<s>
In	O
computer	B-Architecture
networks	I-Architecture
,	O
a	O
reverse	B-Protocol
DNS	I-Protocol
lookup	I-Protocol
or	O
reverse	B-Protocol
DNS	I-Protocol
resolution	I-Protocol
(	O
rDNS	B-Protocol
)	O
is	O
the	O
querying	O
technique	O
of	O
the	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
(	O
DNS	B-Protocol
)	O
to	O
determine	O
the	O
domain	O
name	O
associated	O
with	O
an	O
IP	B-Protocol
address	I-Protocol
–	O
the	O
reverse	O
of	O
the	O
usual	O
"	O
forward	O
"	O
DNS	B-Protocol
lookup	I-Protocol
of	O
an	O
IP	B-Protocol
address	I-Protocol
from	O
a	O
domain	O
name	O
.	O
</s>
<s>
The	O
process	O
of	O
reverse	B-Protocol
resolving	I-Protocol
of	O
an	O
IP	B-Protocol
address	I-Protocol
uses	O
PTR	O
records	O
.	O
</s>
<s>
rDNS	B-Protocol
involves	O
searching	O
domain	O
name	O
registry	O
and	O
registrar	O
tables	O
.	O
</s>
<s>
The	O
reverse	B-Protocol
DNS	I-Protocol
database	O
of	O
the	O
Internet	O
is	O
rooted	O
in	O
the	O
.arpa	O
top-level	O
domain	O
.	O
</s>
<s>
Although	O
the	O
informational	O
RFC	O
1912	O
(	O
Section	O
2.1	O
)	O
recommends	O
that	O
"	O
every	O
Internet-reachable	O
host	O
should	O
have	O
a	O
name	O
"	O
and	O
that	O
"	O
for	O
every	O
IP	B-Protocol
address	I-Protocol
,	O
there	O
should	O
be	O
a	O
matching	O
PTR	O
record	O
,	O
"	O
it	O
is	O
not	O
an	O
Internet	O
Standard	O
requirement	O
,	O
and	O
not	O
all	O
IP	B-Protocol
addresses	I-Protocol
have	O
a	O
reverse	O
entry	O
.	O
</s>
<s>
The	O
modern	O
"	O
reverse	B-Protocol
DNS	I-Protocol
lookup	I-Protocol
"	O
should	O
not	O
be	O
confused	O
with	O
the	O
now-obsolete	O
"	O
inverse	O
query	O
"	O
(	O
IQUERY	O
)	O
mechanism	O
specified	O
in	O
:	O
</s>
<s>
Reverse	B-Protocol
DNS	I-Protocol
lookups	I-Protocol
for	O
IPv4	B-Protocol
addresses	O
use	O
the	O
special	O
domain	O
in-addr.arpa	B-Protocol
.	O
</s>
<s>
In	O
this	O
domain	O
,	O
an	O
IPv4	B-Protocol
address	O
is	O
represented	O
as	O
a	O
concatenated	O
sequence	O
of	O
four	O
decimal	O
numbers	O
,	O
separated	O
by	O
dots	O
,	O
to	O
which	O
is	O
appended	O
the	O
second	O
level	O
domain	O
suffix	O
.in-addr.arpa	O
.	O
</s>
<s>
The	O
four	O
decimal	O
numbers	O
are	O
obtained	O
by	O
splitting	O
the	O
32-bit	O
IPv4	B-Protocol
address	O
into	O
four	O
octets	O
and	O
converting	O
each	O
octet	O
into	O
a	O
decimal	O
number	O
.	O
</s>
<s>
It	O
is	O
important	O
to	O
note	O
that	O
this	O
is	O
the	O
reverse	O
order	O
to	O
the	O
usual	O
dotted-decimal	O
convention	O
for	O
writing	O
IPv4	B-Protocol
addresses	O
in	O
textual	O
form	O
.	O
</s>
<s>
For	O
example	O
,	O
to	O
do	O
a	O
reverse	O
lookup	O
of	O
the	O
IP	B-Protocol
address	I-Protocol
the	O
PTR	O
record	O
for	O
the	O
domain	O
name	O
4.4.8.8.in-addr.arpa	O
would	O
be	O
looked	O
up	O
,	O
and	O
found	O
to	O
point	O
to	O
dns.google	O
.	O
</s>
<s>
If	O
the	O
A	B-Protocol
record	I-Protocol
for	O
dns.google	O
in	O
turn	O
pointed	O
back	O
to	O
then	O
it	O
would	O
be	O
said	O
to	O
be	O
forward-confirmed	B-Protocol
.	O
</s>
<s>
Historically	O
,	O
Internet	O
registries	O
and	O
Internet	O
service	O
providers	O
allocated	O
IP	B-Protocol
addresses	I-Protocol
in	O
blocks	O
of	O
256	O
(	O
for	O
Class	O
C	O
)	O
or	O
larger	O
octet-based	O
blocks	O
for	O
classes	O
B	O
and	O
A	O
.	O
</s>
<s>
The	O
structure	O
of	O
the	O
reverse	B-Protocol
DNS	I-Protocol
domain	O
was	O
based	O
on	O
this	O
definition	O
.	O
</s>
<s>
However	O
,	O
with	O
the	O
introduction	O
of	O
Classless	B-Protocol
Inter-Domain	I-Protocol
Routing	I-Protocol
,	O
IP	B-Protocol
addresses	I-Protocol
were	O
allocated	O
in	O
much	O
smaller	O
blocks	O
,	O
and	O
hence	O
the	O
original	O
design	O
of	O
pointer	O
records	O
was	O
impractical	O
,	O
since	O
autonomy	O
of	O
administration	O
of	O
smaller	O
blocks	O
could	O
not	O
be	O
