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Changes in pf: Packet Filtering
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Is this packet arriving or departing (in, out)?

The next required keyword that appears after either the block (followed by optional reset-icmp or reset-rst keywords) or the pass keyword is the direction keyword. There are two direction keywords you can use: in or out. They are known to cause some confusion, especially when the firewall is equipped with more than one network interface, and when NAT rules are used along with filtering rules.



The key to understanding when a packet matches either the in or the out rule is remembering that these directions are relative to the firewall itself. Iif a packet is sent from an external host to the firewall, it matches the in rule on the firewall's external interface; when it is sent from the firewall itself, it matches the out on the external interface. Similarly, packets sent from internal hosts to the firewall and destined to external host will match in rules on the interface connecting your private network segment to the firewall and out rules on the firewall's external interface.

Do you want to log packets (log, log-all)?

You can tell pf(4) to log packets matching certain rules to the pflog0(4) interface. From there, they are picked up by pflogd(8) and stored in rotated log files located in /var/log.

To start packet logging, use the log or the log-all keywords. The difference between them lies in the way they work with rules that contain either keep state or modulate state rules (more on these later). log logs only the state-making packets, while log-all logs all packets. If you use stateful filtering and want to capture all packets that match the log rule, use log-all, otherwise, use log.

If you want to log all traffic on all interfaces, add either the log or log-all keywords to the rules that cover most traffic. For example, to log all traffic entering and leaving the firewall on the external interface, use these rules:

block  in log on $ext_if
block out log on $ext_if

Similar rules should be added to sections describing packet filtering policy for other interfaces on the firewall. If you would rather log only incoming HTTP traffic, add the log keyword to the rule that lets HTTP packets through.

Although I wrote earlier that the firewall need not be the latest, fastest machine you can get, the more traffic you log and the heavier the traffic that passes through the firewall's interface, the faster the hardware you use the better. This is especially true for the disks that must store the data. As you will learn later in this article, when I discuss the dup-to keyword, you can duplicate packets and send them to a different interface, where a dedicated packet logging machine can sit, listen, store, and analyze traffic. If you use dup-to for logging, then log, and log-all are redundant.

How can I influence the process of matching packets (quick)?

Unlike NAT (nat, binat, rdr) rules, which are processed in the "first matching rule wins" fashion, packet filtering is done in the "last matching rule wins" way. While it is possible to carefully structure your ruleset in a way that avoids letting unwanted packets through, it is more convenient and simpler to put rules that you want to process faster (like very specific blocking rules) at the top of the packet filtering section of the ruleset and add the quick keyword to such rules. Whenever this keyword is used, pf(4) will execute the matching rule and will not try to match the packet against the rest of the ruleset. This saves some processing time, which quickly adds up on a busy link, so I use this keyword frequently.

The quick keyword is added after the log or log-all keywords, or, in the absence of these keywords, after the in or out direction keywords:

pass in log-all quick on $ext_if proto tcp from any to $ext_ad port 80

or

pass in quick on $ext_if proto tcp from any to $ext_ad port 80

Typical applications of quick rules include quickly blocking addresses of problematic sites and blocking packets with spoofed addresses.

Which network interface receives packets (on)?

While the packet filtering rules grammar allows us to write general rules that apply to all interfaces, we can seldom write a good ruleset without adding rules for specific interfaces. The name of the interface is given after the on keyword that appears after the quick keyword. The following examples show a few possible variations of keywords that appear before on:

block in on $ext_if
block in log-all on $ext_if
block in log-all quick on $ext_if

Hint: If you forgot the name of the interface, check the output of dmesg | less. If OpenBSD is not recognizing your network interface, read this article for kernel modification tips. Note that if you are using a device connected to the serial interface (like a modem), such device may not be listed in dmesg output, but should still be recognized by the system. When you are not sure what name your network card falls under in OpenBSD, check the list displayed by apropos driver. Still no luck? Look at this list.

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