Train Bridge at Swarthmore College crossing Crum Creek

Advanced PF Usage

Since my last PF post I've made some changes to my pf.conf file. I've broken the config into its own separate section and displayed them below with some explanation. All together the config is organized and easy to maintain, which is always good.

Macros

Macros can be thought of as variables. They can store either a single scalar value or an arrays of values separated by a comma and surrounded by curly braces. All scalars must be in quotes.

# TCP services to allow, either names from /etc/services or port numbers
tcp_services = "{http, https, ssh, 25565, 8123, 8080, 8000, 12345}"
# UDP Services to allow, either names from /etc/services or port numbers
udp_services = "{domain, 60000:60010}"

# Macro of the primary interface
ext_if="bce1"
jail_if="lo1"

Pretty much the same as before, except now I have a section for mosh. I use PF's colon operator to give me a range of ports, 60000 through 60010.

Tables

Tables are similar to macro groups except they can be edited without restarting PF. Great for blacklists or whitelists. Tables can only store Ip addresses though, limiting there use.

table <whitelist> { 11.22.33.44, 123.45.67.89 }

I've created a whitelist that will be quickly passed though, preventing other rules from being applied. Good for the accidental screw up. I don't have a blacklist table because I use denyhosts.

pfctl -t whitelist -T add 192.168.0.0/16

Cidr notation is also accepted in tables.

Traffic Normalization

Scrub guarantees that all traffic passed into the filter section is not fragmented. Packets that are fragmented are put into a buffer to wait until the rest of the packet arrives.

# Scrubs packets for possible issues. All Trafic is reassembled. Might cause
# issues for games and NFS
scrub in on $ext_if all fragment reassemble

Be cautious with scrub rules. They can cause issues with games and NFS traffic. To log a rules effect add "log" after the in or out keyword as such.

scrub in log on $ext_if all fragment reassemble

Filtering

Not much crazy has changed here. I've added two new rules to allow icmp traffic and to quick pass from the whitelist table. I've also added a rule which drops all traffic with a destination address of 255.255.255.255. It's commented it out though, most of the broadcast traffic I was getting was from UPnP. Not allowing UPnP(udp 1900, tcp 5000) is easier.

# Block all traffic by default
block all

# Ignore local,internal traffic
set skip on lo0

# Quick antispoof check for primary interface
antispoof log quick for $ext_if

# Quick drop on broadcast traffic
#block in quick on $ext_if from any to 255.255.255.255

# Allow incoming and outgoing ICMP
pass inet proto icmp from any to any

# Allow in traffic from services macro
pass in quick from <whitelist> flags S/SA synproxy state
pass in on $ext_if proto tcp to any port $tcp_services
pass in on $ext_if proto udp to any port $udp_services

# Allow ALL outbound traffic
pass out on $ext_if proto tcp from any to any keep state
pass out on $ext_if proto udp from any to any keep state

Other Useful Commands.

Lots of commands are available to do just about everything under the sun with pf. I've listed below some of the more useful commands I've found.

# Realtime logging from pglog0 device
tcpdump -n -e -ttt -i pflog0
# realtime logging from pf log file
tcpdump -n -e -ttt -r /var/log/pflog

# parse ruleset for errors, does not load
pfctl -vnf /etc/pf.conf
# maintain state and reload config
pfctl -f /etc/pf.conf
# same thing as above, just easier to remember
service pf reload

# List loaded rules
pfctl -sr
# View state table
pfctl -ss
# List everything
pfctl -sa