<s>
Netfilter	B-Application
is	O
a	O
framework	B-Architecture
provided	O
by	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
that	O
allows	O
various	O
networking-related	O
operations	O
to	O
be	O
implemented	O
in	O
the	O
form	O
of	O
customized	O
handlers	O
.	O
</s>
<s>
Netfilter	B-Application
offers	O
various	O
functions	O
and	O
operations	O
for	O
packet	B-Application
filtering	I-Application
,	O
network	B-Protocol
address	I-Protocol
translation	I-Protocol
,	O
and	O
port	O
translation	O
,	O
which	O
provide	O
the	O
functionality	O
required	O
for	O
directing	O
packets	O
through	O
a	O
network	O
and	O
prohibiting	B-Application
packets	O
from	O
reaching	O
sensitive	O
locations	O
within	O
a	O
network	O
.	O
</s>
<s>
Netfilter	B-Application
represents	O
a	O
set	O
of	O
hooks	O
inside	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
,	O
allowing	O
specific	O
kernel	B-Application
modules	I-Application
to	O
register	O
callback	O
functions	O
with	O
the	O
kernel	B-Operating_System
's	O
networking	B-Architecture
stack	O
.	O
</s>
<s>
Those	O
functions	O
,	O
usually	O
applied	O
to	O
the	O
traffic	O
in	O
the	O
form	O
of	O
filtering	O
and	O
modification	O
rules	O
,	O
are	O
called	O
for	O
every	O
packet	O
that	O
traverses	O
the	O
respective	O
hook	O
within	O
the	O
networking	B-Architecture
stack	O
.	O
</s>
<s>
Rusty	O
Russell	O
started	O
the	O
netfilter/iptables	B-Application
project	O
in	O
1998	O
;	O
he	O
had	O
also	O
authored	O
the	O
project	O
's	O
predecessor	O
,	O
ipchains	B-Application
.	O
</s>
<s>
As	O
the	O
project	O
grew	O
,	O
he	O
founded	O
the	O
Netfilter	B-Application
Core	O
Team	O
(	O
or	O
simply	O
coreteam	O
)	O
in	O
1999	O
.	O
</s>
<s>
The	O
software	O
they	O
produced	O
(	O
called	O
netfilter	B-Application
hereafter	O
)	O
uses	O
the	O
GNU	B-License
General	I-License
Public	I-License
License	I-License
(	O
GPL	B-License
)	O
license	O
,	O
and	O
in	O
March	O
2000	O
it	O
was	O
merged	O
into	O
version	O
2.4.x	O
of	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
mainline	O
.	O
</s>
<s>
In	O
April	O
2004	O
,	O
following	O
a	O
crack-down	O
by	O
the	O
project	O
on	O
those	O
distributing	O
the	O
project	O
's	O
software	O
embedded	B-Architecture
in	O
routers	B-Protocol
without	O
complying	O
with	O
the	O
GPL	B-License
,	O
a	O
German	O
court	O
granted	O
Welte	O
an	O
historic	O
injunction	O
against	O
Sitecom	O
Germany	O
,	O
which	O
refused	O
to	O
follow	O
the	O
GPL	B-License
's	O
terms	O
(	O
see	O
GPL-related	O
disputes	O
)	O
.	O
</s>
<s>
Prior	O
to	O
iptables	B-Application
,	O
the	O
predominant	O
software	O
packages	O
for	O
creating	O
Linux	B-Operating_System
firewalls	B-Application
were	O
ipchains	B-Application
in	O
Linux	B-Operating_System
kernel	O
2.2.x	O
and	O
ipfwadm	O
in	O
Linux	B-Operating_System
kernel	O
2.0.x	O
,	O
which	O
in	O
turn	O
was	O
based	O
on	O
BSD	B-Operating_System
's	O
ipfw	B-Protocol
.	O
</s>
<s>
Both	O
ipchains	B-Application
and	O
ipfwadm	O
alter	O
the	O
networking	B-Architecture
code	O
so	O
they	O
can	O
manipulate	O
packets	O
,	O
as	O
Linux	B-Operating_System
