<s>
PF	B-Protocol
(	O
Packet	B-Protocol
Filter	I-Protocol
,	O
also	O
written	O
pf	B-Protocol
)	O
is	O
a	O
BSD	O
licensed	O
stateful	O
packet	B-Protocol
filter	I-Protocol
,	O
a	O
central	O
piece	O
of	O
software	O
for	O
firewalling	B-Application
.	O
</s>
<s>
It	O
is	O
comparable	O
to	O
netfilter	B-Application
(	O
iptables	B-Application
)	O
,	O
ipfw	B-Protocol
,	O
and	O
ipfilter	B-Protocol
.	O
</s>
<s>
PF	B-Protocol
was	O
developed	O
for	O
OpenBSD	B-Operating_System
,	O
but	O
has	O
been	O
ported	O
to	O
many	O
other	O
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
PF	B-Protocol
was	O
originally	O
designed	O
as	O
replacement	O
for	O
Darren	O
Reed	O
's	O
IPFilter	B-Protocol
,	O
from	O
which	O
it	O
derives	O
much	O
of	O
its	O
rule	O
syntax	O
.	O
</s>
<s>
IPFilter	B-Protocol
was	O
removed	O
from	O
OpenBSD	B-Operating_System
's	O
CVS	B-Application
tree	O
on	O
30	O
May	O
2001	O
due	O
to	O
OpenBSD	B-Operating_System
developers	O
 '	O
concerns	O
with	O
its	O
license	O
.	O
</s>
<s>
The	O
initial	O
version	O
of	O
PF	B-Protocol
was	O
written	O
by	O
Daniel	O
Hartmeier	O
.	O
</s>
<s>
It	O
appeared	O
in	O
OpenBSD	B-Operating_System
3.0	O
,	O
which	O
was	O
released	O
on	O
1	O
December	O
2001	O
.	O
</s>
<s>
Henning	O
Brauer	O
is	O
currently	O
the	O
main	O
developer	O
of	O
PF	B-Protocol
.	O
</s>
<s>
The	O
filtering	O
syntax	O
is	O
similar	O
to	O
IPFilter	B-Protocol
,	O
with	O
some	O
modifications	O
to	O
make	O
it	O
clearer	O
.	O
</s>
<s>
Network	B-Protocol
address	I-Protocol
translation	I-Protocol
(	O
NAT	O
)	O
and	O
quality	O
of	O
service	O
(	O
QoS	O
)	O
have	O
been	O
integrated	O
into	O
PF	B-Protocol
.	O
</s>
<s>
Features	O
such	O
as	O
pfsync	B-Protocol
and	O
CARP	B-Application
for	O
failover	B-General_Concept
and	O
redundancy	O
,	O
authpf	O
for	O
session	O
authentication	O
,	O
and	O
ftp-proxy	O
to	O
ease	O
firewalling	B-Application
the	O
difficult	O
FTP	B-Protocol
protocol	O
,	O
have	O
also	O
extended	O
PF	B-Protocol
.	O
</s>
<s>
Also	O
PF	B-Protocol
supports	O
SMP	O
(	O
Symmetric	B-Operating_System
multiprocessing	I-Operating_System
)	O
&	O
STO	O
(	O
Stateful	O
Tracking	O
Options	O
)	O
.	O
</s>
<s>
One	O
of	O
the	O
many	O
innovative	O
features	O
is	O
PF	B-Protocol
's	O
logging	O
.	O
</s>
<s>
PF	B-Protocol
's	O
logging	O
is	O
configurable	O
per	O
rule	O
within	O
the	O
pf.conf	O
and	O
logs	O
are	O
provided	O
from	O
PF	B-Protocol
by	O
a	O
pseudo-network	O
interface	O
called	O
pflog	O
,	O
which	O
is	O
the	O
only	O
way	O
to	O
lift	O
data	O
from	O
kernel-level	O
mode	O
for	O
user-level	O
programs	O
.	O
</s>
<s>
Logs	O
may	O
be	O
monitored	O
using	O
standard	O
utilities	O
such	O
as	O
tcpdump	B-Protocol
,	O
which	O
in	O
OpenBSD	B-Operating_System
has	O
been	O
extended	O
especially	O
for	O
the	O
purpose	O
,	O
or	O
saved	O
to	O
disk	O
in	O
the	O
tcpdump/pcap	O
binary	O
format	O
using	O
the	O
pflogd	O
daemon	O
.	O
</s>
<s>
Apart	O
from	O
running	O
on	O
its	O
home	O
platform	O
OpenBSD	B-Operating_System
,	O
PF	B-Protocol
has	O
been	O
ported	O
to	O
many	O
other	O
operating	B-General_Concept
systems	I-General_Concept
,	O
however	O
there	O
are	O
major	O
differences	O
in	O
capabilities	O
.	O
</s>
<s>
OpenBSD	B-Operating_System
always	O
has	O
the	O
latest	O
version	O
with	O
the	O
most	O
features	O
.	O
</s>
<s>
PF	B-Protocol
is	O
currently	O
used	O
in	O
:	O
</s>
