<s>
Extension	B-Protocol
Mechanisms	I-Protocol
for	I-Protocol
DNS	I-Protocol
(	O
EDNS	B-Protocol
)	O
is	O
a	O
specification	O
for	O
expanding	O
the	O
size	O
of	O
several	O
parameters	O
of	O
the	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
(	O
DNS	B-Protocol
)	O
protocol	O
which	O
had	O
size	O
restrictions	O
that	O
the	O
Internet	O
engineering	O
community	O
deemed	O
too	O
limited	O
for	O
increasing	O
functionality	O
of	O
the	O
protocol	O
.	O
</s>
<s>
The	O
first	O
set	O
of	O
extensions	O
was	O
published	O
in	O
1999	O
by	O
the	O
Internet	O
Engineering	O
Task	O
Force	O
as	O
,	O
also	O
known	O
as	O
EDNS0	B-Protocol
which	O
was	O
updated	O
by	O
in	O
2013	O
changing	O
abbreviation	O
slightly	O
to	O
EDNS(0 )	O
.	O
</s>
<s>
The	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
was	O
first	O
developed	O
in	O
the	O
early	O
1980s	O
.	O
</s>
<s>
The	O
restrictions	O
in	O
the	O
size	O
of	O
several	O
flags	O
fields	O
,	O
return	O
codes	O
and	O
label	O
types	O
available	O
in	O
the	O
basic	O
DNS	B-Protocol
protocol	I-Protocol
prevented	O
the	O
support	O
of	O
some	O
desirable	O
features	O
.	O
</s>
<s>
Moreover	O
,	O
DNS	B-Protocol
messages	O
carried	O
by	O
UDP	B-Protocol
were	O
restricted	O
to	O
512	O
bytes	O
,	O
not	O
considering	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
(	O
IP	O
)	O
and	O
transport	B-Architecture
layer	I-Architecture
headers	O
.	O
</s>
<s>
Resorting	O
to	O
a	O
virtual	B-Protocol
circuit	I-Protocol
transport	O
,	O
using	O
the	O
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
TCP	O
)	O
,	O
would	O
greatly	O
increase	O
overhead	O
.	O
</s>
<s>
This	O
presented	O
a	O
major	O
obstacle	O
to	O
adding	O
new	O
features	O
to	O
DNS	B-Protocol
.	O
</s>
<s>
In	O
1999	O
,	O
Paul	O
Vixie	O
proposed	O
extending	O
DNS	B-Protocol
to	O
allow	O
for	O
new	O
flags	O
and	O
response	O
codes	O
and	O
to	O
provide	O
support	O
for	O
longer	O
responses	O
in	O
a	O
framework	O
that	O
is	O
backwards	B-General_Concept
compatible	I-General_Concept
with	O
previous	O
implementations	O
.	O
</s>
<s>
Since	O
no	O
new	O
flags	O
could	O
be	O
added	O
in	O
the	O
DNS	B-Protocol
header	O
,	O
EDNS	B-Protocol
adds	O
information	O
to	O
DNS	B-Protocol
messages	O
in	O
the	O
form	O
of	O
pseudo-Resource	O
Records	O
(	O
"	O
pseudo-RR	O
"	O
s	O
)	O
included	O
in	O
the	O
"	O
additional	O
data	O
"	O
section	O
of	O
a	O
DNS	B-Protocol
message	O
.	O
</s>
<s>
EDNS	B-Protocol
introduces	O
a	O
single	O
pseudo-RR	O
type	O
:	O
OPT	O
.	O
</s>
<s>
As	O
pseudo-RRs	O
,	O
OPT	O
type	O
RRs	O
never	O
appear	O
in	O
any	O
zone	O
file	O
;	O
they	O
exist	O
only	O
in	O
messages	O
,	O
fabricated	O
by	O
the	O
DNS	B-Protocol
participants	O
.	O
</s>
<s>
The	O
mechanism	O
is	O
backward	B-General_Concept
compatible	I-General_Concept