kernel	I-Operating_System
lacked	O
a	O
general	O
packets	O
control	O
framework	B-Architecture
until	O
the	O
introduction	O
of	O
Netfilter	B-Application
.	O
</s>
<s>
Whereas	O
ipchains	B-Application
and	O
ipfwadm	O
combine	O
packet	B-Application
filtering	I-Application
and	O
NAT	B-Protocol
(	O
particularly	O
three	O
specific	O
kinds	O
of	O
NAT	B-Protocol
,	O
called	O
masquerading	O
,	O
port	O
forwarding	O
,	O
and	O
redirection	O
)	O
,	O
Netfilter	B-Application
separates	O
packet	O
operations	O
into	O
multiple	O
parts	O
,	O
described	O
below	O
.	O
</s>
<s>
Each	O
connects	O
to	O
the	O
Netfilter	B-Application
hooks	O
at	O
different	O
points	O
to	O
access	O
packets	O
.	O
</s>
<s>
The	O
connection	O
tracking	O
and	O
NAT	B-Protocol
subsystems	O
are	O
more	O
general	O
and	O
more	O
powerful	O
than	O
the	O
rudimentary	O
versions	O
within	O
ipchains	B-Application
and	O
ipfwadm	O
.	O
</s>
<s>
In	O
2017	O
IPv4	B-Protocol
and	O
IPv6	B-Protocol
flow	O
offload	O
infrastructure	O
was	O
added	O
,	O
allowing	O
a	O
speedup	O
of	O
software	O
flow	O
table	O
forwarding	O
and	O
hardware	O
offload	O
support	O
.	O
</s>
<s>
The	O
kernel	B-Application
modules	I-Application
named	O
ip_tables	O
,	O
ip6_tables	O
,	O
arp_tables	O
(	O
the	O
underscore	O
is	O
part	O
of	O
the	O
name	O
)	O
,	O
and	O
ebtables	O
comprise	O
the	O
legacy	O
packet	B-Application
filtering	I-Application
portion	O
of	O
the	O
Netfilter	B-Application
hook	O
system	O
.	O
</s>
<s>
They	O
provide	O
a	O
table-based	O
system	O
for	O
defining	O
firewall	B-Application
rules	O
that	O
can	O
filter	O
or	O
transform	O
packets	O
.	O
</s>
<s>
The	O
tables	O
can	O
be	O
administered	O
through	O
the	O
user-space	O
tools	O
iptables	B-Application
,	O
ip6tables	B-Application
,	O
arptables	O
,	O
and	O
ebtables	O
.	O
</s>
<s>
Notice	O
that	O
although	O
both	O
the	O
kernel	B-Application
modules	I-Application
and	O
userspace	O
utilities	O
have	O
similar	O
names	O
,	O
each	O
of	O
them	O
is	O
a	O
different	O
entity	O
with	O
different	O
functionality	O
.	O
</s>
<s>
As	O
far	O
as	O
Netfilter	B-Application
is	O
concerned	O
,	O
it	O
runs	O
a	O
particular	O
table	O
in	O
a	O
specific	O
order	O
with	O
respect	O
to	O
other	O
tables	O
.	O
</s>
<s>
These	O
chain	O
titles	O
help	O
describe	O
the	O
origin	O
in	O
the	O
Netfilter	B-Application
stack	O
.	O
</s>
<s>
Netfilter	B-Application
modules	B-Application
not	O
organized	O
into	O
tables	O
(	O
see	O
below	O
)	O
are	O
capable	O
of	O
checking	O
for	O
the	O
origin	O
to	O
select	O
their	O
mode	O
of	O
operation	O
.	O
</s>
<s>
When	O
loaded	O
,	O
registers	O
a	O
hook	O
that	O
will	O
be	O
called	O
before	O
any	O
other	O
Netfilter	B-Application
hook	O
.	O
</s>
<s>
This	O
enables	O
additional	O
modifications	O
by	O
rules	O
that	O
follow	O
,	O
such	O
as	O
NAT	B-Protocol
or	O
further	O
filtering	O
.	O
</s>
<s>
The	O
network	B-Protocol
address	I-Protocol
translation	I-Protocol
table	O
(	O
or	O
"	O
nat	B-Protocol
"	O
)	O
that	O
is	O
made	O
available	O
to	O
iptables	B-Application
is	O
merely	O
a	O
"	O
configuration	O
database	O
