<s>
In	O
computer	B-Architecture
networking	I-Architecture
,	O
the	O
multicast	B-Protocol
DNS	I-Protocol
(	O
mDNS	B-Protocol
)	O
protocol	O
resolves	O
hostnames	B-Protocol
to	O
IP	O
addresses	O
within	O
small	O
networks	O
that	O
do	O
not	O
include	O
a	O
local	O
name	B-Protocol
server	I-Protocol
.	O
</s>
<s>
It	O
is	O
a	O
zero-configuration	B-Application
service	O
,	O
using	O
essentially	O
the	O
same	O
programming	O
interfaces	O
,	O
packet	O
formats	O
and	O
operating	O
semantics	O
as	O
unicast	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
(	O
DNS	B-Protocol
)	O
.	O
</s>
<s>
It	O
was	O
designed	O
to	O
work	O
as	O
either	O
a	O
stand-alone	O
protocol	O
or	O
compatibly	O
with	O
standard	O
DNS	B-Protocol
servers	I-Protocol
.	O
</s>
<s>
It	O
uses	O
IP	B-Protocol
multicast	I-Protocol
User	B-Protocol
Datagram	I-Protocol
Protocol	I-Protocol
(	O
UDP	O
)	O
packets	O
,	O
and	O
is	O
implemented	O
by	O
the	O
Apple	B-Application
Bonjour	I-Application
and	O
open	O
source	O
Avahi	B-Language
software	O
packages	O
,	O
included	O
in	O
most	O
Linux	O
distributions	O
.	O
</s>
<s>
Although	O
the	O
Windows	B-Operating_System
10	I-Operating_System
implementation	O
was	O
limited	O
to	O
discovering	O
networked	O
printers	O
,	O
subsequent	O
releases	O
resolved	O
hostnames	B-Protocol
as	O
well	O
.	O
</s>
<s>
mDNS	B-Protocol
can	O
work	O
in	O
conjunction	O
with	O
DNS	B-Protocol
Service	O
Discovery	O
(	O
DNS-SD	O
)	O
,	O
a	O
companion	O
zero-configuration	B-Application
networking	I-Application
technique	O
specified	O
separately	O
in	O
RFC	O
6763	O
.	O
</s>
<s>
Multicast	B-Protocol
DNS	I-Protocol
was	O
first	O
proposed	O
by	O
Bill	O
Woodcock	O
and	O
Bill	O
Manning	O
in	O
the	O
IETF	O
in	O
2000	O
,	O
and	O
was	O
eventually	O
published	O
as	O
standards-track	O
IETF	O
RFC	O
6762	O
by	O
Stuart	O
Cheshire	O
and	O
Marc	O
Krochmal	O
thirteen	O
years	O
later	O
.	O
</s>
<s>
When	O
an	O
mDNS	B-Protocol
client	O
needs	O
to	O
resolve	O
a	O
hostname	B-Protocol
,	O
it	O
sends	O
an	O
IP	B-Protocol
multicast	I-Protocol
query	O
message	O
that	O
asks	O
the	O
host	O
having	O
that	O
name	O
to	O
identify	O
itself	O
.	O
</s>
<s>
All	O
machines	O
in	O
that	O
subnet	O
can	O
then	O
use	O
that	O
information	O
to	O
update	O
their	O
mDNS	B-Protocol
caches	O
.	O
</s>
<s>
Any	O
host	O
can	O
relinquish	O
its	O
claim	O
to	O
a	O
name	O
by	O
sending	O
a	O
response	O
packet	O
with	O
a	O
time	B-Protocol
to	I-Protocol
live	I-Protocol
(	O
TTL	O
)	O
equal	O
to	O
zero	O
.	O
</s>
<s>
By	O
default	O
,	O
mDNS	B-Protocol
exclusively	O
resolves	O
hostnames	B-Protocol
ending	O
with	O
the	O
.local	B-Protocol
top-level	O
domain	O
.	O
</s>
<s>
This	O
can	O
cause	O
problems	O
if	O
.local	B-Protocol
includes	O
hosts	O
that	O
do	O
not	O
implement	O
mDNS	B-Protocol
but	O
that	O
can	O
be	O
found	O
via	O
a	O
conventional	O
unicast	O
DNS	B-Protocol
server	I-Protocol
.	O
</s>
<s>
Resolving	O
such	O
conflicts	O
requires	O
network-configuration	O
changes	O
that	O
mDNS	B-Protocol
was	O
designed	O
to	O
avoid	O
.	O
</s>
<s>
An	O
mDNS	B-Protocol
message	O
is	O
a	O
multicast	O
UDP	B-Protocol
packet	I-Protocol
sent	O
using	O
the	O
