Creating Firewall Object using SNMP discovery

If your firewall runs SNMP daemon, you can save yourself some time by using SNMP discovery to automatically create interfaces of the new firewall object.

Start by checking checkbox "Use SNMP" on the second page of the wizard and enter SNMP "read" community, then click button "Discover interfaces using SNMP".

Firewall Builder will run series of SNMP queries to the firewall to read the list of interfaces and their addresses. Both IPv4 and IPv6 address can be imported. For IPv6 the firewall must support IP-MIB RFC4293. Once discovery process finishes, click "Next".

Next page of the wizard offers an opportunity to review discovered interfaces and make adjustments if necessary. To change something, highlight an interface, edit its parameters in the dialog, then click "Update". Finally when the process is done and you click "Finish" in the wizard, the program creates new firewall object in the tree and adds all configured interfaces and their addresses.

Editing Firewall Object

The Firewall Object represents the firewall machine and is the most complex object in Firewall Builder. It has three sets of controls that you can modify, not including the policy rule sets. All these controls become available when you double-click the firewall object in the tree.

The base controls let you specify the basic settings of the firewall, such as the name and firewall platform.

• Name: Specify/change the name of the firewall object.
• Platform: Specify/change the firewall software.
• Version: Specify/change the version number of the firewall software. In most cases, you can leave this set to any.
• Host OS: Specify/change the host operating system of the firewall device.
• Inactive firewall: Check this box to make the firewall inactive. The firewall name will change to a regular font (instead of bold) to indicate that it is inactive, and the firewall will not be available for compiling or installation. Essentially, it’s a way to "comment out" the firewall without deleting it.
• Host OS Settings: Opens the Advanced Settings dialog for the indicated Host OS.
• Firewall Settings: Opens the Advanced Settings dialog for the platform/firewall software.

Host OS Settings Dialog

For explanations of the various controls, click the Help button in the dialog.

Firewall Settings Dialog

For explanations of the various controls, click the Help button in the dialog.

Interface Object

Interface objects belong to firewall or host objects. Interface objects cannot exist alone. The dialog for the interface object that belongs to the firewall or host provides controls for the parameters described here.

• Name: The name of the interface object in Firewall Builder must match exactly the name of the interface of the firewall machine it represents. This will be something like "eth0", "eth1", "en0", "br0", and so on.
• Label: On most OS’s this field is not used and serves the purpose of a descriptive label. Firewall Builder GUI uses a label, if it is not blank, to show interfaces in the tree. One of the suggested uses for this field is to mark interfaces to reflect the network topology (’outside’, ’inside’) or the purpose (’web frontend’ or ’backup subnet’). The label is mandatory for Cisco PIX though, where it must reflect the network topology.
• Management interface: When firewall has several network interfaces, one of them can be marked as the ’management interface’. The management interface is used for all communication between Firewall Builder and the firewall. For example, built-in policy installer uses address of the management interface to connect to the firewall via ssh when it copies generated script or configuration file to it.
• External interface (insecure): Marks an interface that connects to the Internet.
• Unprotected interface: Marks interface to which Firewall Builder should not assign any access lists or firewall rules. Unprotected interfaces are recognized by policy compilers for Cisco IOS access lists and PF. Compiler for IOS ACL just skips unprotected interfaces and does not assign any ACL to them when it choses which interface to associate given ACL with. Compiler for PF generates "set skip on <interface_name>" clause for unprotected interfaces.
• Regular Interface: Use this option if the interface has an IP address assigned to it manually (static IP address).
• Address is assigned dynamically: Use this option if the interface has a dynamic address (obtained by means of DHCP or PPP or another protocol). In this case an address is unknown at the moment when Firewall Builder generates the Firewall policy. Some firewalls allow for using the interface name in the policy instead of its IP address; the firewall engine then picks its address either when the policy is activated or even at run-time. Some other firewalls support special syntax for rules that are supposed to match packets headed to or from the firewall machine. Examples of these two cases are OpenBSD PF and Netfilter. PF rules can be constructed using interface names; PF automatically uses the current interface address when it loads rules into the memory. Netfilter supports special "chains" called "INPUT" and "OUPUT" that are guaranteed to inspect only packets headed for the firewall machine ("INPUT") or originated on it ("OUTPUT"). Both methods allow Firewall Builder to build correct firewall policy rules that affect the interface with a dynamic IP address, however the interface must be marked as such for the policy compiler to use proper technique depending on the target firewall platform. In cases where the rule has to use actual IP address of the interface (example: anti-spoofing rules), compiler emulates this feature by adding shell script fragment to determine the address at the time when firewall script is executed and then uses the address in rules. Such emulation is only possible on platforms where firewall configuration is in the form of the shell script, most notably this is iptables script on Linux.
• Unnumbered interface: Use this option if the interface can never have an IP address, such as the Ethernet interface used to run PPPoE communication on some ADSL connections, or tunnel endpoint interface. Although unnumbered interface does not have an address, firewall policy rules or access lists can be associated with it.
• Bridge port: This option is used for port of bridged firewall. Compilers skip bridge ports when they pick interfaces to attach policy and nat rules to. For target firewall platforms that support bridging and require special configuration parameters to match bridged packets, compilers use this attribute to generate proper configuration. For example, in case of iptables compiler uses -m physdev --physdev-in or -m physdev --physdev-out for bridge port interfaces.
• Security level: Depending on the firewall platform, the security level is either External/Internal or a numeric value between 0 and 100, with 0 being least secure and 100 being most secure levels. This field in the GUI dialog automatically shows controls appropriate to the current firewall. Not all firewall support the concept of a security zone.
• Network zone: Network zone of this interface, used only with Cisco PIX (ASA). Network zone drop-down list shows all network objects and groups of addresses and networks present in the tree. Choose one of them to tell the compiler which networks and blocks of addresses can be reached through this interface. Usually the external interface (the one which connects your firewall to the Internet) has the Network Zone set to Any. It is also recommended that you create a group of objects to represent Network Zones for all other interfaces on the firewall. The compiler uses this information to decide which interface each ACL rule should be associated with based on the addresses used in the destination of the rule.

We will take a look at other object types in the next article.