,	O
because	O
older	O
DNS	B-Protocol
responders	O
ignore	O
any	O
RR	O
of	O
the	O
unknown	O
OPT	O
type	O
in	O
a	O
request	O
and	O
a	O
newer	O
DNS	B-Protocol
responder	O
never	O
includes	O
an	O
OPT	O
in	O
a	O
response	O
unless	O
there	O
was	O
one	O
in	O
the	O
request	O
.	O
</s>
<s>
The	O
overall	O
size	O
of	O
the	O
UDP	B-Protocol
packet	I-Protocol
and	O
the	O
version	O
number	O
(	O
at	O
present	O
0	O
)	O
are	O
contained	O
in	O
the	O
OPT	O
record	O
.	O
</s>
<s>
The	O
original	O
DNS	B-Protocol
protocol	I-Protocol
provided	O
two	O
label	O
types	O
,	O
which	O
are	O
defined	O
by	O
the	O
first	O
two	O
bits	O
in	O
DNS	B-Protocol
packets	O
(	O
RFC	O
1035	O
)	O
:	O
00	O
(	O
standard	O
label	O
)	O
and	O
11	O
(	O
compressed	O
label	O
)	O
.	O
</s>
<s>
EDNS	B-Protocol
introduces	O
the	O
label	O
type	O
01	O
as	O
extended	O
label	O
.	O
</s>
<s>
An	O
example	O
of	O
an	O
OPT	O
pseudo-record	O
,	O
as	O
displayed	O
by	O
the	O
dig	B-Protocol
command	I-Protocol
:	O
</s>
<s>
The	O
result	O
of	O
"	O
EDNS	B-Protocol
:	O
version	O
:	O
0	O
"	O
indicates	O
full	O
conformance	O
with	O
EDNS0	B-Protocol
.	O
</s>
<s>
The	O
result	O
"	O
flags	O
:	O
do	O
"	O
indicates	O
that	O
"	O
DNSSEC	B-Protocol
OK	O
"	O
is	O
set	O
.	O
</s>
<s>
EDNS	B-Protocol
is	O
essential	O
for	O
the	O
implementation	O
of	O
DNS	B-Protocol
Security	I-Protocol
Extensions	I-Protocol
(	O
DNSSEC	B-Protocol
)	O
.	O
</s>
<s>
EDNS	B-Protocol
is	O
also	O
used	O
for	O
sending	O
general	O
information	O
from	O
resolvers	O
to	O
name	O
servers	O
about	O
clients	O
 '	O
geographic	O
location	O
in	O
the	O
form	O
of	O
the	O
EDNS	B-Protocol
Client	I-Protocol
Subnet	I-Protocol
(	O
ECS	O
)	O
option	O
.	O
</s>
<s>
There	O
are	O
proposals	O
for	O
using	O
EDNS	B-Protocol
to	O
set	O
how	O
much	O
padding	O
should	O
be	O
around	O
a	O
DNS	B-Protocol
message	O
and	O
for	O
indicating	O
how	O
long	O
a	O
TCP	O
connection	O
should	O
be	O
kept	O
alive	O
.	O
</s>
<s>
In	O
practice	O
,	O
difficulties	O
can	O
arise	O
when	O
using	O
EDNS	B-Protocol
traversing	O
firewalls	O
,	O
since	O
some	O
firewalls	O
assume	O
a	O
maximum	O
DNS	B-Protocol
message	O
length	O
of	O
512	O
bytes	O
and	O
block	O
longer	O
DNS	B-Protocol
packets	O
.	O
</s>
<s>
The	O
introduction	O
of	O
EDNS	B-Protocol
made	O
feasible	O
the	O
DNS	B-Protocol
amplification	O
attack	O
,	O
a	O
type	O
of	O
reflected	O
denial-of-service	O
attack	O
,	O
since	O
EDNS	B-Protocol
facilitates	O
very	O
large	O
response	O
packets	O
compared	O
to	O
relatively	O
small	O
request	O
packets	O
.	O
</s>
<s>
The	O
IETF	O
DNS	B-Protocol
Extensions	I-Protocol
working	O
group	O
(	O
dnsext	O
)	O
has	O
finished	O
work	O
on	O
a	O
refinement	O
of	O
EDNS0	B-Protocol
,	O
which	O
has	O
been	O
published	O
as	O
RFC	O
6891	O
.	O
</s>
