OpenBSD PF - Packet Tagging (Policy Filtering) [Contents]



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

Tagging follows these rules:

Take the following ruleset as an example.
pass in on $int_if tag INT_NET
pass in quick on $int_if proto tcp to port 80 tag INT_NET_HTTP
pass in quick on $int_if from 192.168.1.5

Checking for applied tags

To check for previously applied tags, use the tagged keyword:
pass out on egress tagged INT_NET
Outgoing packets on the external interface 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 egress ! 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: 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 egress inet 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 egress proto tcp to $www_server port 80 tag INET_DMZ
pass in  on egress 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 egress tagged SPAMD
pass out quick on egress 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 egress 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:

int_if      = "dc0"
dmz_if      = "dc1"
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 egress inet tag LAN_INET_NAT tagged LAN_INET nat-to (egress)
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 egress proto tcp to $www_server port 80 tag INET_DMZ
pass in on egress 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 egress tagged SPAMD
pass out quick on egress 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