"	O
for	O
NAT	B-Protocol
mappings	O
only	O
,	O
and	O
not	O
intended	O
for	O
filtering	O
of	O
any	O
kind	O
.	O
</s>
<s>
Used	O
for	O
Mandatory	O
Access	O
Control	O
(	O
MAC	O
)	O
networking	B-Architecture
rules	O
,	O
such	O
as	O
those	O
enabled	O
by	O
the	O
SECMARK	O
and	O
CONNSECMARK	O
targets	O
.	O
</s>
<s>
(	O
These	O
so-called	O
"	O
targets	O
"	O
refer	O
to	O
Security-Enhanced	O
Linux	B-Operating_System
markers	O
.	O
)	O
</s>
<s>
Mandatory	O
Access	O
Control	O
is	O
implemented	O
by	O
Linux	B-Operating_System
Security	O
Modules	B-Application
such	O
as	O
SELinux	O
.	O
</s>
<s>
nftables	O
is	O
the	O
new	O
packet-filtering	O
portion	O
of	O
Netfilter	B-Application
.	O
</s>
<s>
nft	O
is	O
the	O
new	O
userspace	O
utility	O
that	O
replaces	O
iptables	B-Application
,	O
ip6tables	B-Application
,	O
arptables	O
and	O
ebtables	O
.	O
</s>
<s>
nftables	O
kernel	B-Operating_System
engine	O
adds	O
a	O
simple	O
virtual	B-Architecture
machine	I-Architecture
into	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
,	O
which	O
is	O
able	O
to	O
execute	O
bytecode	O
to	O
inspect	O
a	O
network	O
packet	O
and	O
make	O
decisions	O
on	O
how	O
that	O
packet	O
should	O
be	O
handled	O
.	O
</s>
<s>
The	O
operations	O
implemented	O
by	O
this	O
virtual	B-Architecture
machine	I-Architecture
are	O
intentionally	O
made	O
basic	O
:	O
it	O
can	O
get	O
data	O
from	O
the	O
packet	O
itself	O
,	O
have	O
a	O
look	O
at	O
the	O
associated	O
metadata	O
(	O
inbound	O
interface	O
,	O
for	O
example	O
)	O
,	O
and	O
manage	O
connection	O
tracking	O
data	O
.	O
</s>
<s>
The	O
virtual	B-Architecture
machine	I-Architecture
is	O
also	O
capable	O
of	O
manipulating	O
sets	O
of	O
data	O
(	O
typically	O
IP	B-Protocol
addresses	I-Protocol
)	O
,	O
allowing	O
multiple	O
comparison	O
operations	O
to	O
be	O
replaced	O
with	O
a	O
single	O
set	O
lookup	O
.	O
</s>
<s>
This	O
is	O
in	O
contrast	O
to	O
the	O
legacy	O
Xtables	O
(	O
iptables	B-Application
,	O
etc	O
.	O
)	O
</s>
<s>
code	O
,	O
which	O
has	O
protocol	O
awareness	O
so	O
deeply	O
built	O
into	O
the	O
code	O
that	O
it	O
has	O
had	O
to	O
be	O
replicated	O
four	O
timesfor	O
IPv4	B-Protocol
,	O
IPv6	B-Protocol
,	O
ARP	O
,	O
and	O
Ethernet	O
bridgingas	O
the	O
firewall	B-Application
engines	O
are	O
too	O
protocol-specific	O
to	O
be	O
used	O
in	O
a	O
generic	O
manner	O
.	O
</s>
<s>
The	O
main	O
advantages	O
over	O
iptables	B-Application
are	O
simplification	O
of	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
ABI	B-Operating_System
,	O
reduction	O
of	O
code	O
duplication	O
,	O
improved	O
error	B-Error_Name
reporting	I-Error_Name
,	O
and	O
more	O
efficient	O
execution	O
,	O
storage	O
,	O
and	O
incremental	O
,	O
atomic	O
changes	O
of	O
filtering	O
rules	O
.	O
</s>
<s>
The	O
nf_defrag_ipv4	O
module	B-Application
will	O
defragment	O
IPv4	B-Protocol
packets	O
before	O
they	O
reach	O
Netfilter	B-Application
's	O
connection	O
tracking	O
(	O
nf_conntrack_ipv4	O
module	B-Application
)	O
.	O
</s>
<s>
This	O
is	O
necessary	O
for	O
the	O
in-kernel	O
connection	O
tracking	O
and	O
NAT	B-Protocol