granted	O
.	O
</s>
<s>
RFC	O
2317	O
devised	O
a	O
methodology	O
to	O
address	O
this	O
problem	O
by	O
using	O
CNAME	B-Protocol
records	I-Protocol
.	O
</s>
<s>
Reverse	B-Protocol
DNS	I-Protocol
lookups	I-Protocol
for	O
IPv6	B-Protocol
addresses	O
use	O
the	O
special	O
domain	O
ip6.arpa	O
(	O
previously	O
ip6.int	O
)	O
.	O
</s>
<s>
An	O
IPv6	B-Protocol
address	O
appears	O
as	O
a	O
name	O
in	O
this	O
domain	O
as	O
a	O
sequence	O
of	O
nibbles	O
in	O
reverse	O
order	O
,	O
represented	O
as	O
hexadecimal	O
digits	O
as	O
subdomains	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
pointer	O
domain	O
name	O
corresponding	O
to	O
the	O
IPv6	B-Protocol
address	O
is	O
b.a.9.8.7.6.5.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.b.d.0.1.0.0.2.ip6.arpa	O
.	O
</s>
<s>
While	O
most	O
rDNS	B-Protocol
entries	O
only	O
have	O
one	O
PTR	O
record	O
,	O
DNS	B-Protocol
does	O
not	O
restrict	O
the	O
number	O
.	O
</s>
<s>
However	O
,	O
DNS	B-Protocol
lookups	I-Protocol
typically	O
occur	O
over	O
UDP	O
,	O
and	O
since	O
UDP	O
has	O
a	O
limited	O
message	O
size	O
,	O
in	O
extreme	O
cases	O
,	O
multiple	O
PTRs	O
could	O
cause	O
a	O
DNS	B-Protocol
response	O
to	O
exceed	O
those	O
UDP	O
limits	O
.	O
</s>
<s>
Record	O
types	O
other	O
than	O
PTR	O
records	O
may	O
also	O
appear	O
in	O
the	O
reverse	B-Protocol
DNS	I-Protocol
tree	O
.	O
</s>
<s>
For	O
example	O
,	O
encryption	O
keys	O
may	O
be	O
placed	O
there	O
for	O
IPsec	B-Protocol
,	O
SSH	B-Protocol
and	O
IKE	B-Protocol
.	O
</s>
<s>
DNS-Based	O
Service	O
Discovery	O
uses	O
specially-named	O
records	O
in	O
the	O
reverse	B-Protocol
DNS	I-Protocol
tree	O
to	O
provide	O
hints	O
to	O
clients	O
about	O
subnet-specific	O
service	O
discovery	O
domains	O
.	O
</s>
<s>
Less	O
standardized	O
usages	O
include	O
comments	O
placed	O
in	O
TXT	B-Protocol
records	I-Protocol
and	O
LOC	B-Protocol
records	I-Protocol
to	O
identify	O
the	O
geophysical	O
location	O
of	O
an	O
IP	B-Protocol
address	I-Protocol
.	O
</s>
<s>
The	O
most	O
common	O
uses	O
of	O
the	O
reverse	B-Protocol
DNS	I-Protocol
include	O
:	O
</s>
<s>
The	O
original	O
use	O
of	O
the	O
rDNS	B-Protocol
:	O
network	O
troubleshooting	O
via	O
tools	O
such	O
as	O
traceroute	B-Protocol
,	O
ping	B-Protocol
,	O
and	O
the	O
"	O
Received	O
:	O
"	O
trace	O
header	O
field	O
for	O
SMTP	B-Protocol
e-mail	O
,	O
web	O
sites	O
tracking	O
users	O
(	O
especially	O
on	O
Internet	B-Application
forums	I-Application
)	O
,	O
etc	O
.	O
</s>
<s>
One	O
e-mail	O
anti-spam	O
technique	O
:	O
checking	O
the	O
domain	O
names	O
in	O
the	O
rDNS	B-Protocol
to	O
see	O
if	O
they	O
are	O
likely	O
from	O
dialup	O
users	O
,	O
or	O
dynamically	O
assigned	O
addresses	O
unlikely	O
to	O
be	O
used	O
by	O
legitimate	O
mail	O
servers	O
.	O
</s>
<s>
Owners	O
of	O
such	O
IP	B-Protocol
addresses	I-Protocol
typically	O
assign	O
them	O
generic	O
rDNS	B-Protocol
names	O
such	O
as	O
"	O
1-2-3-4-dynamic-ip.example.com	O
.	O
"	O
</s>
<s>
A	O
forward-confirmed	B-Protocol
reverse	I-Protocol
DNS	I-Protocol
(	O
FCrDNS	B-Protocol
)	O
verification	O
can	O
create	O
a	O
form	O
of	O
authentication	O
showing	O
a	O
valid	O
relationship	O
between	O
the	O
owner	O
of	O
a	O
domain	O
name	O
and	O
the	O
owner	O
of	O
the	O
server	O
that	O
has	O
been	O
given	O
an	O
IP	B-Protocol
address	I-Protocol
.	O
</s>
<s>
System	O
logging	O
or	O
monitoring	O
tools	O
often	O
receive	O
entries	O
with	O
the	O
relevant	O
devices	O
specified	O
only	O
by	O
IP	B-Protocol
addresses	I-Protocol
.	O
</s>
<s>
To	O
provide	O
more	O
human-usable	O
data	O
,	O
these	O
programs	O
often	O
perform	O
a	O
reverse	O
lookup	O
before	O
writing	O
the	O
log	O
,	O
thus	O
writing	O
a	O
name	O
rather	O
than	O
the	O
IP	B-Protocol
address	I-Protocol
.	O
</s>
