<s>
A	O
wildcard	B-Protocol
DNS	I-Protocol
record	I-Protocol
is	O
a	O
record	O
in	O
a	O
DNS	B-Protocol
zone	I-Protocol
that	O
will	O
match	O
requests	O
for	O
non-existent	O
domain	O
names	O
.	O
</s>
<s>
A	O
wildcard	B-Protocol
DNS	I-Protocol
record	I-Protocol
is	O
specified	O
by	O
using	O
a	O
*	O
as	O
the	O
leftmost	O
label	O
(	O
part	O
)	O
of	O
a	O
domain	O
name	O
,	O
e.g.	O
</s>
<s>
The	O
exact	O
rules	O
for	O
when	O
a	O
wildcard	B-Algorithm
will	O
match	O
are	O
specified	O
in	O
,	O
but	O
the	O
rules	O
are	O
neither	O
intuitive	O
nor	O
clearly	O
specified	O
.	O
</s>
<s>
A	O
wildcard	B-Protocol
DNS	I-Protocol
record	I-Protocol
in	O
a	O
zone	B-Protocol
file	I-Protocol
looks	O
similar	O
to	O
this	O
example	O
:	O
</s>
<s>
This	O
wildcard	B-Protocol
DNS	I-Protocol
record	I-Protocol
will	O
cause	O
DNS	O
lookups	O
on	O
domain	O
names	O
ending	O
in	O
example.com	O
that	O
do	O
not	O
exist	O
to	O
have	O
MX	O
records	O
synthesized	O
for	O
them	O
.	O
</s>
<s>
Wildcards	B-Algorithm
in	O
the	O
DNS	O
are	O
much	O
more	O
limited	O
than	O
other	O
wildcard	B-Algorithm
characters	I-Algorithm
used	O
in	O
other	O
computer	O
systems	O
.	O
</s>
<s>
Wildcard	B-Protocol
DNS	I-Protocol
records	I-Protocol
have	O
a	O
single	O
*	O
(	O
asterisk	O
)	O
as	O
the	O
leftmost	O
DNS	O
label	O
,	O
such	O
as	O
*	O
.example.com	O
.	O
</s>
<s>
Asterisks	O
at	O
other	O
places	O
in	O
the	O
domain	O
will	O
not	O
work	O
as	O
a	O
wildcard	B-Algorithm
,	O
so	O
neither	O
*	O
abc.example.com	O
nor	O
abc	O
.	O
</s>
<s>
*	O
.example.com	O
work	O
as	O
wildcard	B-Protocol
DNS	I-Protocol
records	I-Protocol
.	O
</s>
<s>
Moreover	O
,	O
the	O
wildcard	B-Algorithm
is	O
matched	O
only	O
when	O
a	O
domain	O
does	O
not	O
exist	O
,	O
not	O
just	O
when	O
there	O
are	O
no	O
matching	O
records	O
of	O
the	O
type	O
that	O
has	O
been	O
queried	O
for	O
.	O
</s>
<s>
Even	O
the	O
definition	O
of	O
"	O
does	O
not	O
exist	O
"	O
as	O
defined	O
in	O
the	O
search	O
algorithm	O
of	O
section	O
4.3.3	O
can	O
result	O
in	O
the	O
wildcard	B-Algorithm
not	O
matching	O
cases	O
that	O
one	O
might	O
expect	O
with	O
other	O
types	O
of	O
wildcards	B-Algorithm
.	O
</s>
<s>
The	O
original	O
definition	O
of	O
how	O
a	O
DNS	O
wildcard	B-Algorithm
behaves	O
is	O
specified	O
in	O
sections	O
4.3.2	O
and	O
4.3.3	O
,	O
but	O
only	O
indirectly	O
by	O
certain	O
steps	O
in	O
a	O
search	O
algorithm	O
and	O
as	O
a	O
result	O
,	O
the	O
rules	O
are	O
neither	O
intuitive	O
nor	O
clearly	O
specified	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
20	O
years	O
