PF, quick reference guide

pf.conf is divided into the sections:

• Macros - Variables are defined in this section. This simplifies changing hardware, or makes it easier to list a lot of arguments as a variable. IP's are not set here, but instead in the next section.

• Tables - Variables for IP's are defined here. This can be also set by the overload argument from rules in the filtering section.

• Options - Options lines start with "set"

• Scrub - Scrub packets

• Queueing -

• Translation - Nat translation settings

• Filter rules - Here is where rules are set up. PF rules work where the last line takes effect over the lines before it.

Macros

Macros are defined with an = equal sign with arguments to be used in the rules in " quotes.

wireless_card_to_internet="ath0"
tcp="{ http ntp 443 }" # this is often used, but doesn't limit protocols to tcp

To reference these later use $, for instance $tcp. This simply allows lists to be kept neatly instead of having to retype the same lines inconveniently, or allows a simple change to be made without replacing all references to the change.

characters

These are used throughout the rules file, not only in the macros section.

• { } is used for lists, a space must be present between parenthesis and each list item.
• Greater than > and less than < are useful for port definitions. Their equal to corespondents >=, and <= can also be used.
• >< is used to define a range. To define numbers outside of a range (inverse range) use <>.

Tables

IP table variable settings are kept here, and not in the macros section. This has uses for your network gateways, and for ips generated by PF filter rules, for instance tracking brute force attempts. These rules start with table, and use < > to define the variables.

table <ips>
table <ipvariable>
table <ipfile> file "/dir/file"

Files can also be used to set table variables as shown in the example with the file argument. overload < > from the "filter rules" section can also set variable ips here, for instance for mapping brute force ips.

Options

Options start with "set". set skip lo0 is an important option to prevent PF from blocking local services.

Scrub

scrub in all
antispoof for $interface

Filter rules

Syntax

[action] [direction] log quick on $interface [af] proto $protocol from $ip port $port to $ip port $port flags $tcpflags [state] ([state options])

• action - block, pass
• direction - in, out
• log - log, log (all)
• interface - $network_interface
• af - inet, inet6
• proto - tcp, udp, sctp, icmp6; see /etc/protocols. Can be a number or named.
• port - see /etc/services.

Some arguments are optional, depending on circumstances.

Notice

Rule instructions may vary according to newer versions of PF. Newer versions of PF such as on OpenBSD contain more abilities, and some syntax changes compared to PF on FreeBSD.

Resources

References

• PF: The OpenBSD Packet Filter
• Firewalling with PF / Brannmur med PF
• Network Administration with FreeBSD 7: Network Configuration - Firewalls
• FreeBSD Documentation

Further Reading

• /usr/share/examples/pf/
• Book of PF