Design and Performance of the

OpenBSD Stateful Packet Filter (pf)

Daniel Hartmeier

[e-mail]

Systor AG

Usenix 2002 – p.1/22

Introduction

part of a firewall, working on IP packet level (vs.

application level proxies or ethernet level bridges)
packet filter intercepting each IP packet that passes

through the kernel (in and out on each interface),

passing or blocking it
stateless inspection based on the fields of each

packet
stateful filtering keeping track of connections,

additional information makes filtering more

powerful (sequence number checks) and easier

(replies, random client ports)

Usenix 2002 – p.2/22

Motivation

OpenBSD included IPFilter in the default install
what appeared to be a BSD license turned out to be

non-free
unlike other license problems discovered by the

ongoing license audit, this case couldn’t be resolved,

IPFilter removed from the tree
existing alternatives were considered (ipfw), larger

code base, kernel dependencies
rewrite offers additional options, integrates better

with existing kernel features

Usenix 2002 – p.3/22

Overview

Introduction
Motivation
Filter rules, skip steps
State table, trees, lookups, translations (NAT,

redirections)
Benchmarks
Conclusions

Usenix 2002 – p.4/22

Filter rules

linear linked list, evaluated top to bottom for each

packet (unlike netfilter’s chains tree)
rules contain parameters that match/mismatch a

packet
rules pass or block a packet
last matching rule wins (except for ’quick’, which

aborts rule evaluation)
rules can create state, further state matching packets

are passed without rule set evaluation

Usenix 2002 – p.5/22

Skip steps

transparent optimization of rule set evaluation,

improve performance without affecting semantics
example: ten consecutive rules apply only to packets

from source address X, packet has source address Y,

first rule evaluated, next nine skipped
skipping is done on most parameters, in pre-defined

order
parameters like direction (in, out), interface or

address family (IPv4/IPv6) partition the rule set a

lot, performance increase is significant
worst case: consecutive rules have no equal

parameters, every rule must be evaluated, no

additional cost (linked list traversal)

Usenix 2002 – p.6/22

State table

TCP (sequence number checks on each packet),

ICMP error messages match referred to packet

(simplifies rules without breaking PMTU etc.)
UDP, ICMP queries/replies, other protocols,

pseudo-connections with timeouts
adjustable timeouts, pseudo-connections for

non-TCP protocols
binary search tree (AVL, now Red-Black), O(log n)

even in worst-case
key is two address/port pairs

Usenix 2002 – p.7/22

Translations (NAT, redirections)

translating source addresses: NAT/PAT to one

address using proxy ports
translating destination: redirections (based on

addresses/ports)
mapping stored in state table
application level proxies (ftp) in userland

Usenix 2002 – p.8/22

State table keys

one state entry per connection, stored in two trees
example: 10.1.1.1:20000 -> 62.65.145.30:50001 ->

129.128.5.191:80
outgoing packets: 10.1.1.1:20000 ->

129.128.5.191:80, replace source address/port with

gateway
incoming packets: 129.128.5.191:80 ->

62.65.145.30:50001, replace destination

address/port with local host
three address/port pairs of one connection: lan, gwy,

ext
without translation, two pairs are equal

Usenix 2002 – p.9/22

State table keys

two trees: tree-lan-ext (outgoing) and tree-ext-gwy

(incoming), contain the same state pointers
no addition translation map (and lookup) needed

Usenix 2002 – p.10/22

Normalization

IP normalization (scrubbing) to remove

interpretation ambiguities, like overlapping

fragments (confusing IDSs)
reassembly (caching) of fragments before filtering,

only complete packets are filtered
sequence number modulation

Usenix 2002 – p.11/22

Logging

through bpf, virtual network interface pflog0
link layer header used for pf related information

(rule, action)
binary log files, readable with tcpdump and other

tools

Usenix 2002 – p.12/22

Benchmarks: Setup

two (old) i386 machines with two network interface

cards each, connected with two crosswire Cat5

cables, 10 mbit/s unidirectional
tester: generate TCP packets on ethernet level

through first NIC, capture incoming ethernet frames

on second NIC
firewall: OpenBSD and GNU/Linux (equal

hardware), IP forwarding enabled, packet filter

enabled, no other services, no other network traffic

(static arp table)

Usenix 2002 – p.13/22

Benchmarks: Packet generation

TCP packets of variable size, random

source/destination addresses and ports
embedded timestamp to calculate latency,

incremental serial number to detect packet loss
send packets of specified size at specified rate for

several seconds, print throughput, latency and loss
verify that setup can handle maximum link rate

correctly

Usenix 2002 – p.14/22

Local, reaching link limit

0

100

200

300

400

500

600

700

800

900

0

100

200

300

400

500

600

700

800

900

receiving rate (packets/s)

sending rate (packets/s)