later	O
,	O
,	O
"	O
The	O
Role	O
of	O
Wildcards	B-Algorithm
in	O
the	O
Domain	O
Name	O
System	O
"	O
was	O
written	O
to	O
help	O
clarify	O
the	O
rules	O
.	O
</s>
<s>
To	O
quote	O
,	O
"	O
A	O
common	O
mistake	O
is	O
thinking	O
that	O
a	O
wildcard	B-Algorithm
MX	O
for	O
a	O
zone	O
will	O
apply	O
to	O
all	O
hosts	O
in	O
the	O
zone	O
.	O
</s>
<s>
A	O
wildcard	B-Algorithm
MX	O
will	O
apply	O
only	O
to	O
names	O
in	O
the	O
zone	O
which	O
are	O
n't	O
listed	O
in	O
the	O
DNS	O
at	O
all.	O
"	O
</s>
<s>
That	O
is	O
,	O
if	O
there	O
is	O
a	O
wildcard	B-Algorithm
MX	O
for	O
*	O
.example.com	O
,	O
and	O
an	O
A	O
record	O
(	O
but	O
no	O
MX	O
record	O
)	O
for	O
www.example.com	O
,	O
the	O
correct	O
response	O
(	O
as	O
per	O
)	O
to	O
an	O
MX	O
request	O
for	O
www.example.com	O
is	O
"	O
no	O
error	O
,	O
but	O
no	O
data	O
"	O
;	O
this	O
is	O
in	O
contrast	O
to	O
the	O
possibly	O
expected	O
response	O
of	O
the	O
MX	O
record	O
attached	O
to	O
*	O
.example.com	O
.	O
</s>
<s>
The	O
following	O
example	O
is	O
from	O
section	O
2.2.1	O
and	O
is	O
useful	O
in	O
clarifying	O
how	O
wildcards	B-Algorithm
work	O
.	O
</s>
<s>
Say	O
there	O
is	O
a	O
DNS	B-Protocol
zone	I-Protocol
with	O
the	O
following	O
resource	O
records	O
:	O
</s>
<s>
The	O
following	O
responses	O
would	O
be	O
synthesized	O
from	O
one	O
of	O
the	O
wildcards	B-Algorithm
in	O
the	O
zone	O
:	O
</s>
<s>
does	O
not	O
exist	O
,	O
but	O
the	O
wildcard	B-Algorithm
does	O
.	O
</s>
<s>
The	O
following	O
responses	O
would	O
not	O
be	O
synthesized	O
from	O
any	O
of	O
the	O
wildcards	B-Algorithm
in	O
the	O
zone	O
:	O
</s>
<s>
MX	O
No	O
wildcard	B-Algorithm
will	O
match	O
because	O
host1.example	O
.	O
</s>
<s>
The	O
wildcard	B-Algorithm
MX	O
record	O
does	O
not	O
provide	O
MX	O
records	O
for	O
domains	O
that	O
otherwise	O
exist	O
.	O
</s>
<s>
MX	O
No	O
wildcard	B-Algorithm
will	O
match	O
because	O
sub.*	O
.example	O
.	O
</s>
<s>
will	O
never	O
act	O
as	O
a	O
wildcard	B-Algorithm
,	O
even	O
though	O
it	O
has	O
an	O
asterisk	O
in	O
it	O
.	O
</s>
<s>
SRV	O
No	O
wildcard	B-Algorithm
will	O
match	O
because	O
_tcp.host1.example	O
.	O
</s>
<s>
A	O
No	O
wildcard	B-Algorithm
will	O
match	O
because	O
subdel.example	O
.	O
</s>
<s>
into	O
a	O
different	O
DNS	B-Protocol
zone	I-Protocol
.	O
</s>
<s>
does	O
not	O
exist	O
in	O
the	O
other	O
zone	O
,	O
a	O
wildcard	B-Algorithm
will	O
not	O
be	O
used	O
from	O
the	O
parent	O
zone	O
.	O
</s>
<s>
MX	O
No	O
wildcard	B-Algorithm
will	O
match	O
because	O
*	O
.example	O
.	O
</s>
<s>
exists	O
,	O
it	O
is	O
a	O
