for commonly used objects like TCP services.

6. Configuring iptables Policy Rules

After you have created a firewall object and network objects you can start to configure the firewall's rules.

When you create a firewall object, for example linux-1 from our previous example, it will be opened

automatically in the object tree and its Policy object is opened in the main window for editing. The Policy

object is where access list rules are configured.

To add a new rule to the Policy, click on the green icon at the top left of the main window. This

creates a new rule with default values set to deny all.

Figure 11. Default Rule

Every rule includes the following sections:

Source - this can be one or more IP objects. Default value is Any which is the same as the "0/0" iptables

parameter that matches all IP addresses.

Destination - this can be one or more IP objects. Default value is Any which is the same as the "0/0"

parameter in iptables that matches all IP addresses.

Service - this can be one or more Service object. Example services include the TCP and UDP protocols

like HTTP and DNS. Default value is Any which matches any IP service.

Interface - this can be one or more interfaces configured on the firewall object. Default value is All

which means the rule will be applied to all interfaces.

Direction - options are Inbound, Outbound and Both. This roughly translates to the iptables INPUT,

OUTPUT and FORWARD chains. Default value is Both.

Action - Options are:

Accept - matches iptables -j ACCEPT target

Deny - matches iptables -j DROP target


10/17



7

Reject - matches iptables -j REJECT target

Accounting - traffic goes through additional user defined chain with -j RETURN target

Queue - matches iptables -j QUEUE target

Tag - matches iptables -j MARK target with --set-mark set to tag value

Classify - matches iptables -j CLASSIFY target with --set-class set to user defined string

Custom - user can define a custom string

Branch - matches iptables target of user defined chain

Routing - matches iptables target -j ROUTE

Continue - continue processing even if rule matches

Options - options are Logging On and Logging Off. Setting Logging On matches the iptables -j LOG

target. Default value is Logging On.

Configuring a Rule

In the examples below the fields in the rule will be set to the values that match the first rule from our

example scenario (scenario rules shown in figure below). This rule controls SSH access to the firewall

itself.

Figure 12. Scenario Rules

Setting the Source

To set the Source of a rule, drag-and-drop at least one IP object from the tree to the Source field of your

rule. For example, drag the Network object called Internal Network that you created earlier to the Source

column of the rule as shown below.

Figure 13. Setting the Source

After you drop the network object into the rule the Source fields will change from Any to Internal Network.

Figure 14. After Source is Set


11/17



8

Note

You can have more than one IP object in the Source and Destination fields. When Firewall

Builder generates the iptables commands it will automatically split the rule in to multiple lines

if necessary.

Setting the DestinationSetting the Destination is exactly the same as setting the Source, except you drag-and-drop IP objects in

to the Destination field of the rule. For our first example rule we want the Destination to be the "inside"

interface of the firewall object. Drag-and-drop the eth1 object from the object tree to the Destination

column.

Figure 15. Setting the Destination

After you drop the interface object into the rule the Destination field will change from Any to "inside"

which is the label of the eth1 interface.

Figure 16. After Destination is Set

Setting the Service

Firewall Builder comes with hundreds ofpredefinedobjects including Service objects for almost all

standard protocols. To access these objects switch to the Standard library by selecting it from the drop

down at the top of the Object tree window.

Figure 17. Switching Libraries

Services are located in the Services folder. In this rule we want to set the service to SSH, so you would

navigate to the SSH service by opening the Services folder, then opening the TCP folder and scrolling

down until you find the "ssh" object.

Once you find the ssh object, drag-and-drop from the tree on the left in to the Service section of the rule

in the Rules window.


12/17



9

Figure 18. Setting the Service

Note

To switch back to the User library, which contains the objects that you created, click on the drop

down menu that says Standard and select User from the list of libraries.

Setting the Interface

If desired, set the Interface for the rule by dragging-and-dropping an interface object from the firewall

object to the Interface section of the rule. This, combined with the Direction setting, will cause thegenerated iptables rules to include -i or -o .

Figure 19. Setting the Interface

Setting the Direction

The direction of the rule is based on the traffic you want to filter. Traffic coming in to an interface should

have the rule Direction set to Inbound and traffic going out of an interface should have the rule Direction

set to Outbound. In our example the direction of the rule will be Inbound since it is controlling access to

the firewall itself on the "inside" interface.

Setting the Action

The action controls whether traffic matching the rule should be permitted or denied. Firewall Builder setsthe default action for INPUT, OUTPUT and FORWARD chains to deny, so any traffic that has not matched

a rule that permits the traffic will be dropped. Right-click and set the action to "Accept" to allow the SSH

traffic from the local network to the firewall.

Setting the Options

Logging for rule matches is set in the Options section. By default logging is turned on. To turn logging

off, right-click in the Options section and select Logging Off.


13/17



10

Example of a Complete Rule

The following is the first rule from our example which allows traffic from the internal network to the

firewall's inside interface that has a traffic type of SSH.