helper	O
modules	B-Application
(	O
which	O
are	O
a	O
form	O
of	O
"	O
mini-ALGs	O
"	O
)	O
that	O
only	O
work	O
reliably	O
on	O
entire	O
packets	O
,	O
not	O
necessarily	O
on	O
fragments	O
.	O
</s>
<s>
The	O
IPv6	B-Protocol
defragmenter	O
is	O
not	O
a	O
module	B-Application
in	O
its	O
own	O
right	O
,	O
but	O
is	O
integrated	O
into	O
the	O
nf_conntrack_ipv6	O
module	B-Application
.	O
</s>
<s>
One	O
of	O
the	O
important	O
features	O
built	O
on	O
top	O
of	O
the	O
Netfilter	B-Application
framework	B-Architecture
is	O
connection	O
tracking	O
.	O
</s>
<s>
Connection	O
tracking	O
allows	O
the	O
kernel	B-Operating_System
to	O
keep	O
track	O
of	O
all	O
logical	O
network	O
connections	O
or	O
sessions	B-Protocol
,	O
and	O
thereby	O
relate	O
all	O
of	O
the	O
packets	O
which	O
may	O
make	O
up	O
that	O
connection	O
.	O
</s>
<s>
NAT	B-Protocol
relies	O
on	O
this	O
information	O
to	O
translate	O
all	O
related	O
packets	O
in	O
the	O
same	O
way	O
,	O
and	O
iptables	B-Application
can	O
use	O
this	O
information	O
to	O
act	O
as	O
a	O
stateful	O
firewall	B-Application
.	O
</s>
<s>
Even	O
connectionless-mode	B-Protocol
transmissions	I-Protocol
such	O
as	O
UDP	B-Protocol
,	O
IPsec	B-Protocol
(	O
AH/ESP	O
)	O
,	O
GRE	B-Protocol
and	O
other	O
tunneling	B-Protocol
protocols	I-Protocol
have	O
,	O
at	O
least	O
,	O
a	O
pseudo	O
connection	O
state	O
.	O
</s>
<s>
The	O
heuristic	O
for	O
such	O
protocols	O
is	O
often	O
based	O
upon	O
a	O
preset	O
timeout	O
value	O
for	O
inactivity	O
,	O
after	O
whose	O
expiration	O
a	O
Netfilter	B-Application
connection	O
is	O
dropped	O
.	O
</s>
<s>
Each	O
Netfilter	B-Application
connection	O
is	O
uniquely	O
identified	O
by	O
a	O
(	O
layer-3	O
protocol	O
,	O
source	O
address	O
,	O
destination	O
address	O
,	O
layer-4	O
protocol	O
,	O
layer-4	O
key	O
)	O
tuple	O
.	O
</s>
<s>
The	O
layer-4	O
key	O
depends	O
on	O
the	O
transport	O
protocol	O
;	O
for	O
TCP/UDP	O
it	O
is	O
the	O
port	O
numbers	O
,	O
for	O
tunnels	O
it	O
can	O
be	O
their	O
tunnel	O
ID	O
,	O
but	O
otherwise	O
is	O
just	O
zero	O
,	O
as	O
if	O
it	O
were	O
not	O
part	O
of	O
the	O
tuple	O
.	O
</s>
<s>
Netfilter	B-Application
connections	O
can	O
be	O
manipulated	O
with	O
the	O
user-space	O
tool	O
conntrack	O
.	O
</s>
<s>
iptables	B-Application
can	O
make	O
use	O
of	O
checking	O
the	O
connection	O
's	O
information	O
such	O
as	O
states	O
,	O
statuses	O
and	O
more	O
to	O
make	O
packet	B-Application
filtering	I-Application
rules	O
more	O
powerful	O
and	O
easier	O
to	O
manage	O
.	O
</s>
<s>
INVALID	B-Protocol
the	O
packet	O
was	O
found	O
to	O
be	O
invalid	B-Protocol
,	O
e.g.	O
</s>
<s>
A	O
normal	O
example	O
would	O
be	O
that	O
the	O
first	O
packet	O
the	O
conntrack	O
subsystem	O
sees	O
will	O
be	O
classified	O
"	O
new	O
"	O
,	O
the	O
reply	O
would	O
be	O
classified	O
"	O
established	O
"	O
and	O
an	O
ICMP	B-Protocol
error	O
would	O
be	O
"	O
related	O
"	O
.	O
</s>
<s>
An	O
ICMP	B-Protocol
error	O
packet	O
which	O
did	O
not	O
match	O
any	O
known	O
connection	O
would	O
be	O
"	O
invalid	B-Protocol
"	O
.	O
</s>
<s>
Through	O
