<s>
In	O
networking	B-Architecture
,	O
a	O
black	B-Application
hole	I-Application
refers	O
to	O
a	O
place	O
in	O
the	O
network	O
where	O
incoming	O
or	O
outgoing	O
traffic	O
is	O
silently	O
discarded	O
(	O
or	O
"	O
dropped	O
"	O
)	O
,	O
without	O
informing	O
the	O
source	O
that	O
the	O
data	O
did	O
not	O
reach	O
its	O
intended	O
recipient	O
.	O
</s>
<s>
When	O
examining	O
the	O
topology	B-Architecture
of	I-Architecture
the	I-Architecture
network	I-Architecture
,	O
the	O
black	B-Application
holes	I-Application
themselves	O
are	O
invisible	O
,	O
and	O
can	O
only	O
be	O
detected	O
by	O
monitoring	O
the	O
lost	O
traffic	O
;	O
hence	O
the	O
name	O
as	O
astronomical	O
Black	B-Application
holes	I-Application
cannot	O
be	O
directly	O
observed	O
.	O
</s>
<s>
The	O
most	O
common	O
form	O
of	O
black	B-Application
hole	I-Application
is	O
simply	O
an	O
IP	B-Protocol
address	I-Protocol
that	O
specifies	O
a	O
host	O
machine	O
that	O
is	O
not	O
running	O
or	O
an	O
address	O
to	O
which	O
no	O
host	O
has	O
been	O
assigned	O
.	O
</s>
<s>
Even	O
though	O
TCP/IP	B-Protocol
provides	O
a	O
means	O
of	O
communicating	O
the	O
delivery	O
failure	O
back	O
to	O
the	O
sender	O
via	O
ICMP	B-Protocol
,	O
traffic	O
destined	O
for	O
such	O
addresses	O
is	O
often	O
just	O
dropped	O
.	O
</s>
<s>
Note	O
that	O
a	O
dead	O
address	O
will	O
be	O
undetectable	O
only	O
to	O
protocols	O
that	O
are	O
both	O
connectionless	O
and	O
unreliable	O
(	O
e.g.	O
,	O
UDP	B-Protocol
)	O
.	O
</s>
<s>
Connection-oriented	O
or	O
reliable	O
protocols	O
(	O
TCP	O
,	O
RUDP	B-Protocol
)	O
will	O
either	O
fail	O
to	O
connect	O
to	O
a	O
dead	O
address	O
or	O
will	O
fail	O
to	O
receive	O
expected	O
acknowledgements	O
.	O
</s>
<s>
For	O
IPv6	B-Protocol
,	O
the	O
black	B-Application
hole	I-Application
prefix	O
is	O
described	O
by	O
.	O
</s>
<s>
For	O
IPv4	B-Protocol
,	O
no	O
black	B-Application
hole	I-Application
address	O
is	O
explicitly	O
defined	O
,	O
however	O
the	O
reserved	B-Protocol
IP	I-Protocol
addresses	I-Protocol
can	O
help	O
achieve	O
a	O
similar	O
effect	O
.	O
</s>
<s>
Most	O
firewalls	B-Application
(	O
and	O
routers	B-Protocol
for	O
household	O
use	O
)	O
can	O
be	O
configured	O
to	O
silently	O
discard	O
packets	B-Protocol
addressed	O
to	O
forbidden	O
hosts	O
or	O
ports	O
,	O
resulting	O
in	O
small	O
or	O
large	O
"	O
black	B-Application
holes	I-Application
"	O
in	O
the	O
network	O
.	O
</s>
<s>
Personal	B-Protocol
firewalls	I-Protocol
that	O
do	O
not	O
respond	O
to	O
ICMP	B-Protocol
echo	O
requests	O
(	O
"	O
ping	O
"	O
)	O
have	O
been	O
designated	O
by	O
some	O
vendors	O
as	O
being	O
in	O
"	O
stealth	O
mode	O
"	O
.	O
</s>
<s>
Despite	O
this	O
,	O
in	O
most	O
networks	O
the	O
IP	B-Protocol
addresses	I-Protocol
of	O
hosts	O
with	O
