<s>
TCP	B-Protocol
Wrappers	I-Protocol
(	O
also	O
known	O
as	O
tcp_wrappers	O
)	O
is	O
a	O
host-based	O
networking	O
ACL	O
system	O
,	O
used	O
to	O
filter	B-Application
network	O
access	O
to	O
Internet	B-Protocol
Protocol	I-Protocol
servers	O
on	O
(	O
Unix-like	B-Operating_System
)	O
operating	B-General_Concept
systems	I-General_Concept
such	O
as	O
Linux	B-Application
or	O
BSD	B-Operating_System
.	O
</s>
<s>
It	O
allows	O
host	O
or	O
subnetwork	B-Protocol
IP	B-Protocol
addresses	I-Protocol
,	O
names	B-Protocol
and/or	O
ident	O
query	O
replies	O
,	O
to	O
be	O
used	O
as	O
tokens	O
on	O
which	O
to	O
filter	B-Application
for	O
access	O
control	O
purposes	O
.	O
</s>
<s>
The	O
original	O
code	O
was	O
written	O
by	O
Wietse	O
Venema	O
in	O
1990	O
to	O
monitor	O
a	O
cracker	O
's	O
activities	O
on	O
the	O
Unix	B-Application
workstations	O
at	O
the	O
Department	O
of	O
Math	O
and	O
Computer	O
Science	O
at	O
the	O
Eindhoven	O
University	O
of	O
Technology	O
.	O
</s>
<s>
He	O
maintained	O
it	O
until	O
1995	O
,	O
and	O
on	O
June	O
1	O
,	O
2001	O
,	O
released	O
it	O
under	O
its	O
own	O
BSD-style	B-Operating_System
license	I-Operating_System
.	O
</s>
<s>
The	O
tarball	B-Application
includes	O
a	O
library	B-Library
named	O
libwrap	B-Protocol
that	O
implements	O
the	O
actual	O
functionality	O
.	O
</s>
<s>
Initially	O
,	O
only	O
services	O
that	O
were	O
spawned	O
for	O
each	O
connection	O
from	O
a	O
super-server	B-Protocol
(	O
such	O
as	O
inetd	B-Protocol
)	O
got	O
wrapped	O
,	O
utilizing	O
the	O
tcpd	B-Protocol
program	O
.	O
</s>
<s>
However	O
most	O
common	O
network	O
service	O
daemons	B-Operating_System
today	O
can	O
be	O
linked	B-Application
against	O
libwrap	B-Protocol
directly	O
.	O
</s>
<s>
This	O
is	O
used	O
by	O
daemons	B-Operating_System
that	O
operate	O
without	O
being	O
spawned	O
from	O
a	O
super-server	B-Protocol
,	O
or	O
when	O
a	O
single	O
process	O
handles	O
multiple	O
connections	O
.	O
</s>
<s>
When	O
compared	O
to	O
host	O
access	O
control	O
directives	O
often	O
found	O
in	O
daemons	B-Operating_System
 '	O
configuration	O
files	O
,	O
TCP	B-Protocol
Wrappers	I-Protocol
have	O
the	O
benefit	O
of	O
runtime	B-Library
ACL	O
reconfiguration	O
(	O
i.e.	O
,	O
services	O
do	O
n't	O
have	O
to	O
be	O
reloaded	O
or	O
restarted	O
)	O
and	O
a	O
generic	O
approach	O
to	O
network	O
administration	O
.	O
</s>
<s>
This	O
makes	O
it	O
easy	O
to	O
use	O
for	O
anti-worm	O
scripts	O
,	O
such	O
as	O
DenyHosts	B-Application
or	O
Fail2ban	B-Protocol
,	O
to	O
add	O
and	O
expire	O
client-blocking	O
rules	O
,	O
when	O
excessive	O
connections	O
and/or	O
many	O
failed	O
login	O
attempts	O
are	O
encountered	O
.	O
</s>
<s>
While	O
originally	O
written	O
to	O
protect	O
TCP	B-Protocol
and	O
UDP	B-Protocol
accepting	O
services	O
,	O
examples	O
of	O
usage	O
to	O
filter	B-Application
on	O
certain	O
ICMP	B-Protocol
packets	O
exist	O
too	O
,	O
such	O
as	O
'	O
pingd	O
 '	O
the	O
userspace	B-Operating_System
ping	B-Protocol
request	O
responder	O
.	O
</s>
