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PF: Packet Tagging (Policy Filtering)

Introduction
Assigning Tags to Packets
Checking for Applied Tags
Policy Filtering
Tagging Ethernet Frames

Introduction

Packet tagging is a way of marking packets with an internal identifier that can later be used in filter and translation rule criteria. With tagging, it's possible to do such things as create "trusts" between interfaces and determine if packets have been processed by translation rules. It's also possible to move away from rule-based filtering and to start doing policy-based filtering.

Assigning Tags to Packets

To add a tag to a packet, use the tag keyword:

pass in on $int_if all tag INTERNAL_NET keep state

The tag INTERNAL_NET will be added to any packet which matches the above rule.

A tag can also be assigned using a macro. For instance:

name = "INTERNAL_NET" pass in on $int_if all tag $name

There are a set of predefined macros which can also be used.

$if - The interface
$srcaddr - Source IP address
$dstaddr - Destination IP address
$srcport - The source port specification
$dstport - The destination port specification
$proto - The protocol
$nr - The rule number

These macros are expanded at ruleset load time and NOT at runtime.

Tagging follows these rules:

Tags are "sticky". Once a tag is applied to a packet by a matching rule it is never removed. It can, however, be replaced with a different tag.
Because of a tag's "stickiness", a packet can have a tag even if the last matching rule doesn't use the tag keyword.
A packet is only ever assigned a maximum of one tag at a time.
Tags are internal identifiers. Tags are not sent out over the wire.
Tag names can be up to 63 characters long. In OpenBSD 4.0 and earlier, tag names are limited to 15 characters.

Take the following ruleset as an example.

(1) pass in on $int_if tag INT_NET (2) pass in quick on $int_if proto tcp to port 80 tag INT_NET_HTTP (3) pass in quick on $int_if from 192.168.1.5

Packets coming in on $int_if will be assigned a tag of INT_NET by rule #1.
TCP packets coming in on $int_if and destined for port 80 will first be assigned a tag of INT_NET by rule #1. That tag will then be replaced with the INT_NET_HTTP tag by rule #2.
Packets coming in on $int_if from 192.168.1.5 will be tagged one of two ways. If the packet is destined for TCP port 80 it will match rule #2 and be tagged with INT_NET_HTTP. Otherwise, the packet will match rule #3 but will be tagged with INT_NET. Because the packet matches rule #1, the INT_NET tag is applied and is not removed unless a subsequently matching rule specifies a tag (this is the "stickiness" of a tag).

Checking for Applied Tags

To check for previously applied tags, use the tagged keyword:

pass out on $ext_if tagged INT_NET

Outgoing packets on $ext_if must be tagged with the INT_NET tag in order to match the above rule. Inverse matching can also be done by using the ! operator:

pass out on $ext_if ! tagged WIFI_NET

Policy Filtering

Policy filtering takes a different approach to writing a filter ruleset. A policy is defined which sets the rules for what types of traffic is passed and what types are blocked. Packets are then classified into the policy based on the traditional criteria of source/destination IP address/port, protocol, etc. For example, examine the following firewall policy:

Traffic from the internal LAN to the Internet is permitted (LAN_INET) and must be translated (LAN_INET_NAT)
Traffic from the internal LAN to the DMZ is permitted (LAN_DMZ)
Traffic from the Internet to servers in the DMZ is permitted (INET_DMZ)
Traffic from the Internet that's being redirected to spamd(8) is permitted (SPAMD)
All other traffic is blocked

Note how the policy covers all traffic that will be passing through the firewall. The item in parenthesis indicates the tag that will be used for that policy item.

Rules now need to be written to classify packets into the policy.

block all pass out on $ext_if tag LAN_INET_NAT tagged LAN_INET nat-to ($ext_if) pass in on $int_if from $int_net tag LAN_INET pass in on $int_if from $int_net to $dmz_net tag LAN_DMZ pass in on $ext_if proto tcp to $www_server port 80 tag INET_DMZ pass in on $ext_if proto tcp from <spamd> to port smtp \ tag SPAMD rdr-to 127.0.0.1 port 8025

Now the rules that define the policy are set.

pass in quick on $ext_if tagged SPAMD pass out quick on $ext_if tagged LAN_INET_NAT pass out quick on $dmz_if tagged LAN_DMZ pass out quick on $dmz_if tagged INET_DMZ

Now that the whole ruleset is setup, changes are a matter of modifying the classification rules. For example, if a POP3/SMTP server is added to the DMZ, it will be necessary to add classification rules for POP3 and SMTP traffic, like so:

mail_server = "192.168.0.10" ... pass in on $ext_if proto tcp to $mail_server port { smtp, pop3 } \ tag INET_DMZ

Email traffic will now be passed as part of the INET_DMZ policy entry.

The complete ruleset:

# macros int_if = "dc0" dmz_if = "dc1" ext_if = "ep0" int_net = "10.0.0.0/24" dmz_net = "192.168.0.0/24" www_server = "192.168.0.5" mail_server = "192.168.0.10" table <spamd> persist file "/etc/spammers" # classification -- classify packets based on the defined firewall # policy. block all pass out on $ext_if tag LAN_INET_NAT tagged LAN_INET nat-to ($ext_if)
pass in on $int_if from $int_net tag LAN_INET
pass in on $int_if from $int_net to $dmz_net tag LAN_DMZ
pass in on $ext_if proto tcp to $www_server port 80 tag INET_DMZ pass in on $ext_if proto tcp from <spamd> to port smtp \
tag SPAMD rdr-to 127.0.0.1 port 8025
# policy enforcement -- pass/block based on the defined firewall policy. pass in quick on $ext_if tagged SPAMD pass out quick on $ext_if tagged LAN_INET_NAT pass out quick on $dmz_if tagged LAN_DMZ pass out quick on $dmz_if tagged INET_DMZ

Tagging Ethernet Frames

Tagging can be performed at the Ethernet level if the machine doing the tagging/filtering is also acting as a bridge(4). By creating bridge(4) filter rules that use the tag keyword, PF can be made to filter based on the source or destination MAC address. Bridge(4) rules are created using the ifconfig(8) command. Example:

# ifconfig bridge0 rule pass in on fxp0 src 0:de:ad:be:ef:0 \ tag USER1

And then in pf.conf:

pass in on fxp0 tagged USER1