firewalls	B-Application
configured	O
in	O
this	O
way	O
are	O
easily	O
distinguished	O
from	O
invalid	O
or	O
otherwise	O
unreachable	O
IP	B-Protocol
addresses	I-Protocol
:	O
On	O
encountering	O
the	O
latter	O
,	O
a	O
router	B-Protocol
will	O
generally	O
respond	O
with	O
an	O
ICMP	B-Protocol
network	O
rsp	O
.	O
</s>
<s>
Network	B-Protocol
address	I-Protocol
translation	I-Protocol
(	O
NAT	O
)	O
,	O
as	O
used	O
in	O
home	O
and	O
office	O
routers	B-Protocol
,	O
is	O
generally	O
a	O
more	O
effective	O
way	O
of	O
obscuring	O
the	O
layout	O
of	O
an	O
internal	O
network	O
.	O
</s>
<s>
A	O
null	B-Protocol
route	I-Protocol
or	O
black	B-Application
hole	I-Application
route	O
is	O
a	O
network	O
route	O
(	O
routing	B-Protocol
table	I-Protocol
entry	O
)	O
that	O
goes	O
nowhere	O
.	O
</s>
<s>
Matching	O
packets	B-Protocol
are	O
dropped	O
(	O
ignored	O
)	O
rather	O
than	O
forwarded	O
,	O
acting	O
as	O
a	O
kind	O
of	O
very	O
limited	O
firewall	B-Application
.	O
</s>
<s>
The	O
act	O
of	O
using	O
null	B-Protocol
routes	I-Protocol
is	O
often	O
called	O
blackhole	B-Protocol
filtering	I-Protocol
.	O
</s>
<s>
The	O
rest	O
of	O
this	O
article	O
deals	O
with	O
null	B-Protocol
routing	I-Protocol
in	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
(	O
IP	O
)	O
.	O
</s>
<s>
Black	B-Application
hole	I-Application
filtering	O
refers	O
specifically	O
to	O
dropping	O
packets	B-Protocol
at	O
the	O
routing	O
level	O
,	O
usually	O
using	O
a	O
routing	B-Protocol
protocol	I-Protocol
to	O
implement	O
the	O
filtering	O
on	O
several	O
routers	B-Protocol
at	O
once	O
,	O
often	O
dynamically	O
to	O
respond	O
quickly	O
to	O
distributed	O
denial-of-service	O
attacks	O
.	O
</s>
<s>
Remote	O
Triggered	O
Black	B-Application
Hole	I-Application
Filtering	O
(	O
RTBH	O
)	O
is	O
a	O
technique	O
that	O
provides	O
the	O
ability	O
to	O
drop	O
undesirable	O
traffic	O
before	O
it	O
enters	O
a	O
protected	O
network	O
.	O
</s>
<s>
Null	B-Protocol
routes	I-Protocol
are	O
typically	O
configured	O
with	O
a	O
special	O
route	O
flag	O
,	O
but	O
can	O
also	O
be	O
implemented	O
by	O
forwarding	O
packets	B-Protocol
to	O
an	O
illegal	O
IP	B-Protocol
address	I-Protocol
such	O
as	O
,	O
or	O
the	O
loopback	B-Protocol
address	I-Protocol
.	O
</s>
<s>
Null	B-Protocol
routing	I-Protocol
has	O
an	O
advantage	O
over	O
classic	O
firewalls	B-Application
since	O
it	O
is	O
available	O
on	O
every	O
potential	O
network	B-Protocol
router	I-Protocol
(	O
including	O
all	O
modern	O
operating	O
systems	O
)	O
,	O
and	O
adds	O
virtually	O
no	O
performance	O
impact	O
.	O
</s>
<s>
Due	O
to	O
the	O
nature	O
of	O
high-bandwidth	O
routers	B-Protocol
,	O
null	B-Protocol
routing	I-Protocol
can	O
often	O
sustain	O
higher	O
throughput	O
than	O
conventional	O
