<s>
inetd	B-Protocol
(	O
internet	O
service	O
daemon	B-Operating_System
)	O
is	O
a	O
super-server	B-Protocol
daemon	B-Operating_System
on	O
many	O
Unix	B-Application
systems	I-Application
that	O
provides	O
Internet	O
services	O
.	O
</s>
<s>
Requests	O
are	O
served	O
by	O
spawning	O
a	O
process	O
which	O
runs	O
the	O
appropriate	O
executable	O
,	O
but	O
simple	O
services	O
such	O
as	O
echo	B-Protocol
are	O
served	O
by	O
inetd	B-Protocol
itself	O
.	O
</s>
<s>
First	O
appearing	O
in	O
4.3BSD	B-Operating_System
,	O
it	O
is	O
generally	O
located	O
at	O
/usr/sbin/inetd	O
.	O
</s>
<s>
Often	O
called	O
a	O
super-server	B-Protocol
,	O
inetd	B-Protocol
listens	O
on	O
designated	O
ports	B-Protocol
used	O
by	O
Internet	O
services	O
such	O
as	O
FTP	B-Protocol
,	O
POP3	B-Protocol
,	O
and	O
telnet	B-Protocol
.	O
</s>
<s>
When	O
a	O
TCP	B-Protocol
packet	O
or	O
UDP	B-Protocol
packet	I-Protocol
arrives	O
with	O
a	O
particular	O
destination	O
port	B-Protocol
number	I-Protocol
,	O
inetd	B-Protocol
launches	O
the	O
appropriate	O
server	O
program	O
to	O
handle	O
the	O
connection	O
.	O
</s>
<s>
Furthermore	O
,	O
in	O
inetd	B-Protocol
's	O
"	O
nowait	O
"	O
mode	O
of	O
service	O
management	O
,	O
no	O
network	O
code	O
is	O
required	O
in	O
the	O
service-specific	O
programs	O
,	O
as	O
inetd	B-Protocol
hooks	O
the	O
network	O
stream	O
directly	O
to	O
stdin	O
and	O
stdout	O
of	O
the	O
spawned	O
process	O
.	O
</s>
<s>
For	O
protocols	O
that	O
have	O
frequent	O
traffic	O
,	O
such	O
as	O
HTTP	B-Protocol
and	O
POP3	B-Protocol
,	O
either	O
inetd	B-Protocol
's	O
"	O
wait	O
"	O
mode	O
of	O
operation	O
,	O
or	O
a	O
dedicated	O
server	O
that	O
intercepts	O
the	O
traffic	O
directly	O
may	O
be	O
preferable	O
.	O
</s>
<s>
The	O
list	O
of	O
services	O
that	O
will	O
be	O
serviced	O
is	O
given	O
in	O
a	O
configuration	O
file	O
,	O
usually	O
/etc/inetd.conf	O
.	O
</s>
<s>
A	O
GUI	B-Application
for	O
managing	O
the	O
configuration	O
file	O
is	O
an	O
optional	O
accessory	O
.	O
</s>
<s>
The	O
daemon	B-Operating_System
may	O
need	O
a	O
signal	O
in	O
order	O
to	O
re-read	O
its	O
configuration	O
.	O
</s>
<s>
For	O
an	O
example	O
,	O
telnet	B-Protocol
can	O
be	O
configured	O
as	O
follows	O
(	O
line	O
taken	O
from	O
a	O
machine	O
running	O
AIX	B-Application
version	O
5.1	O
)	O
:	O
</s>
<s>
The	O
first	O
word	O
,	O
telnet	B-Protocol
,	O
is	O
the	O
official	O
name	O
of	O
the	O
service	O
.	O
</s>
<s>
It	O
is	O
resolved	O
using	O
the	O
system	O
database	O
to	O
map	O
port	B-Protocol
numbers	I-Protocol
and	O
protocols	O
to	O
service	O
names	O
.	O
</s>
<s>
A	O
single-threaded	O
server	O
expects	O
inetd	B-Protocol
to	O
wait	O
until	O
it	O
finishes	O
reading	O
all	O
the	O
data	O
.	O
</s>
<s>
Otherwise	O
inetd	B-Protocol
lets	O
the	O
server	O
run	O
and	O
spawns	O
new	O
,	O
concurrent	O
processes	O
for	O
new	O
requests	O
.	O
</s>
<s>
In	O
the	O
example	O
,	O
inetd	B-Protocol
is	O
told	O
to	O
launch	O
the	O
program	O
/usr/sbin/telnetd	O
with	O
the	O
command	O
line	O
arguments	O
telnetd	B-Protocol
-a	O
.	O
inetd	B-Protocol
automatically	O
hooks	O
the	O
socket	O
to	O
stdin	O
,	O
stdout	O
,	O
and	O
stderr	O
of	O
the	O
server	O
