<s>
Primarily	O
it	O
enables	O
the	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
(	O
DNS	B-Protocol
)	O
to	O
authenticate	O
updates	O
to	O
a	O
DNS	B-Protocol
database	O
.	O
</s>
<s>
It	O
is	O
most	O
commonly	O
used	O
to	O
update	O
Dynamic	B-Protocol
DNS	I-Protocol
or	O
a	O
secondary/slave	O
DNS	B-Protocol
server	O
.	O
</s>
<s>
TSIG	B-Protocol
uses	O
shared	O
secret	O
keys	O
and	O
one-way	O
hashing	O
to	O
provide	O
a	O
cryptographically	O
secure	O
means	O
of	O
authenticating	O
each	O
endpoint	O
of	O
a	O
connection	O
as	O
being	O
allowed	O
to	O
make	O
or	O
respond	O
to	O
a	O
DNS	B-Protocol
update	O
.	O
</s>
<s>
Although	O
queries	O
to	O
DNS	B-Protocol
may	O
usually	O
be	O
made	O
without	O
authentication	O
,	O
updates	O
to	O
DNS	B-Protocol
must	O
be	O
authenticated	O
,	O
since	O
they	O
make	O
lasting	O
changes	O
to	O
the	O
structure	O
of	O
the	O
Internet	O
naming	O
system	O
.	O
</s>
<s>
The	O
use	O
of	O
a	O
key	O
shared	O
by	O
the	O
client	O
making	O
the	O
update	O
and	O
the	O
DNS	B-Protocol
server	O
helps	O
to	O
ensure	O
the	O
authenticity	O
and	O
integrity	O
of	O
the	O
update	O
request	O
.	O
</s>
<s>
A	O
timestamp	O
is	O
included	O
in	O
the	O
TSIG	B-Protocol
protocol	O
to	O
prevent	O
recorded	O
responses	O
from	O
being	O
reused	O
,	O
which	O
would	O
allow	O
an	O
attacker	O
to	O
breach	O
the	O
security	O
of	O
TSIG	B-Protocol
.	O
</s>
<s>
This	O
places	O
a	O
requirement	O
on	O
dynamic	B-Protocol
DNS	I-Protocol
servers	O
and	O
TSIG	B-Protocol
clients	O
to	O
contain	O
an	O
accurate	O
clock	O
.	O
</s>
<s>
Since	O
DNS	B-Protocol
servers	O
are	O
connected	O
to	O
a	O
network	O
,	O
the	O
Network	B-Protocol
Time	I-Protocol
Protocol	I-Protocol
can	O
provide	O
an	O
accurate	O
time	O
source	O
.	O
</s>
<s>
DNS	B-Protocol
updates	O
,	O
like	O
queries	O
,	O
are	O
normally	O
transported	O
via	O
UDP	B-Protocol
since	O
it	O
requires	O
lower	O
overhead	O
than	O
TCP	B-Protocol
.	O
</s>
<s>
However	O
,	O
DNS	B-Protocol
servers	O
support	O
both	O
UDP	B-Protocol
and	O
TCP	B-Protocol
requests	O
.	O
</s>
<s>
An	O
update	O
,	O
as	O
specified	O
in	O
RFC	O
2136	O
,	O
is	O
a	O
set	O
of	O
instructions	O
to	O
a	O
DNS	B-Protocol
server	O
.	O
</s>
<s>
TSIG	B-Protocol
adds	O
a	O
final	O
record	O
,	O
which	O
includes	O
a	O
timestamp	O
and	O
the	O
hash	O
of	O
the	O
request	O
.	O
</s>
<s>
The	O
response	O
to	O
a	O
successful	O
TSIG	B-Protocol
update	O
will	O
also	O
be	O
signed	O
with	O
a	O
TSIG	B-Protocol
record	O
.	O
</s>
<s>
Failures	O
are	O
not	O
signed	O
to	O
prevent	O
an	O
attacker	O
from	O
learning	O
anything	O
about	O
the	O
TSIG	B-Protocol
key	O
using	O
specially	O
crafted	O
update	O
"	O
probes	O
"	O
.	O
</s>
<s>
The	O
nsupdate	B-Protocol
program	O
can	O
use	O
TSIG	B-Protocol
to	O
do	O
DNS	B-Protocol
updates	O
.	O
</s>
<s>
The	O
TSIG	B-Protocol
record	O
is	O
in	O
the	O
same	O
format	O
as	O
the	O
other	O
records	O
in	O
the	O
update	O
request	O
.	O
</s>
<s>
+	O
TSIG	B-Protocol
record	O
fields	O
Field	O
Bytes	O
Value	O
Description	O
NAME	O
Max	O
.	O
</s>
<s>
Although	O
TSIG	B-Protocol
is	O
widely	O
deployed	O
,	O
there	O
are	O
several	O
problems	O
with	O