following	O
addressing	O
:	O
</s>
<s>
The	O
payload	O
structure	O
is	O
based	O
on	O
the	O
unicast	O
DNS	B-Protocol
packet	O
format	O
,	O
consisting	O
of	O
two	O
parts	O
—	O
the	O
header	O
and	O
the	O
data	O
.	O
</s>
<s>
The	O
header	O
is	O
identical	O
to	O
that	O
found	O
in	O
unicast	O
DNS	B-Protocol
,	O
as	O
are	O
the	O
sub-sections	O
in	O
the	O
data	O
part	O
:	O
queries	O
,	O
answers	O
,	O
authoritative-nameservers	O
,	O
and	O
additional	O
records	O
.	O
</s>
<s>
The	O
wire	O
format	O
for	O
records	O
in	O
the	O
query	O
section	O
is	O
slightly	O
modified	O
from	O
that	O
in	O
unicast	O
DNS	B-Protocol
,	O
adding	O
the	O
single-bit	O
UNICAST-RESPONSE	O
field	O
.	O
</s>
<s>
+	O
mDNS	B-Protocol
Query	O
section	O
fields	O
Field	O
Description	O
Length	O
bits	O
QNAME	O
Name	O
of	O
the	O
node	O
to	O
which	O
the	O
query	O
pertains	O
Variable	O
QTYPE	B-Protocol
The	O
type	O
of	O
the	O
query	O
,	O
i.e.	O
</s>
<s>
As	O
in	O
unicast	O
DNS	B-Protocol
,	O
the	O
QNAME	O
field	O
consists	O
of	O
a	O
series	O
of	O
length/value	O
sub-fields	O
called	O
"	O
labels	O
"	O
.	O
</s>
<s>
Each	O
label	O
represents	O
one	O
of	O
the	O
dot-separated	O
substrings	O
in	O
a	O
fully	B-Protocol
qualified	I-Protocol
domain	I-Protocol
name	I-Protocol
(	O
FQDN	B-Protocol
)	O
.	O
</s>
<s>
The	O
list	O
is	O
terminated	O
by	O
either	O
a	O
single	O
null-byte	O
representing	O
the	O
"	O
root	O
"	O
of	O
the	O
DNS	B-Protocol
,	O
or	O
by	O
a	O
byte	O
with	O
the	O
two	O
high-order	O
bits	O
set	O
(	O
value	O
192	O
)	O
to	O
signal	O
an	O
indirect	O
pointer	O
to	O
another	O
location	O
in	O
the	O
message	O
.	O
</s>
<s>
The	O
QCLASS	O
field	O
is	O
identical	O
to	O
that	O
found	O
in	O
unicast	O
DNS	B-Protocol
.	O
</s>
<s>
All	O
records	O
in	O
the	O
answers	O
,	O
authoritative-nameservers	O
,	O
and	O
additional	O
records	O
sections	O
have	O
the	O
same	O
format	O
and	O
are	O
collectively	O
known	O
as	O
Resource	O
Records	O
(	O
RR	O
)	O
.	O
</s>
<s>
Resource	O
Records	O
in	O
mDNS	B-Protocol
also	O
have	O
a	O
slightly	O
modified	O
general	O
format	O
compared	O
to	O
unicast	O
DNS	B-Protocol
:	O
</s>
<s>
+	O
mDNS	B-Protocol
Resource	O
Record	O
fields	O
Field	O
Description	O
Length	O
bits	O
RRNAME	O
Name	O
of	O
the	O
node	O
to	O
which	O
the	O
record	O
pertains	O
Variable	O
RRTYPE	B-Protocol
The	O
type	O
of	O
the	O
Resource	O
Record	O
16	O
CACHE-FLUSH	O
Boolean	O
flag	O
indicating	O
whether	O
outdated	O
cached	O
records	O
should	O
be	O
purged	O
1	O
RRCLASS	O
Class	O
code	O
,	O
1	O
a.k.a.	O
</s>
<s>
The	O
CACHE-FLUSH	O
bit	O
is	O
used	O
to	O
instruct	O
neighbor	O
nodes	O
that	O
the	O
record	O
should	O
overwrite	O
,	O
rather	O
than	O
be	O
appended	O
onto	O
,	O
any	O
existing	O
cached	O
entries	O
for	O
this	O
RRNAME	O
and	O
RRTYPE	B-Protocol
.	O
</s>
<s>
The	O
formats	O
of	O
the	O
RDATA	O
fields	O
are	O
the	O
same	O
as	O
those	O
found	O
in	O
unicast	O
DNS	B-Protocol
.	O
</s>
<s>
However	O
,	O
DNS	B-Protocol
Service	O
Discovery	O
(	O
DNS-SD	O
)	O
,	O
the	O
most	O
common	O
use-case	O
for	O
mDNS	B-Protocol
,	O
specifies	O
slight	O
modifications	O
to	O
some	O
of	O
their	O
formats	O
(	O
notably	O
TXT	O
records	O
)	O
.	O
</s>