the	O
use	O
of	O
plugin	O
modules	B-Application
,	O
connection	O
tracking	O
can	O
be	O
given	O
knowledge	O
of	O
application-layer	O
protocols	O
and	O
thus	O
understand	O
that	O
two	O
or	O
more	O
distinct	O
connections	O
are	O
"	O
related	O
"	O
.	O
</s>
<s>
For	O
example	O
,	O
consider	O
the	O
FTP	B-Protocol
protocol	O
.	O
</s>
<s>
When	O
the	O
nf_conntrack_ftp	O
module	B-Application
is	O
loaded	O
,	O
the	O
first	O
packet	O
of	O
an	O
FTP	B-Protocol
data	O
connection	O
will	O
be	O
classified	O
as	O
"	O
related	O
"	O
instead	O
of	O
"	O
new	O
"	O
,	O
as	O
it	O
is	O
logically	O
part	O
of	O
an	O
existing	O
connection	O
.	O
</s>
<s>
The	O
helpers	O
only	O
inspect	O
one	O
packet	O
at	O
a	O
time	O
,	O
so	O
if	O
vital	O
information	O
for	O
connection	O
tracking	O
is	O
split	O
across	O
two	O
packets	O
,	O
either	O
due	O
to	O
IP	B-Protocol
fragmentation	I-Protocol
or	O
TCP	O
segmentation	O
,	O
the	O
helper	O
will	O
not	O
necessarily	O
recognize	O
patterns	O
and	O
therefore	O
not	O
perform	O
its	O
operation	O
.	O
</s>
<s>
IP	B-Protocol
fragmentation	I-Protocol
is	O
dealt	O
with	O
the	O
connection	O
tracking	O
subsystem	O
requiring	O
defragmentation	O
,	O
though	O
TCP	O
segmentation	O
is	O
not	O
handled	O
.	O
</s>
<s>
In	O
case	O
of	O
FTP	B-Protocol
,	O
segmentation	O
is	O
deemed	O
not	O
to	O
happen	O
"	O
near	O
"	O
a	O
command	O
like	O
PASV	O
with	O
standard	O
segment	O
sizes	O
,	O
so	O
is	O
not	O
dealt	O
with	O
in	O
Netfilter	B-Application
either	O
.	O
</s>
<s>
NAT	B-Protocol
in	O
Netfilter	B-Application
is	O
implemented	O
by	O
simply	O
changing	O
the	O
reply	O
address	O
,	O
and	O
where	O
desired	O
,	O
port	O
.	O
</s>
<s>
Being	O
fragment-free	O
is	O
also	O
a	O
requirement	O
for	O
NAT	B-Protocol
.	O
</s>
<s>
(	O
If	O
need	O
be	O
,	O
IPv4	B-Protocol
packets	O
may	O
be	O
refragmented	O
by	O
the	O
normal	O
,	O
non-Netfilter	O
,	O
IPv4	B-Protocol
stack	O
.	O
)	O
</s>
<s>
Similar	O
to	O
connection	O
tracking	O
helpers	O
,	O
NAT	B-Protocol
helpers	O
will	O
do	O
a	O
packet	O
inspection	O
and	O
substitute	O
original	O
addresses	O
by	O
reply	O
addresses	O
in	O
the	O
payload	O
.	O
</s>
<s>
Though	O
not	O
being	O
kernel	B-Application
modules	I-Application
that	O
make	O
use	O
of	O
Netfilter	B-Application
code	O
directly	O
,	O
the	O
Netfilter	B-Application
project	O
hosts	O
a	O
few	O
more	O
noteworthy	O
software	O
.	O
</s>
<s>
conntrack-tools	O
is	O
a	O
set	O
of	O
user-space	O
tools	O
for	O
Linux	B-Operating_System
that	O
allow	O
system	O
administrators	O
to	O
interact	O
with	O
the	O
Connection	O
Tracking	O
entries	O
and	O
tables	O
.	O
</s>
<s>
The	O
userspace	O
daemon	O
conntrackd	O
can	O
be	O
used	O
to	O
enable	O
high	O
availability	O
cluster-based	O
stateful	O
firewalls	B-Application
and	O
collect	O
statistics	O
of	O
the	O
stateful	O
firewall	B-Application
use	O
.	O
</s>
<s>
Unlike	O
other	O
extensions	O
such	O
as	O
Connection	O
Tracking	O
,	O
ipset	B-Application
is	O
more	O
related	O
to	O
iptables	B-Application
than	O
it	O
is	O
to	O
the	O
core	O
Netfilter	B-Application
code	O
.	O
</s>
<s>
