<s>
Proxy	B-Protocol
ARP	I-Protocol
is	O
a	O
technique	O
by	O
which	O
a	O
proxy	B-Application
server	I-Application
on	O
a	O
given	O
network	O
answers	O
the	O
Address	O
Resolution	O
Protocol	O
(	O
ARP	O
)	O
queries	O
for	O
an	O
IP	B-Protocol
address	I-Protocol
that	O
is	O
not	O
on	O
that	O
network	O
.	O
</s>
<s>
The	O
traffic	O
directed	O
to	O
the	O
proxy	O
address	O
is	O
then	O
typically	O
routed	O
by	O
the	O
proxy	O
to	O
the	O
intended	O
destination	O
via	O
another	O
interface	O
or	O
via	O
a	O
tunnel	B-Protocol
.	O
</s>
<s>
The	O
process	O
,	O
which	O
results	O
in	O
the	O
proxy	B-Application
server	I-Application
responding	O
with	O
its	O
own	O
MAC	O
address	O
to	O
an	O
ARP	O
request	O
for	O
a	O
different	O
IP	B-Protocol
address	I-Protocol
for	O
proxying	O
purposes	O
,	O
is	O
sometimes	O
referred	O
to	O
as	O
publishing	O
.	O
</s>
<s>
Below	O
are	O
some	O
typical	O
uses	O
for	O
proxy	B-Protocol
ARP	I-Protocol
:	O
</s>
<s>
Joining	O
a	O
broadcast	O
LAN	O
with	O
serial	B-Protocol
links	I-Protocol
(	O
e.g.	O
,	O
dialup	O
or	O
VPN	B-Application
connections	O
)	O
.	O
</s>
<s>
One	O
or	O
more	O
of	O
the	O
nodes	O
is	O
an	O
access	B-Protocol
router	I-Protocol
accepting	O
dialup	O
or	O
VPN	B-Application
connections	O
.	O
</s>
<s>
The	O
access	B-Protocol
router	I-Protocol
gives	O
the	O
dial-up	O
nodes	O
IP	B-Protocol
addresses	I-Protocol
in	O
the	O
range	O
192.168.0.128	O
–	O
192.168.0.254	O
;	O
for	O
this	O
example	O
,	O
assume	O
a	O
dial-up	O
node	O
gets	O
IP	B-Protocol
address	I-Protocol
192.168.0.254	O
.	O
</s>
<s>
The	O
access	B-Protocol
router	I-Protocol
uses	O
proxy	B-Protocol
ARP	I-Protocol
to	O
make	O
the	O
dial-up	O
node	O
present	O
in	O
the	O
subnet	O
without	O
being	O
wired	O
into	O
the	O
Ethernet	O
:	O
the	O
access	B-Protocol
router	I-Protocol
'	O
publishes	O
 '	O
its	O
own	O
MAC	O
address	O
for	O
192.168.0.254	O
.	O
</s>
<s>
Now	O
,	O
when	O
another	O
node	O
wired	O
into	O
the	O
Ethernet	O
wants	O
to	O
talk	O
to	O
the	O
dial-up	O
node	O
,	O
it	O
will	O
ask	O
on	O
the	O
network	O
for	O
the	O
MAC	O
address	O
of	O
192.168.0.254	O
and	O
find	O
the	O
access	B-Protocol
router	I-Protocol
's	O
MAC	O
address	O
.	O
</s>
<s>
It	O
will	O
therefore	O
send	O
its	O
IP	O
packets	O
to	O
the	O
access	B-Protocol
router	I-Protocol
,	O
and	O
the	O
access	B-Protocol
router	I-Protocol
will	O
know	O
to	O
pass	O
them	O
on	O
to	O
the	O
particular	O
dial-up	O
node	O
.	O
</s>
<s>
Certain	O
applications	O
may	O
require	O
multiple	O
IP	B-Protocol
addresses	I-Protocol
on	O
the	O
server	O
.	O
</s>
<s>
Provided	O
the	O
addresses	O
have	O
to	O
be	O
from	O
the	O
10.0.0.0/24	O
range	O
,	O
the	O
way	O
the	O
problem	O
is	O
solved	O
is	O
through	O
proxy	B-Protocol
ARP	I-Protocol
.	O
</s>
<s>
Additional	O
addresses	O
(	O
say	O
,	O
10.0.0.230-10.0.0.240	O
)	O
are	O
aliased	B-Protocol
to	O
the	O
loopback	B-Protocol
interface	O
of	O