Figure 20. New Rule with Fields Set

6.1. Additional Tips For Working with Rules

Adding a Rule

To add a new rule click the icon at the top of the Rules Editor window. This will insert a new rule

above the current rule. To add a new rule below the current rule right-click on a rule and select "Add New

Rule Below".

Figure 21. Adding Rules

Copy-and-Paste

In addition to drag-and-drop you can also copy-and-paste objects. For example, you can right-click on the

Internal Network object in the first rule and select Copy. Navigate to the Source section of the new rule

you just created and right-click and select Paste.

Using Filters to Find Objects

Filters provide a way to quickly find objects in the tree without having to open multiple folders and scroll.

For example, if you wanted to use the POP3 protocol in a rule you could use the filter to find it.

The POP3 protocol object is located in the Standard library, so select it from the dropdown menu at the top

of the Object Window. Type pop3 in to the filter field. This will display all objects in the current library

that contain pop3.

Figure 22. Using Filter to Find Objects


14/17



11

Note

After you are done with the filtered object, clear the filter field by clicking the X to the right of

the input box and then switch back to the User library by selecting it in the dropdown menu at

the top of the object panel.

Example of Completed RulesFor our example we needed to create two firewall rules for traffic going through or to the firewall. We

also create a special rule at the top of the Policy that allows the firewall to communicate with itself using

the loopback interface. The completed firewalll rules are shown in the diagram below.

Figure 23. Three Rules

Note

Many Linux processes use the loopback interface to communicate between processes, so it is

very important to have a rule at the top of the policy that allows the firewall to communicate with

itself using the loopback interface.

7. Configuring NAT Rules

Now that the basic firewall rules have been configured we need to define our NAT policy. Open the NAT

object for editing by double-clicking on it in the object tree as shown in the diagram below.

Figure 24. Open the NAT Object for Editing

For our example we need to create a single NAT rule that will translate the source IP address of any traffic

coming from the inside 10.10.10.0/24 network going to the Internet. The source IP should be translated to

the IP address of the "outside" interface of the firewall.

To edit NAT rules we will use the same concepts that were used to edit the firewall Policy rules. Start by

clicking the green '+' sign at the top of the Rules panel to add a new NAT rule.

Drag-and-drop the "Internal Network" object you created earlier to the Original Src column of the NAT

rule. This identifies the traffic that will have its source IP address translated. Now, drag-and-drop the

"outside" interface from the linux-1 firewall object to the Translated Src column of the rule. This field

identifies the IP address that the traffic will be translated to. After you are done the NAT rule should like

the diagram below.


15/17



12

Figure 25. Completed NAT Rule

That's it, now we are ready to generate the configuration file and use the built-in installer to deploy it to

the firewall.

8. Compile and Install

In Firewall Builder the process of converting the rules from the Firewall Builder GUI syntax to the target

device commands is called compiling the configuration.

To compile, click on the Compile icon which looks like a hammer . If you havent saved your

configuration file yet you will be asked to do so. After you save your file a wizard will be displayed that

lets you select which firewall(s) you want to compile. In this example we are going to complie the firewall

called linux-1 configured with the rules above.

If there arent any errors, you should see some messages scroll by in the main window and a message at

the top left stating Success.

To view the output of the compile, click on the button that says Inspect Generated Files. This will open the

file that contains the script file that Firewall Builder generates. The script file can be quite long, it includes

functions to update interface IP addresses, static routes, etc. Near the bottom of the script you will find the

section where the iptables commands are generated. Note that any line that starts with # is a comment.

Figure 26. Example Compiler Output

The output from the compiler is automatically saved in a file in the same directory as the data file that was

used to create it. The generated files are named with the firewall name and a .fw extension. In our examplethe generated configuration file is called linux-1.fw. You could copy this script file to the firewall, or you

can use the built-in Firewall Builder installer.

Installing

Firewall Builder can install the generated configuration file for you using SSH and SCP. To use the installer

we need to identify one of the router interfaces as the Management Interface. This tells Firewall Builder

which IP address to connect to on the router.


16/17



13

Do this by double-clicking the firewall object to expand it, and then double-clicking on the interface name

that you want to assign as the management interface. In this example this is interface eth1 which is the

interface connected to the internal network.

Figure 27. Setting the Managment Interface

Note

Any time you are changing access lists on your firewall you face the risk of locking yourself out

of the device. Please be careful to always inspect your firewall rules closely and make sure that

you will be able to access the firewall after the iptables rules have been installed.

To install your access lists on the firewall, click on the install icon . This will bring up a wizard where

you will select the firewall to install. Click Next > to install the selected firewall.

Figure 28. Setting Compile and Install Actions

Firewall Builder will compile your rules converting them in to iptables command format. After the compile

completes successfully click Next >. Enter your username and password.

Figure 29. Install Dialog

After the access list configuration is installed you see a message at the bottom of the main window and

the status indicator in the upper left corner of the wizard will indicate if the installation was successful.


17/17



14

Figure 30. Successful Install

By default Firewall Builder will use SCP to copy the firewall script file to the firewall. Once the script file

is copied to the firewall, Firewall Builder will connect using SSH to run the script.

getting started iptables

Documents