ipset	B-Application
does	O
not	O
make	O
use	O
of	O
Netfilter	B-Application
hooks	O
for	O
instance	O
,	O
but	O
actually	O
provides	O
an	O
iptables	B-Application
module	B-Application
to	O
match	O
and	O
do	O
minimal	O
modifications	O
(	O
set/clear	O
)	O
to	O
IP	O
sets	O
.	O
</s>
<s>
The	O
user-space	O
tool	O
called	O
ipset	B-Application
is	O
used	O
to	O
set	O
up	O
,	O
maintain	O
and	O
inspect	O
so	O
called	O
"	O
IP	O
sets	O
"	O
in	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
.	O
</s>
<s>
An	O
IP	O
set	O
usually	O
contains	O
a	O
set	O
of	O
IP	B-Protocol
addresses	I-Protocol
,	O
but	O
can	O
also	O
contain	O
sets	O
of	O
other	O
network	O
numbers	O
,	O
depending	O
on	O
its	O
"	O
type	O
"	O
.	O
</s>
<s>
These	O
sets	O
are	O
much	O
more	O
lookup-efficient	O
than	O
bare	O
iptables	B-Application
rules	O
,	O
but	O
of	O
course	O
may	O
come	O
with	O
a	O
greater	O
memory	O
footprint	O
.	O
</s>
<s>
Different	O
storage	O
algorithms	O
(	O
for	O
the	O
data	O
structures	O
in	O
memory	O
)	O
are	O
provided	O
in	O
ipset	B-Application
for	O
the	O
user	O
to	O
select	O
an	O
optimum	O
solution	O
.	O
</s>
<s>
A	O
set	O
can	O
only	O
be	O
removed	O
(	O
destroyed	O
)	O
if	O
there	O
are	O
no	O
iptables	B-Application
rules	O
or	O
other	O
sets	O
referring	O
to	O
it	O
.	O
</s>
<s>
By	O
redirecting	O
initial	O
SYN	O
requests	O
to	O
the	O
SYNPROXY	O
target	O
,	O
connections	O
are	O
not	O
registered	O
within	O
the	O
connection	O
tracking	O
until	O
they	O
reach	O
a	O
validated	O
final	O
ACK	O
state	O
,	O
freeing	O
up	O
connection	O
tracking	O
from	O
accounting	O
large	O
numbers	O
of	O
potentially	O
invalid	B-Protocol
connections	O
.	O
</s>
<s>
On	O
3	O
November	O
2013	O
,	O
SYN	O
proxy	O
functionality	O
was	O
merged	O
into	O
the	O
Netfilter	B-Application
,	O
with	O
the	O
release	O
of	O
version	O
3.12	O
of	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
mainline	O
.	O
</s>
<s>
ulogd	O
is	O
a	O
user-space	O
daemon	O
to	O
receive	O
and	O
log	O
packets	O
and	O
event	O
notifications	O
from	O
the	O
Netfilter	B-Application
subsystems	O
.	O
</s>
<s>
ip_tables	O
can	O
deliver	O
packets	O
via	O
the	O
userspace	O
queueing	O
mechanism	O
to	O
it	O
,	O
and	O
connection	O
tracking	O
can	O
interact	O
with	O
ulogd	O
to	O
exchange	O
further	O
information	O
about	O
packets	O
or	O
events	O
(	O
such	O
as	O
connection	O
teardown	O
,	O
NAT	B-Protocol
setup	O
)	O
.	O
</s>
<s>
Netfilter	B-Application
also	O
provides	O
a	O
set	O
of	O
libraries	O
having	O
libnetfilter	O
as	O
a	O
prefix	O
of	O
their	O
names	O
,	O
that	O
can	O
be	O
used	O
to	O
perform	O
different	O
tasks	O
from	O
the	O
userspace	O
.	O
</s>
<s>
These	O
libraries	O
are	O
released	O
under	O
the	O
GNU	B-License
GPL	I-License
version	O
2	O
.	O
</s>
<s>
The	O
Netfilter	B-Application
project	O
organizes	O
an	O
annual	O
meeting	O
for	O
developers	O
,	O
which	O
is	O
used	O
to	O
discuss	O
ongoing	O
research	O
and	O
development	O
efforts	O
.	O
</s>
<s>
The	O
2018	O
Netfilter	B-Application
workshop	O
took	O
place	O
in	O
Berlin	O
,	O
Germany	O
,	O
in	O
June	O
2018	O
.	O
</s>
