<s>
A	B-Protocol
Service	O
record	O
(	O
SRV	B-Protocol
record	I-Protocol
)	O
is	O
a	B-Protocol
specification	O
of	O
data	O
in	O
the	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
defining	O
the	O
location	O
,	O
i.e.	O
,	O
the	O
hostname	O
and	O
port	O
number	O
,	O
of	O
servers	O
for	O
specified	O
services	O
.	O
</s>
<s>
Some	O
Internet	O
protocols	O
such	O
as	O
the	O
Session	B-Application
Initiation	I-Application
Protocol	I-Application
(	O
SIP	O
)	O
and	O
the	O
Extensible	B-Protocol
Messaging	I-Protocol
and	I-Protocol
Presence	I-Protocol
Protocol	I-Protocol
(	O
XMPP	B-Protocol
)	O
often	O
require	O
SRV	O
support	O
by	O
network	O
elements	O
.	O
</s>
<s>
A	B-Protocol
SRV	B-Protocol
record	I-Protocol
has	O
the	O
form	O
:	O
</s>
<s>
proto	O
:	O
the	O
transport	O
protocol	O
of	O
the	O
desired	O
service	O
;	O
this	O
is	O
usually	O
either	O
TCP	B-Protocol
or	O
UDP	B-Protocol
.	O
</s>
<s>
name	O
:	O
the	O
domain	O
name	O
for	O
which	O
this	O
record	O
is	O
valid	O
,	O
ending	O
in	O
a	B-Protocol
dot	O
.	O
</s>
<s>
ttl	O
:	O
standard	O
DNS	B-Protocol
time	B-Protocol
to	I-Protocol
live	I-Protocol
field	O
.	O
</s>
<s>
IN	O
:	O
standard	O
DNS	B-Protocol
class	O
field	O
(	O
this	O
is	O
always	O
IN	O
)	O
.	O
</s>
<s>
weight	O
:	O
A	B-Protocol
relative	O
weight	O
for	O
records	O
with	O
the	O
same	O
priority	O
,	O
higher	O
value	O
means	O
higher	O
chance	O
of	O
getting	O
picked	O
.	O
</s>
<s>
port	O
:	O
the	O
TCP	B-Protocol
or	O
UDP	B-Protocol
port	O
on	O
which	O
the	O
service	O
is	O
to	O
be	O
found	O
.	O
</s>
<s>
target	O
:	O
the	O
canonical	O
hostname	O
of	O
the	O
machine	O
providing	O
the	O
service	O
,	O
ending	O
in	O
a	B-Protocol
dot	O
.	O
</s>
<s>
An	O
example	O
SRV	B-Protocol
record	I-Protocol
in	O
textual	O
form	O
that	O
might	O
be	O
found	O
in	O
a	B-Protocol
zone	B-Protocol
file	I-Protocol
might	O
be	O
the	O
following	O
:	O
</s>
<s>
86400	O
IN	O
SRV	O
0	O
5	O
5060	B-Application
sipserver.example.com	O
.	O
</s>
<s>
This	O
points	O
to	O
a	B-Protocol
server	O
named	O
sipserver.example.com	O
listening	O
on	O
TCP	B-Protocol
port	B-Application
5060	I-Application
for	O
Session	B-Application
Initiation	I-Application
Protocol	I-Application
(	O
SIP	O
)	O
protocol	O
services	O
.	O
</s>
<s>
As	O
in	O
MX	B-Protocol
records	I-Protocol
,	O
the	O
target	O
in	O
SRV	B-Protocol
records	I-Protocol
must	O
point	O
to	O
hostname	O
with	O
an	O
address	O
record	O
(	O
A	B-Protocol
or	O
AAAA	B-Protocol
record	I-Protocol
)	O
.	O
</s>
<s>
Pointing	O
to	O
a	B-Protocol
hostname	O
with	O
a	B-Protocol
CNAME	B-Protocol
record	I-Protocol
is	O
not	O
a	B-Protocol
valid	O
configuration	O
.	O
</s>
<s>
Clients	O
should	O
use	O
the	O
SRV	B-Protocol
records	I-Protocol
with	O
the	O
lowest-numbered	O
priority	O
value	O
first	O
,	O
and	O
fall	O
back	O
to	O
records	O
of	O
higher	O
value	O
if	O
the	O
connection	O
fails	O
.	O
</s>
<s>
If	O
a	B-Protocol
service	O
has	O
multiple	O
SRV	B-Protocol
records	I-Protocol
with	O
the	O
same	O
priority	O
value	O
,	O
clients	O
should	O
load	B-Application