1518 bytes/packet

Usenix 2002 – p.15/22

Local, reaching link limit

0

100

200

300

400

500

600

700

800

900

0

100

200

300

400

500

600

700

800

900

receiving rate (packets/s)

sending rate (packets/s)

812

812

1518 bytes/packet

Usenix 2002 – p.15/22

Local, varying packet sizes

0

200000

400000

600000

800000

1e+06

1.2e+06

1.4e+06

0

2000

4000

6000

8000

10000

12000

14000

16000

throughput (bytes/s)

sending rate (packets/s)

812

1518 bytes

Usenix 2002 – p.16/22

Local, varying packet sizes

0

200000

400000

600000

800000

1e+06

1.2e+06

1.4e+06

0

2000

4000

6000

8000

10000

12000

14000

16000

throughput (bytes/s)

sending rate (packets/s)

961

1280 bytes

Usenix 2002 – p.16/22

Local, varying packet sizes

0

200000

400000

600000

800000

1e+06

1.2e+06

1.4e+06

0

2000

4000

6000

8000

10000

12000

14000

16000

throughput (bytes/s)

sending rate (packets/s)

1197

1024 bytes

Usenix 2002 – p.16/22

Local, varying packet sizes

0

200000

400000

600000

800000

1e+06

1.2e+06

1.4e+06

0

2000

4000

6000

8000

10000

12000

14000

16000

throughput (bytes/s)

sending rate (packets/s)

1586

768 bytes

Usenix 2002 – p.16/22

Local, varying packet sizes

0

200000

400000

600000

800000

1e+06

1.2e+06

1.4e+06

0

2000

4000

6000

8000

10000

12000

14000

16000

throughput (bytes/s)

sending rate (packets/s)

2349

512 bytes

Usenix 2002 – p.16/22

Local, varying packet sizes

0

200000

400000

600000

800000

1e+06

1.2e+06

1.4e+06

0

2000

4000

6000

8000

10000

12000

14000

16000

throughput (bytes/s)

sending rate (packets/s)

4528

256 bytes

Usenix 2002 – p.16/22

Local, varying packet sizes

0

200000

400000

600000

800000

1e+06

1.2e+06

1.4e+06

0

2000

4000

6000

8000

10000

12000

14000

16000

throughput (bytes/s)

sending rate (packets/s)

8445

128 bytes

Usenix 2002 – p.16/22

Local, varying packet sizes

0

200000

400000

600000

800000

1e+06

1.2e+06

1.4e+06

0

2000

4000

6000

8000

10000

12000

14000

16000

throughput (bytes/s)

sending rate (packets/s)

14880

64 bytes

Usenix 2002 – p.16/22

Local, varying packet sizes

0

200000

400000

600000

800000

1e+06

1.2e+06

1.4e+06

0

2000

4000

6000

8000

10000

12000

14000

16000

throughput (bytes/s)

sending rate (packets/s)

Local

OpenBSD

GNU/Linux

Usenix 2002 – p.16/22

Stateless, 100 rules, throughput

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

0

1000

2000

3000

4000

5000

throughput (packets/s)

sending rate (packets/s)

iptables

Usenix 2002 – p.17/22

Stateless, 100 rules, throughput

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

0

1000

2000

3000

4000

5000

throughput (packets/s)

sending rate (packets/s)

iptables

ipf

Usenix 2002 – p.17/22

Stateless, 100 rules, throughput

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

0

1000

2000

3000

4000

5000

throughput (packets/s)

sending rate (packets/s)

iptables

ipf

pf

Usenix 2002 – p.17/22

Maximum throughput vs. rules

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

0

200

400

600

800

1000

maximum throughput (packets/s)

number of rules

iptables

ipf

pf

Usenix 2002 – p.18/22

Maximum throughput vs. states

3000

3500

4000

4500

5000

5500

6000

6500

7000

7500

0

5000

10000

15000

20000

maximum throughput (packets/s)

number of states

ipf

pf

Usenix 2002 – p.19/22

Conclusions

rule set evaluation is expensive. State lookups are

cheap
filtering statefully not only improves filter decision

quality, it actually increases performance
memory cost: 64000 states with 64MB RAM

(without tuning), increasing linearly
binary search tree for states scales with O(log n)

Usenix 2002 – p.20/22

Production results

Duron 700MHz, 128MB RAM, 3x DEC 21143

NICs
25000-40000 concurrent states
average of 5000 packets/s
fully stateful filtering (no stateless passing)
CPU load doesn’t exceed 10 percent
(same box and filter policy with IPFilter was 90

percent load average)

Usenix 2002 – p.21/22

Questions?

The OpenBSD Project: http://www.openbsd.org/
Paper and slides: http://www.benzedrine.cx/pf.html
[e-mail]

Usenix 2002 – p.22/22