Packet Filters

Packet filters enhance network security by specifying whether a BIG-IP^® system interface should accept or reject certain packets based on criteria that you specify. Packet filters enforce an access policy on incoming traffic. They apply to incoming traffic only.

You implement packet filtering by creating packet filter rules, using the BIG-IP Configuration utility. The primary purpose of a packet filter rule is to define the criteria that you want the BIG-IP system to use when filtering packets. Examples of criteria that you can specify in a packet filter rule are:

• The source IP address of a packet
• The destination IP address of a packet
• The destination port of a packet

You specify the criteria for applying packet filter rules within an expression. When creating a packet filter rule, you can instruct the BIG-IP system to build an expression for you, in which case you need only choose the criteria from predefined lists, or you can write your own expression text, using the syntax of the tcpdump utility. For more information on the tcpdump utility, see the online man page for the tcpdump command.

You can also configure global packet filtering that applies to all packet filter rules that you create.

Global settings for packet filtering are divided into two categories: Properties and Exemptions. The BIG-IP^® system applies global settings to all packets coming into the BIG-IP system.

You can configure three specific global properties for packet filtering.

There are a number of exemptions you can set for packet filtering. When filtering packets, the BIG-IP^® system always applies these exemptions, effectively overriding certain criteria you might have previously set within an individual packet filter rule.

You use the Order setting to specify the order in which you want the BIG-IP^® system to apply existing packet filter rules. This setting is required. Possible values for this setting are:

First

Select this value if you want this packet filter rule to be the first rule that the BIG-IP system applies.

Last

Select this value if you want this packet filter rule to be the last rule that the BIG-IP system applies.

After

Select this value, and then select a packet filter rule from the list, if you want the system to apply this packet filter after the packet filter that you select from the list. Note that this setting is most useful when you have more than three packet filter rules configured.

When a packet matches the criteria that you have specified in a packet filter rule, the BIG-IP^® system can take a specific action. You define this action using the Action setting. You can choose one of these actions:

Accept

Select Accept if you want the system to accept the packet, and stop processing additional packet filter rules, if any exist. This is the default setting.

Discard

Select Discard if you want the system to drop the packet, and stop processing additional packet filter rules, if any exist.

Reject

Select Reject if you want the system to drop the packet, and also send a rejection packet to the sender, indicating that the packet was refused. Note that the behavior of the system when you select the Reject action depends on how you configured the general packet filter Options property, Send ICMP Error on Packet Reject.

Continue

Select Continue if you simply want the system to acknowledge the packet for logging or statistical purposes. Setting the Action value to Continue does not affect the way that the BIG-IP system handles the packet; the system continues to evaluate traffic matching a rule, starting with the next packet filter rule listed.

Using the Rate Class setting, you can assign a rate class to traffic that matches the criteria defined in a packet filter rule. Note that this setting applies only when you have the rate shaping feature enabled.

The default value for this setting is None. If you previously created rate classes using the rate shaping feature, you can choose one of those rate classes from the Rate Class list.

You use the Apply to VLAN setting to display a list of VLANs and then select a VLAN or VLAN group name. Selecting a VLAN from the list means that the packet filter rule filters ingress traffic from that VLAN only. For example, if you select the value *All VLANS, the BIG-IP^® system applies the packet filter rule to all traffic coming into the BIG-IP system.

Similarly, if you select the VLAN internal, the BIG-IP system applies the packet filter rule to traffic from VLAN internal only. The default value is *All VLANS.

If you select the name of a VLAN group instead of an individual VLAN, the packet filter rule applies to all VLANs in that VLAN group.

If you want to generate a log message each time a packet matches a rule, you can enable logging for the packet filter rule. With this configuration, you can then display the Logging screen in the BIG-IP Configuration utility and view events related to packet filtering.

To match incoming packets, the BIG-IP^® system must use a filter expression. A filter expression specifies the criteria that you want the BIG-IP system to use when filtering packets. For example, the BIG-IP system can filter packets based on the source or destination IP address in the header of a packet.

Using the BIG-IP Configuration utility, you can create a filter expression in either of two ways:

• You can write your own expression, using a Filter Expression box.
• You can specify a set of criteria (such as source or destination IP addresses) that you want the BIG-IP system to use when filtering packets. When you use this method, the BIG-IP system builds a filter expression for you.

You can have as many rules as you want, limited only by the available memory. Of course, the more statements you have, the more challenging it is to understand and maintain your packet filters.