firewalls	B-Application
.	O
</s>
<s>
For	O
this	O
reason	O
,	O
null	B-Protocol
routes	I-Protocol
are	O
often	O
used	O
on	O
high-performance	O
core	B-Protocol
routers	I-Protocol
to	O
mitigate	O
large-scale	O
denial-of-service	O
attacks	O
before	O
the	O
packets	B-Protocol
reach	O
a	O
bottleneck	O
,	O
thus	O
avoiding	O
collateral	O
damage	O
from	O
DDoS	O
attacks	O
although	O
the	O
target	O
of	O
the	O
attack	O
will	O
be	O
inaccessible	O
to	O
anyone	O
.	O
</s>
<s>
Blackhole	B-Protocol
filtering	I-Protocol
can	O
also	O
be	O
abused	O
by	O
malicious	O
attackers	O
on	O
compromised	O
routers	B-Protocol
to	O
filter	O
out	O
traffic	O
destined	O
to	O
a	O
certain	O
address	O
.	O
</s>
<s>
Routing	O
typically	O
only	O
works	O
on	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
layer	O
and	O
is	O
very	O
limited	O
in	O
packet	B-Protocol
classification	O
.	O
</s>
<s>
It	O
is	O
bound	O
to	O
be	O
stateless	B-Application
due	O
to	O
the	O
nature	O
of	O
IP	B-Protocol
routers	I-Protocol
.	O
</s>
<s>
Typically	O
,	O
classification	O
is	O
limited	O
to	O
the	O
destination	O
IP	B-Protocol
address	I-Protocol
prefix	I-Protocol
,	O
source	O
IP	B-Protocol
address	I-Protocol
and	O
incoming	O
network	B-Protocol
interface	I-Protocol
.	O
</s>
<s>
A	O
DNS-based	O
Blackhole	O
List	O
(	O
DNSBL	O
)	O
or	O
Real-time	O
Blackhole	O
List	O
(	O
RBL	O
)	O
is	O
a	O
list	O
of	O
IP	B-Protocol
addresses	I-Protocol
published	O
through	O
the	O
Internet	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
(	O
DNS	B-Protocol
)	O
either	O
as	O
a	O
zone	O
file	O
that	O
can	O
be	O
used	O
by	O
DNS	B-Protocol
server	O
software	O
,	O
or	O
as	O
a	O
live	O
DNS	B-Protocol
zone	O
that	O
can	O
be	O
queried	O
in	O
real-time	O
.	O
</s>
<s>
DNSBLs	O
are	O
most	O
often	O
used	O
to	O
publish	O
the	O
addresses	O
of	O
computers	O
or	O
networks	O
linked	O
to	O
spamming	O
;	O
most	O
mail	B-Protocol
server	I-Protocol
software	O
can	O
be	O
configured	O
to	O
reject	O
or	O
flag	O
messages	O
which	O
have	O
been	O
sent	O
from	O
a	O
site	O
listed	O
on	O
one	O
or	O
more	O
such	O
lists	O
.	O
</s>
<s>
Some	O
firewalls	B-Application
incorrectly	O
discard	O
all	O
ICMP	B-Protocol
packets	B-Protocol
,	O
including	O
the	O
ones	O
needed	O
for	O
Path	B-Protocol
MTU	I-Protocol
discovery	I-Protocol
to	O
work	O
correctly	O
.	O
</s>
<s>
This	O
causes	O
TCP	O
connections	O
from/to/through	O
hosts	O
with	O
a	O
lower	O
MTU	B-Protocol
to	O
hang	O
.	O
</s>
<s>
A	O
black	B-Application
hole	I-Application
e-mail	O
address	O
is	O
an	O
e-mail	O
address	O
which	O
is	O
valid	O
(	O
messages	O
sent	O
to	O
it	O
will	O
not	O
generate	O
errors	O
)	O
,	O
but	O
all	O
the	O
received	O
messages	O
are	O
automatically	O
deleted	O
,	O
and	O
never	O
stored	O
or	O
seen	O
by	O
humans	O
.	O
</s>