wildcard	B-Algorithm
domain	O
,	O
but	O
it	O
still	O
exists	O
.	O
</s>
<s>
The	O
final	O
example	O
highlights	O
one	O
common	O
misconception	O
about	O
wildcards	B-Algorithm
.	O
</s>
<s>
A	O
wildcard	B-Algorithm
"	O
blocks	O
itself	O
"	O
in	O
the	O
sense	O
that	O
a	O
wildcard	B-Algorithm
does	O
not	O
match	O
its	O
own	O
subdomains	O
.	O
</s>
<s>
zone	O
;	O
it	O
fails	O
to	O
match	O
the	O
names	O
below	O
*	O
example	O
..	O
To	O
cover	O
names	O
under	O
*	O
.example.	O
,	O
another	O
wildcard	B-Algorithm
domain	O
name	O
is	O
needed*	O
.	O
</s>
<s>
To	O
quote	O
from	O
,	O
many	O
DNS	O
implementations	O
diverge	O
,	O
in	O
different	O
ways	O
,	O
from	O
the	O
original	O
definition	O
of	O
wildcards	B-Algorithm
.	O
</s>
<s>
With	O
djbdns	B-Protocol
,	O
in	O
addition	O
to	O
checking	O
for	O
wildcards	B-Algorithm
at	O
the	O
current	O
level	O
,	O
the	O
server	O
checks	O
for	O
wildcards	B-Algorithm
in	O
all	O
enclosing	O
superdomains	O
,	O
all	O
of	O
the	O
way	O
up	O
to	O
the	O
root	O
.	O
</s>
<s>
In	O
the	O
examples	O
listed	O
above	O
,	O
the	O
query	O
for	O
_telnet._tcp.host1.example	O
for	O
an	O
MX	O
record	O
would	O
match	O
a	O
wildcard	B-Algorithm
despite	O
the	O
domain	O
_tcp.host1.example	O
existing	O
.	O
</s>
<s>
Microsoft	B-Protocol
's	I-Protocol
DNS	I-Protocol
server	I-Protocol
(	O
if	O
configured	O
to	O
do	O
so	O
)	O
and	O
MaraDNS	B-Protocol
(	O
by	O
default	O
)	O
have	O
wildcards	B-Algorithm
also	O
match	O
all	O
requests	O
for	O
empty	O
resource	O
record	O
sets	O
;	O
i.e.	O
,	O
domain	O
names	O
for	O
which	O
there	O
are	O
no	O
records	O
of	O
the	O
desired	O
type	O
.	O
</s>
<s>
*	O
.example	O
explicitly	O
existing	O
with	O
only	O
a	O
TXT	B-Protocol
Record	I-Protocol
.	O
</s>
<s>
Wildcard	B-Algorithm
domains	O
are	O
widely	O
used	O
by	O
blogging	O
websites	O
that	O
allow	O
users	O
to	O
create	O
sub-domains	O
upon	O
demand	O
;	O
e.g.	O
,	O
sites	O
such	O
as	O
WordPress	B-Application
or	O
Blogspot	B-Application
.	O
</s>
<s>
Another	O
popular	O
use	O
is	O
by	O
Free	O
Dynamic	B-Protocol
DNS	I-Protocol
websites	O
that	O
allow	O
users	O
to	O
create	O
a	O
DNS	O
name	O
that	O
changes	O
to	O
match	O
their	O
host	O
IP	O
as	O
the	O
IP	O
address	O
is	O
changed	O
periodically	O
by	O
their	O
ISP	O
's	O
DHCP	O
server	O
.	O
</s>
<s>
New	O
gTLDs	O
are	O
prohibited	O
from	O
publishing	O
wildcards	B-Algorithm
(	O
or	O
using	O
equivalent	O
name	O
server	O
mechanisms	O
)	O
by	O
of	O
the	O
ICANN	B-Application
New	O
gTLD	O
Base	O
Registry	O
agreement	O
.	O
</s>
<s>
However	O
,	O
ICANN	B-Application
