<s>
Forward-confirmed	B-Protocol
reverse	I-Protocol
DNS	I-Protocol
(	O
FCrDNS	B-Protocol
)	O
,	O
also	O
known	O
as	O
full-circle	B-Protocol
reverse	I-Protocol
DNS	I-Protocol
,	O
double-reverse	B-Protocol
DNS	I-Protocol
,	O
or	O
iprev	B-Protocol
,	O
is	O
a	O
networking	O
parameter	O
configuration	O
in	O
which	O
a	O
given	O
IP	O
address	O
has	O
both	O
forward	O
(	O
name-to-address	O
)	O
and	O
reverse	O
(	O
address-to-name	O
)	O
Domain	O
Name	O
System	O
(	O
DNS	O
)	O
entries	O
that	O
match	O
each	O
other	O
.	O
</s>
<s>
A	O
FCrDNS	B-Protocol
verification	O
can	O
create	O
a	O
weak	O
form	O
of	O
authentication	O
that	O
there	O
is	O
a	O
valid	O
relationship	O
between	O
the	O
owner	O
of	O
a	O
domain	O
name	O
and	O
the	O
owner	O
of	O
the	O
network	O
that	O
has	O
been	O
given	O
an	O
IP	O
address	O
.	O
</s>
<s>
Using	O
an	O
ISP	O
's	O
mail	B-Protocol
server	I-Protocol
as	O
a	O
relay	O
may	O
solve	O
the	O
reverse	O
DNS	O
problem	O
,	O
because	O
the	O
requirement	O
is	O
the	O
forward	O
and	O
reverse	O
lookup	O
for	O
the	O
sending	O
relay	O
have	O
to	O
match	O
,	O
it	O
does	O
not	O
have	O
to	O
be	O
related	O
to	O
the	O
from-field	O
or	O
sending	O
domain	O
of	O
messages	O
it	O
relays	O
.	O
</s>
<s>
Other	O
methods	O
for	O
establishing	O
a	O
relation	O
between	O
an	O
IP	O
address	O
and	O
a	O
domain	O
in	O
email	O
are	O
the	O
Sender	B-Protocol
Policy	I-Protocol
Framework	I-Protocol
(	O
SPF	O
)	O
and	O
the	O
MX	B-Protocol
record	I-Protocol
.	O
</s>
<s>
Most	O
e-mail	O
mail	B-Protocol
transfer	I-Protocol
agents	I-Protocol
(	O
server	O
software	O
)	O
use	O
a	O
FCrDNS	B-Protocol
verification	O
and	O
if	O
there	O
is	O
a	O
valid	O
domain	O
name	O
,	O
put	O
it	O
into	O
the	O
"	O
Received	O
:	O
"	O
trace	O
header	O
field	O
.	O
</s>
<s>
Some	O
e-mail	O
mail	B-Protocol
transfer	I-Protocol
agents	I-Protocol
will	O
perform	O
FCrDNS	B-Protocol
verification	O
on	O
the	O
domain	O
name	O
given	O
on	O
the	O
SMTP	O
HELO	O
and	O
EHLO	O
commands	O
.	O
</s>
<s>
The	O
Sender	B-Protocol
Policy	I-Protocol
Framework	I-Protocol
email	O
anti-forgery	O
system	O
uses	O
a	O
FCrDNS	B-Protocol
check	O
in	O
its	O
"	O
ptr	O
:	O
"	O
mechanism	O
.	O
</s>
<s>
Some	O
e-mail	O
spam	O
filters	O
use	O
FCrDNS	B-Protocol
checks	O
as	O
an	O
authentication	O
method	O
for	O
domain	O
names	O
or	O
for	O
whitelisting	O
purposes	O
,	O
according	O
to	O
,	O
for	O
example	O
.	O
</s>
<s>
SpamCop	O
uses	O
the	O
FCrDNS	B-Protocol
check	O
,	O
which	O
sometimes	O
causes	O
problems	O
for	O
SpamCop	O
users	O
who	O
are	O
also	O
customers	O
of	O
Internet	O
service	O
providers	O
who	O
do	O
not	O
provide	O
properly	O
matching	O
DNS	O
and	O
rDNS	O
records	O
for	O
mail	B-Protocol
servers	I-Protocol
.	O
</s>
<s>
Some	O
FTP	B-Protocol
,	O
Telnet	B-Protocol
and	O
TCP	B-Protocol
Wrapper	I-Protocol
servers	O
perform	O
FCrDNS	B-Protocol
checks	O
.	O
</s>
<s>
Some	O
IRC	B-Protocol
Servers	O
perform	O
FCrDNS	B-Protocol
checks	O
to	O
prevent	O
abuse	O
.	O
</s>