balance	I-Application
them	O
in	O
proportion	O
to	O
the	O
values	O
of	O
their	O
weight	O
fields	O
.	O
</s>
<s>
In	O
the	O
following	O
example	O
,	O
both	O
the	O
priority	O
and	O
weight	O
fields	O
are	O
used	O
to	O
provide	O
a	B-Protocol
combination	O
of	O
load	B-Application
balancing	I-Application
and	O
backup	O
service	O
.	O
</s>
<s>
86400	O
IN	O
SRV	O
10	O
60	O
5060	B-Application
bigbox.example.com	O
.	O
</s>
<s>
86400	O
IN	O
SRV	O
10	O
20	O
5060	B-Application
smallbox1.example.com	O
.	O
</s>
<s>
86400	O
IN	O
SRV	O
10	O
20	O
5060	B-Application
smallbox2.example.com	O
.	O
</s>
<s>
86400	O
IN	O
SRV	O
20	O
0	O
5060	B-Application
backupbox.example.com	O
.	O
</s>
<s>
The	O
first	O
three	O
records	O
share	O
a	B-Protocol
priority	O
of	O
10	O
,	O
so	O
the	O
weight	O
field	O
's	O
value	O
will	O
be	O
used	O
by	O
clients	O
to	O
determine	O
which	O
server	O
(	O
host	O
and	O
port	O
combination	O
)	O
to	O
contact	O
.	O
</s>
<s>
This	O
might	O
be	O
a	B-Protocol
machine	O
in	O
another	O
physical	O
location	O
,	O
presumably	O
not	O
vulnerable	O
to	O
anything	O
that	O
would	O
cause	O
the	O
first	O
three	O
hosts	O
to	O
become	O
unavailable	O
.	O
</s>
<s>
The	O
load	B-Application
balancing	I-Application
provided	O
by	O
SRV	B-Protocol
records	I-Protocol
is	O
inherently	O
limited	O
,	O
since	O
the	O
information	O
is	O
essentially	O
static	O
.	O
</s>
<s>
Current	O
load	O
of	O
servers	O
is	O
not	O
taken	O
into	O
account	O
,	O
unless	O
TTL	O
values	O
are	O
low	O
enough	O
(	O
around	O
a	B-Protocol
minute	O
or	O
lower	O
)	O
that	O
the	O
priority	O
(	O
or	O
weight	O
)	O
values	O
can	O
be	O
quickly	O
updated	O
.	O
</s>
<s>
SRV	B-Protocol
records	I-Protocol
are	O
common	O
in	O
conjunction	O
with	O
the	O
following	O
standardized	O
communications	O
protocols	O
:	O
</s>
<s>
In	O
Microsoft	B-Application
Windows	I-Application
2000	I-Application
clients	O
query	O
for	O
SRV	B-Protocol
records	I-Protocol
to	O
determine	O
the	O
domain	B-Protocol
controller	I-Protocol
for	O
a	B-Protocol
given	O
service	O
.	O
</s>
<s>
SRV	B-Protocol
records	I-Protocol
are	O
also	O
used	O
by	O
Outlook	O
2007	O
,	O
2010	O
and	O
Macintosh	O
10.6	O
mail	O
to	O
locate	O
the	O
Exchange	O
Autodiscover	O
service	O
.	O
</s>
<s>
In	O
Microsoft	O
Windows	O
networks	O
domain	B-Protocol
controllers	I-Protocol
register	O
their	O
network	O
service	O
types	O
for	O
Active	B-Protocol
Directory	I-Protocol
in	O
the	O
DNS	B-Protocol
.	O
</s>
<s>
An	O
older	O
version	O
of	O
the	O
Internet	O
Draft	O
for	O
OpenPGP	O
Web	O
Key	O
Directory	O
uses	O
SRV	B-Protocol
records	I-Protocol
for	O
discovering	O
OpenPGP	O
keys	O
through	O
web	O
servers	O
.	O
</s>
<s>
Usages	O
of	O
SRV	B-Protocol
records	I-Protocol
are	O
no	O
longer	O
part	O
of	O
the	O
Internet	O
Draft	O
in	O
later	O
versions	O
.	O
</s>
<s>
A	B-Protocol
registry	O
of	O
service	O
names	O
for	O
SRV	B-Protocol
records	I-Protocol
&	O
protocols	O
is	O
maintained	O
by	O
the	O
Internet	O
Assigned	O
Numbers	O
Authority	O
(	O
IANA	O
)	O
as	O
defined	O
in	O
RFC	O
6335	O
.	O
</s>
