Rule groups allow you to write your ruleset in a tree structure,
instead of as a linear list, so that if an incoming packet is unrelated
to a set of rules, those rules will never be processed. This reduces
IPFilter processing time on each packet and improves IPFilter system
performance.
The following is a simple rule group example:
block out quick on lan1 all head 10
pass out quick proto tcp from any to 20.20.20.64/26 port = 80 flags S keep state group 10
block out on lan2 all |
In this example, if the packet is not destined for lan1, the head of rule group 10 does not match; IPFilter does not process any
of the rules in group 10. Rules processing continues at the root level
(group 0). If the packet does match lan1, the quick keyword stops further processing at the group 0 level. IPFilter then processes all rules in group 10 against the packet.
Rule groups can be used to break up a complex firewall ruleset.
For example, there are three interfaces in the firewall with interfaces lan0, lan1, and lan2.
lan0 is connected to external network 20.20.20.0/26.
lan1 is connected to DMZ network 20.20.20.64/26.
lan2 is connected to protected network 20.20.20.128/25.
A complete ruleset for this situation would be complex and
significantly slow user connections to the network. To prevent this,
a ruleset is created with rule groups:
 |
block in quick on lan0 all head 1
block in quick on lan0 from 192.168.0.0/16 to any group 1
block in quick on lan0 from 172.16.0.0/12 to any group 1
block in quick on lan0 from 10.0.0.0/8 to any group 1
block in quick on lan0 from 127.0.0.0/8 to any group 1
block in log quick on lan0 from 20.20.20.0/24 to any group 1
block in log quick on lan0 from any to 20.20.20.0/32 group 1
block in log quick on lan0 from any to 20.20.20.63/32 group 1
block in log quick on lan0 from any to 20.20.20.64/32 group 1
block in log quick on lan0 from any to 20.20.20.127/32 group 1
block in log quick on lan0 from any to 20.20.20.128/32 group 1
block in log quick on lan0 from any to 20.20.20.255/32 group 1
pass in on lan0 all group 1
pass out on lan0 all
block out quick on lan1 all head 10
pass out quick on lan1 proto tcp from any to 20.20.20.64/26 port = 80 flags S keep state group 10
pass out quick on lan1 proto tcp from any to 20.20.20.64/26 port = 21 flags S keep state group 10
pass out quick on lan1 proto tcp from any to 20.20.20.64/26 port = 20 flags S keep state group 10
pass out quick on lan1 proto tcp from any to 20.20.20.65/32 port = 53 flags S keep state group 10
pass out quick on lan1 proto udp from any to 20.20.20.65/32 port = 53 keep state group 10
pass out quick on lan1 proto tcp from any to 20.20.20.66/32 port = 53 flags S keep state group 10
pass out quick on lan1 proto udp from any to 20.20.20.66/32 port = 53 keep state group 10 |
|
For a host on the lan2 network, IPFilter bypasses all the rules in group 10 when a packet is not destined for hosts on that
network.
Multi-level
grouping is also supported, allowing IPFilter rules to be arranged
in hierarchical, nested groups. By using the head and group keywords in a rule, multi-level grouping allows the user
to fine tune a range to improve performance. The following is an
example of a multi-level rule grouping:
pass in proto tcp from 1.0.0.0-9.0.0.0 to any port = 23 keep state head 1
pass in proto tcp from 2.0.0.0-8.0.0.0 to any port = 23 keep state head 2 group 1
pass in proto tcp from 3.0.0.0-7.0.0.0 to any port = 23 keep state head 3 group 2
pass in proto tcp from 4.0.0.0-6.0.0.0 to any port = 23 keep state head 4 group 3
pass in proto tcp from 5.0.0.0-5.5.0.0 to any port = 23 keep state group 4 |
You can group your rules by protocol, machine, netblock, or
other logical criteria that help system performance. There is not
a hard limit to the number of group levels you can maintain. For
more information, see Appendix C “Performance
Guidelines”.
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