's	O
)	O
,	O
explicitly	O
requires	O
new	O
gTLDs	O
to	O
publish	O
(	O
for	O
at	O
least	O
90	O
days	O
)	O
special	O
MX	O
,	O
SRV	O
,	O
TXT	O
,	O
and	O
127.0.53.53	O
A	O
record	O
wildcards	B-Algorithm
that	O
warn	O
of	O
potential	O
name	O
collisions	O
due	O
to	O
use	O
of	O
relative	O
domain	O
names	O
with	O
domain	B-Protocol
search	I-Protocol
paths	I-Protocol
.	O
</s>
<s>
Several	O
domain	B-Application
name	I-Application
registrars	I-Application
have	O
,	O
at	O
various	O
times	O
,	O
deployed	O
wildcard	B-Algorithm
records	O
for	O
the	O
top-level	O
domains	O
to	O
provide	O
a	O
platform	O
for	O
advertising	O
,	O
most	O
notably	O
VeriSign	B-Application
for	O
.com	O
and	O
.net	B-Application
with	O
its	O
(	O
now	O
removed	O
)	O
Site	B-Protocol
Finder	I-Protocol
system	O
.	O
</s>
<s>
The	O
.museum	O
TLD	O
also	O
had	O
a	O
wildcard	B-Algorithm
record	O
which	O
has	O
now	O
been	O
removed	O
.	O
</s>
<s>
,	O
top-level	O
domains	O
using	O
a	O
wildcard	B-Algorithm
A	O
record	O
(	O
other	O
than	O
127.0.53.53	O
)	O
are	O
.fm	B-Protocol
,	O
.la	O
,	O
.ph	B-General_Concept
,	O
.pw	O
,	O
.vg	O
and	O
.ws	O
.	O
</s>
<s>
The	O
internationalized	B-Language
TLDs	O
.中国	O
(	O
or	O
for	O
"	O
China	O
"	O
)	O
and	O
.გე	O
(	O
for	O
the	O
Georgian	O
letters	O
for	O
the	O
Georgian	O
country	O
code	O
"	O
GE	O
"	O
)	O
also	O
have	O
wildcard	B-Algorithm
A	O
records	O
.	O
</s>
<s>
The	O
*	O
.中国	O
wildcard	B-Algorithm
resolves	O
to	O
ibaidu.com	O
(	O
flagged	O
by	O
Chrome	O
as	O
unsafe	O
)	O
,	O
and	O
the	O
*	O
.გე	O
wildcard	B-Algorithm
resolves	O
to	O
a	O
website	O
of	O
the	O
.ge	B-General_Concept
TLD	O
.	O
</s>
<s>
It	O
has	O
also	O
become	O
common	O
for	O
ISPs	O
to	O
synthesize	O
address	O
records	O
for	O
typos	O
,	O
for	O
the	O
same	O
person	O
,	O
a	O
practice	O
called	O
"	O
catchall	O
"	O
typosquatting	O
,	O
but	O
these	O
are	O
n't	O
true	O
wildcards	B-Algorithm
,	O
but	O
rather	O
modified	O
caching	O
name	O
servers	O
.	O
</s>
<s>
The	O
Internet	O
Software	O
Consortium	O
produced	O
a	O
version	O
of	O
the	O
BIND	B-Protocol
DNS	O
software	O
that	O
can	O
be	O
configured	O
to	O
filter	O
out	O
wildcard	B-Protocol
DNS	I-Protocol
records	I-Protocol
from	O
specific	O
domains	O
.	O
</s>
<s>
Various	O
developers	O
have	O
produced	O
software	O
patches	O
for	O
BIND	B-Protocol
and	O
for	O
djbdns	B-Protocol
.	O
</s>
<s>
Other	O
DNS	O
server	O
programs	O
have	O
followed	O
suit	O
,	O
providing	O
the	O
ability	O
to	O
ignore	O
wildcard	B-Protocol
DNS	I-Protocol
records	I-Protocol
as	O
configured	O
.	O
</s>