the	O
protocol	O
:	O
</s>
<s>
Although	O
still	O
in	O
common	O
usage	O
,	O
the	O
HMAC-MD5	B-Algorithm
digest	O
is	O
no	O
longer	O
considered	O
very	O
secure	O
.	O
</s>
<s>
HMAC-SHA256	B-Algorithm
is	O
preferred	O
.	O
</s>
<s>
RFC	O
2137	O
specifies	O
an	O
update	O
method	O
using	O
a	O
public	B-Application
key	I-Application
"	O
SIG	O
"	O
DNS	B-Protocol
record	I-Protocol
.	O
</s>
<s>
A	O
client	O
holding	O
the	O
corresponding	O
private	B-Application
key	I-Application
can	O
sign	O
the	O
update	O
request	O
.	O
</s>
<s>
This	O
method	O
matches	O
the	O
DNSSEC	B-Protocol
method	O
for	O
secure	O
queries	O
.	O
</s>
<s>
In	O
,	O
RFC	O
3645	O
proposed	O
extending	O
TSIG	B-Protocol
to	O
allow	O
the	O
Generic	O
Security	O
Service	O
(	O
GSS	O
)	O
method	O
of	O
secure	O
key	O
exchange	O
,	O
eliminating	O
the	O
need	O
for	O
manually	O
distributing	O
keys	O
to	O
all	O
TSIG	B-Protocol
clients	O
.	O
</s>
<s>
The	O
method	O
for	O
distributing	O
public	B-Application
keys	I-Application
as	O
a	O
DNS	B-Protocol
resource	O
record	O
(	O
RR	O
)	O
is	O
specified	O
in	O
RFC	O
2930	O
,	O
with	O
GSS	O
as	O
one	O
mode	O
of	O
this	O
method	O
.	O
</s>
<s>
A	O
modified	B-Protocol
GSS-TSIG	I-Protocol
-	O
using	O
the	O
Windows	O
Kerberos	B-Protocol
Server	O
-	O
was	O
implemented	O
by	O
Microsoft	B-Application
Windows	I-Application
Active	B-Protocol
Directory	I-Protocol
servers	O
and	O
clients	O
called	O
Secure	O
Dynamic	O
Update	O
.	O
</s>
<s>
In	O
combination	O
with	O
poorly	O
configured	O
DNS	B-Protocol
(	O
with	O
no	O
reverse	O
lookup	O
zone	O
)	O
using	O
RFC	O
1918	O
addressing	O
,	O
reverse	O
DNS	B-Protocol
updates	O
using	O
this	O
authentication	O
scheme	O
are	O
forwarded	O
en	O
masse	O
to	O
the	O
root	O
DNS	B-Protocol
servers	O
and	O
thus	O
increase	O
the	O
traffic	O
to	O
root	O
DNS	B-Protocol
servers	O
.	O
</s>
<s>
There	O
is	O
an	O
anycast	B-Protocol
group	O
which	O
deals	O
with	O
this	O
traffic	O
to	O
take	O
it	O
away	O
from	O
the	O
root	O
DNS	B-Protocol
servers	O
.	O
</s>
<s>
RFC	O
2845	O
defines	O
TSIG	B-Protocol
,	O
specifies	O
only	O
one	O
allowed	O
hashing	O
function	O
,	O
the	O
128-bit	O
HMAC-MD5	B-Algorithm
,	O
which	O
is	O
no	O
longer	O
considered	O
to	O
be	O
highly	O
secure	O
.	O
</s>
<s>
RFC	O
4635	O
was	O
circulated	O
to	O
allow	O
RFC	O
3174	O
Secure	O
Hash	O
Algorithm	O
(	O
SHA1	O
)	O
hashing	O
and	O
FIPS	O
PUB	O
180-2	O
SHA-2	B-Algorithm
hashing	O
to	O
replace	O
MD5	B-Algorithm
.	O
</s>
<s>
The	O
160-bit	O
and	O
256-bit	O
digests	O
generated	O
by	O
SHA1	O
and	O
SHA-2	B-Algorithm
are	O
more	O
secure	O
than	O
the	O
128-bit	O
digest	O
generated	O
by	O
MD5	B-Algorithm
.	O
</s>
<s>
RFC	O
2930	O
defines	O
TKEY	B-Protocol
,	O
a	O
DNS	B-Protocol
record	I-Protocol
used	O
to	O
distribute	O
keys	O
automatically	O
from	O
a	O
DNS	B-Protocol
server	O
to	O
DNS	B-Protocol
clients	O
.	O
</s>
<s>
RFC	O
3645	O
defines	O
GSS-TSIG	B-Protocol
,	O
which	O
uses	O
gss-api	O
and	O
TKEY	B-Protocol
to	O
distribute	O
keys	O
automatically	O
in	O
gss-api	O
mode	O
.	O
</s>
<s>
The	O
DNSCurve	B-Protocol
proposal	O
has	O
many	O
similarities	O
to	O
TSIG	B-Protocol
.	O
</s>
