<s>
A	O
Name	O
Authority	O
Pointer	O
(	O
NAPTR	B-Protocol
)	O
is	O
a	O
type	O
of	O
resource	O
record	O
in	O
the	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
of	O
the	O
Internet	O
.	O
</s>
<s>
NAPTR	B-Protocol
records	I-Protocol
are	O
most	O
commonly	O
used	O
for	O
applications	O
in	O
Internet	B-Application
telephony	I-Application
,	O
for	O
example	O
,	O
in	O
the	O
mapping	O
of	O
servers	O
and	O
user	O
addresses	O
in	O
the	O
Session	B-Application
Initiation	I-Application
Protocol	I-Application
(	O
SIP	O
)	O
.	O
</s>
<s>
The	O
combination	O
of	O
NAPTR	B-Protocol
records	I-Protocol
with	O
Service	O
Records	O
(	O
SRV	O
)	O
allows	O
the	O
chaining	O
of	O
multiple	O
records	O
to	O
form	O
complex	O
rewrite	O
rules	O
which	O
produce	O
new	O
domain	O
labels	O
or	O
uniform	B-Protocol
resource	I-Protocol
identifiers	I-Protocol
(	O
URIs	B-Protocol
)	O
.	O
</s>
<s>
The	O
DNS	B-Protocol
type	O
code	O
for	O
the	O
NAPTR	B-Protocol
record	I-Protocol
is	O
35	O
.	O
</s>
<s>
Uniform	B-Protocol
Resource	I-Protocol
Names	I-Protocol
(	O
URNs	B-Protocol
)	O
are	O
a	O
subset	O
of	O
Uniform	B-Protocol
Resource	I-Protocol
Identifiers	I-Protocol
(	O
URIs	B-Protocol
)	O
used	O
for	O
abstract	O
identifiers	O
,	O
such	O
as	O
a	O
person	O
's	O
name	O
or	O
their	O
telephone	O
number	O
.	O
</s>
<s>
For	O
URNs	B-Protocol
to	O
be	O
meaningful	O
,	O
they	O
must	O
be	O
mapped	O
to	O
a	O
concrete	O
resource	O
of	O
some	O
sort	O
.	O
</s>
<s>
Uniform	O
Resource	O
Locators	O
(	O
URLs	O
)	O
are	O
often	O
used	O
to	O
describe	O
such	O
resources	O
,	O
such	O
as	O
a	O
computer	O
hostname	B-Protocol
,	O
or	O
a	O
local	O
file	O
.	O
</s>
<s>
The	O
NAPTR	B-Protocol
record	I-Protocol
aids	O
in	O
the	O
standardization	O
of	O
URNs	B-Protocol
.	O
</s>
<s>
NAPTR	B-Protocol
records	I-Protocol
map	O
between	O
sets	O
of	O
URNs	B-Protocol
,	O
URLs	O
and	O
plain	O
domain	O
names	O
and	O
suggest	O
to	O
clients	O
the	O
protocols	O
available	O
for	O
communication	O
with	O
the	O
mapped	O
resource	O
.	O
</s>
<s>
Each	O
NAPTR	B-Protocol
record	I-Protocol
contains	O
a	O
service	O
name	O
,	O
a	O
set	O
of	O
flags	O
,	O
a	O
regular	B-Language
expression	I-Language
rule	O
,	O
an	O
order	O
value	O
,	O
a	O
preference	O
and	O
a	O
replacement	O
pattern	O
.	O
</s>
<s>
Multiple	O
records	O
can	O
be	O
chained	O
together	O
in	O
a	O
cascade	O
to	O
rewrite	O
URIs	B-Protocol
in	O
deterministic	O
ways	O
.	O
</s>
<s>
A	O
common	O
use	O
for	O
NAPTR	B-Protocol
records	I-Protocol
is	O
in	O
Session	B-Application
Initiation	I-Application
Protocol	I-Application
,	O
where	O
it	O
is	O
used	O
to	O
route	O
telephony	O
sessions	O
over	O
the	O
IP	O
networks	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
SIP	O
URN	B-Protocol
for	O
the	O
US	O
telephone	O
number	O
1-800-555-1234	O
might	O
be	O
and	O
its	O
domain	O
name	O
4.3.2.1.5.5.5.0.0.8.1.e164.arpa	O
.	O
</s>
<s>
IN	O
NAPTR	B-Protocol
100	O
10	O
"	O
U	O
"	O
"	O
E2U+sip	O
"	O
"	O
!	O
^	O
.*	B-Language
$	O
!	O
sip:customer-service	O
@example	O
.com	O
!	O
"	O
</s>
<s>
IN	O
NAPTR	B-Protocol
102	O
