I secure my BSD servers with PF. In FreeBSD 9, PF has been updated to the same version as in OpenBSD 4.5.
I use lists in my PF configuration, as shown in this /etc/pf.conf snippet:
mgmt_hosts="{ 10.0.1.0/24, 172.19.8.0/24}"
...
pass in on $ext_if from $mgmt_hosts
...
When I have new management hosts, I add their IP address or subnets to the mgmt_hosts list. When PF reads this configuration file, every place that a rule references the list, an additional rule is created for each member of the list. Here, every subnet in the mgmt_hosts list gets a “pass in” rule. When I list these rules on a running FreeBSD 8 host, they’ll look something like this:
# pfctl -sr
...
pass in on em0 inet from 10.0.1.0/24 to any flags S/SA keep state
pass in on em0 inet from 172.19.8.0/24 to any flags S/SA keep state
...
Very useful for maintaining a readable rule file.
I updated a host to FreeBSD 9, and saw the following in my rules.
# pfctl -sr
...
pass in on vr0 inet from <__automatic_4c6aed29_0> to any flags S/SA keep state
...
Wait a minute. What is this __automatic crap? And where are my management hosts?
This version of PF automatically converts lists to tables. If you have a big rule set, using a table makes the rules shown by pfctl more readable. (I seem to recall that tables perform better than lists, but I can’t find a reference for that, so take that with a grain of sand.)
You can name tables, but tables created by PF have a name that starts with __automatic. To view all the tables, run:
# pfctl -sT
__automatic_4c6aed29_0
#
To see the hosts in this table, use pfctl’s -t and -T flags.
# pfctl -t __automatic_4c6aed29_0 -T show
10.0.1.0/24
172.19.8.0/24
#
Wow. This works, but it doesn’t look like fun. If I have to routinely type __automatic_4c6aed29_0, I will increase my subvocalized swearing by at least ten percent. But it does not interrupt service. Old rule sets continue to work. (I don’t mind needing to update my rules with a new OS version, but I need to know about it beforehand rather than just blindly updating.)
To make my life easier I can convert my PF rules to use tables instead of lists. Here’s the same pf.conf using a table instead of a list.
table <mgmt_hosts> const {10.0.1.0/24, 172.19.8.0/24}
...
pass in on $ext_if from <mgmt_hosts>
...
Unlike a list, a table is explicitly declared as a table. The name always appears in angle brackets.
I use the const keyword to tell PF that the contents of this table cannot be changed at the command line. PF tables can be adjusted at the command line without reloading the rules, which is a handy feature for, say, automatically blocking port scanners, feeding IDS data to your firewall, or DOSing yourself.
When I look at my parsed rules now, I’ll see:
# pfctl -sr
...
pass in on vr0 from <mgmt_hosts> to any flags S/SA keep state
...
I can now read my rules more easily.
(Bootnote: OpenBSD just came out with 5.0, so FreeBSD 9 is five versions behind. OpenBSD PF develops quickly. But thirty-month-old PF is better than a lot of other firewall software.)