the	O
server	O
(	O
or	O
assigned	O
to	O
special	O
interfaces	O
,	O
the	O
latter	O
typically	O
being	O
the	O
case	O
with	O
VMware/UML/jails/vservers/other	O
virtual	O
server	O
environments	O
)	O
and	O
'	O
published	O
 '	O
on	O
the	O
10.0.0.2	O
interface	O
(	O
although	O
many	O
operating	O
systems	O
allow	O
direct	O
allocation	O
of	O
multiple	O
addresses	O
to	O
one	O
interface	O
,	O
thus	O
eliminating	O
the	O
need	O
for	O
such	O
workarounds	O
)	O
.	O
</s>
<s>
In	O
this	O
scenario	O
a	O
firewall	O
can	O
be	O
configured	O
with	O
a	O
single	O
IP	B-Protocol
address	I-Protocol
.	O
</s>
<s>
A	O
proxy	B-Protocol
ARP	I-Protocol
firewall	O
can	O
be	O
placed	O
in	O
front	O
of	O
the	O
server	O
.	O
</s>
<s>
In	O
case	O
of	O
Mobile-IP	O
the	O
Home	O
Agent	O
uses	O
proxy	B-Protocol
ARP	I-Protocol
in	O
order	O
to	O
receive	O
messages	O
on	O
behalf	O
of	O
the	O
Mobile	O
Node	O
so	O
that	O
it	O
can	O
forward	O
the	O
appropriate	O
message	O
to	O
the	O
actual	O
mobile	O
node	O
's	O
address	O
(	O
Care-of	B-Protocol
address	I-Protocol
)	O
.	O
</s>
<s>
A	O
setup	O
that	O
involves	O
two	O
physical	O
segments	O
sharing	O
the	O
same	O
IP	O
subnet	O
and	O
connected	O
together	O
via	O
a	O
router	B-Protocol
.	O
</s>
<s>
ARP	O
manipulation	O
techniques	O
are	O
the	O
basis	O
for	O
protocols	O
providing	O
redundancy	O
on	O
broadcast	O
networks	O
(	O
e.g.	O
,	O
Ethernet	O
)	O
,	O
most	O
notably	O
Common	B-Application
Address	I-Application
Redundancy	I-Application
Protocol	I-Application
and	O
Virtual	B-Protocol
Router	I-Protocol
Redundancy	I-Protocol
Protocol	I-Protocol
.	O
</s>
<s>
Disadvantage	O
of	O
proxy	B-Protocol
ARP	I-Protocol
include	O
scalability	O
as	O
ARP	O
resolution	O
by	O
a	O
proxy	O
is	O
required	O
for	O
every	O
device	O
routed	O
in	O
this	O
manner	O
,	O
and	O
reliability	O
as	O
no	O
fallback	O
mechanism	O
is	O
present	O
,	O
and	O
masquerading	O
can	O
be	O
confusing	O
in	O
some	O
environments	O
.	O
</s>
<s>
Proxy	B-Protocol
ARP	I-Protocol
can	O
create	O
DoS	O
attacks	O
on	O
networks	O
if	O
misconfigured	O
.	O
</s>
<s>
For	O
example	O
,	O
a	O
misconfigured	O
router	B-Protocol
with	O
proxy	B-Protocol
ARP	I-Protocol
has	O
the	O
ability	O
to	O
receive	O
packets	O
destined	O
for	O
other	O
hosts	O
(	O
as	O
it	O
gives	O
its	O
own	O
MAC	O
address	O
in	O
response	O
to	O
ARP	O
requests	O
for	O
other	O
hosts/routers	O
)	O
,	O
but	O
may	O
not	O
have	O
the	O
ability	O
to	O
correctly	O
forward	O
these	O
packets	O
on	O
to	O
their	O
final	O
destination	O
,	O
thus	O
blackholing	O
the	O
traffic	O
.	O
</s>
<s>
Proxy	B-Protocol
ARP	I-Protocol
can	O
hide	O
device	O
misconfigurations	O
,	O
such	O
as	O
a	O
missing	O
or	O
incorrect	O
default	B-Protocol
gateway	I-Protocol
.	O
</s>
<s>
OpenBSD	B-Operating_System
implements	O
proxy	B-Protocol
ARP	I-Protocol
.	O
</s>
<s>
Linux	B-Application
implements	O
proxy	B-Protocol
ARP	I-Protocol
.	O
</s>