10	O
"	O
U	O
"	O
"	O
E2U+email	O
"	O
"	O
!	O
^	O
.*	B-Language
$	O
!	O
mailto:information	O
@example	O
.com	O
!	O
"	O
</s>
<s>
The	O
service	O
name	O
E2U+sip	O
is	O
an	O
ENUM	B-Language
string	O
indicating	O
that	O
the	O
record	O
can	O
be	O
used	O
in	O
telephone	O
number-to-SIP-URI	O
queries	O
.	O
</s>
<s>
The	O
client	O
applies	O
the	O
regular	B-Language
expression	I-Language
!	O
^	O
.	O
</s>
<s>
*$	O
!	O
sip:customer-service	O
@example	O
.com	O
!,	O
which	O
replaces	O
its	O
entire	O
URN	B-Protocol
tel	O
:	O
+	O
1-800-555-1234	O
with	O
sip:customer-service	O
@example	O
.com	O
.	O
</s>
<s>
The	O
flag	O
U	O
indicates	O
that	O
the	O
replacement	O
string	O
is	O
a	O
SIP	O
URN	B-Protocol
,	O
and	O
that	O
no	O
further	O
rules	O
should	O
be	O
applied	O
.	O
</s>
<s>
To	O
resolve	O
SIP	O
URN	B-Protocol
,	O
the	O
client	O
performs	O
a	O
second	O
NAPTR	B-Protocol
lookupon	O
example.com,	O
yielding	O
:	O
</s>
<s>
IN	O
NAPTR	B-Protocol
100	O
10	O
"	O
S	O
"	O
"	O
SIP+D2U	O
"	O
"	O
!	O
^	O
.*	B-Language
$	O
!	O
sip:customer-service	O
@example	O
.com	O
!	O
"	O
</s>
<s>
IN	O
NAPTR	B-Protocol
102	O
10	O
"	O
S	O
"	O
"	O
SIP+D2T	O
"	O
"	O
!	O
^	O
.*	B-Language
$	O
!	O
sip:customer-service	O
@example	O
.com	O
!	O
"	O
</s>
<s>
The	O
regular	B-Language
expression	I-Language
rule	O
replaces	O
the	O
query	O
URN	B-Protocol
,	O
this	O
time	O
with	O
the	O
domain	O
name	O
_sip._udp.example.com	O
.	O
</s>
<s>
The	O
flag	O
S	O
indicates	O
that	O
the	O
resulting	O
domain	O
name	O
points	O
to	O
a	O
SRV	B-Protocol
record	I-Protocol
.	O
</s>
<s>
The	O
client	O
thus	O
finishes	O
with	O
_sip._udp.example.com	O
,	O
for	O
which	O
it	O
can	O
then	O
fetch	O
a	O
SRV	B-Protocol
record	I-Protocol
to	O
initiate	O
a	O
telephony	O
call	O
.	O
</s>
<s>
Vendor	O
Product	O
NAPTR	B-Protocol
support	O
?	O
</s>
<s>
ISC	O
BIND	B-Protocol
CZ.NIC	B-Protocol
Knot	B-Protocol
DNS	I-Protocol
Cisco	O
Systems	O
CNR	B-Protocol
Daniel	O
J	O
.	O
Bernstein	O
djbdns	B-Protocol
BlueCat	O
Networks	O
Integrity	O
EfficientIP	O
SOLIDserver	O
GoogleGoogle	O
Cloud	O
DNS	B-Protocol
Infoblox	O
Infoblox	O
Trinzic	O
Appliance	O
Microsoft	B-Application
Windows	I-Application
Server	I-Application
2003	I-Application
DNS	B-Protocol
Server	O
Microsoft	O
Windows	B-Device
Server	I-Device
2008	I-Device
R2	I-Device
DNS	B-Protocol
Server	O
Microsoft	B-Application
Azure	I-Application
DNS	B-Protocol
NS1	O
mDNS	O
and	O
DDI	O
PowerDNS/Open	O
-Xchange	O
PowerDNS	B-Language
NLnet	B-General_Concept
Labs	I-General_Concept
NSD	B-Protocol
Amazon	B-Application
Web	I-Application
Services	I-Application
Amazon	B-Protocol
Route	I-Protocol
53	I-Protocol
Sam	O
Trenholme	O
MaraDNS	B-Protocol
Unixservice	O
,	O
LLC	O
.	O
</s>
<s>
NAPTR	B-Protocol
implementations	O
generally	O
also	O
implement	O
EDNS	B-Protocol
as	O
responses	O
returning	O
multiple	O
NAPTR	B-Protocol
records	I-Protocol
are	O
usually	O
larger	O
than	O
the	O
normal	O
512	O
byte	O
packet	O
size	O
limit	O
and	O
would	O
otherwise	O
require	O
a	O
less	O
efficient	O
fallback	O
to	O
TCP	O
rather	O
than	O
UDP	O
.	O
</s>