program	O
.	O
</s>
<s>
Generally	O
TCP	B-Protocol
sockets	O
are	O
handled	O
by	O
spawning	O
a	O
separate	O
server	O
to	O
handle	O
each	O
connection	O
concurrently	O
.	O
</s>
<s>
UDP	B-Protocol
sockets	O
are	O
generally	O
handled	O
by	O
a	O
single	O
server	O
instance	O
that	O
handles	O
all	O
packets	O
on	O
that	O
port	O
.	O
</s>
<s>
Some	O
simple	O
services	O
,	O
such	O
as	O
echo	B-Protocol
,	O
are	O
handled	O
directly	O
by	O
inetd	B-Protocol
,	O
without	O
spawning	O
an	O
external	O
server	O
.	O
</s>
<s>
This	O
is	O
a	O
simple	O
inetd	B-Protocol
service	O
,	O
written	O
in	O
C	B-Language
.	O
It	O
expects	O
a	O
command	O
line	O
argument	O
containing	O
a	O
filename	O
for	O
a	O
log	O
file	O
,	O
and	O
then	O
it	O
logs	O
all	O
strings	O
sent	O
through	O
the	O
socket	O
to	O
the	O
log	O
file	O
.	O
</s>
<s>
The	O
example	O
uses	O
stdio	B-Language
functions	O
and	O
it	O
responds	O
to	O
network	O
traffic	O
coming	O
in	O
on	O
stdin	O
.	O
</s>
<s>
This	O
means	O
UDP	B-Protocol
is	O
the	O
correct	O
protocol	O
to	O
use	O
.	O
</s>
<s>
First	O
,	O
an	O
unused	O
port	B-Protocol
number	I-Protocol
must	O
be	O
selected	O
.	O
</s>
<s>
And	O
the	O
entry	O
in	O
/etc/inetd.conf	O
will	O
look	O
like	O
this	O
:	O
</s>
<s>
This	O
tells	O
inetd	B-Protocol
to	O
run	O
the	O
/usr/local/bin/errlogd	O
program	O
,	O
with	O
the	O
commandline	O
:	O
errlogd	O
/tmp/logfile.txt	O
(	O
refer	O
to	O
the	O
inetd.conf	B-Protocol
man	B-Protocol
page	I-Protocol
for	O
information	O
on	O
the	O
other	O
arguments	O
)	O
.	O
</s>
<s>
inetd	B-Protocol
will	O
run	O
the	O
service	O
when	O
needed	O
,	O
and	O
attach	O
port	O
9999	O
to	O
the	O
input	O
and	O
output	O
streams	O
,	O
and	O
all	O
strings	O
sent	O
to	O
that	O
port	O
will	O
be	O
logged	O
to	O
the	O
file	O
.	O
</s>
<s>
By	O
specifying	O
wait	O
,	O
it	O
tells	O
inetd	B-Protocol
to	O
only	O
use	O
one	O
instance	O
of	O
the	O
server	O
to	O
handle	O
all	O
requests	O
.	O
</s>
<s>
Note	O
:	O
the	O
functionality	O
of	O
the	O
above	O
example	O
is	O
usually	O
implemented	O
by	O
using	O
syslog	B-Protocol
and	O
a	O
process	O
like	O
syslogd	B-Protocol
.	O
</s>
<s>
syslogd	B-Protocol
would	O
normally	O
be	O
started	O
in	O
parallel	O
with	O
inetd	B-Protocol
,	O
not	O
as	O
an	O
inetd	B-Protocol
service	O
.	O
</s>
<s>
In	O
recent	O
years	O
,	O
because	O
of	O
the	O
security	O
limitations	O
in	O
the	O
original	O
design	O
of	O
inetd	B-Protocol
,	O
it	O
has	O
been	O
replaced	O
by	O
xinetd	B-Operating_System
,	O
rlinetd	O
,	O
ucspi-tcp	B-Protocol
,	O
and	O
others	O
in	O
many	O
systems	O
.	O
</s>
<s>
Distributions	O
of	O
Linux	B-Application
especially	O
have	O
many	O
options	O
and	O
Mac	B-Operating_System
OS	I-Operating_System
X	I-Operating_System
(	O
beginning	O
with	O
Mac	B-Operating_System
OS	I-Operating_System
X	I-Operating_System
v10.2	I-Operating_System
)	O
uses	O
xinetd	B-Operating_System
.	O
</s>
<s>
As	O
of	O
version	O
Mac	B-Application
OS	I-Application
X	I-Application
v10.4	I-Application
,	O
Apple	O
has	O
merged	O
the	O
functionality	O
of	O
inetd	B-Protocol
into	O
launchd	B-Application
.	O
</s>
<s>
The	O
services	O
provided	O
by	O
inetd	B-Protocol
can	O
be	O
omitted	O
entirely	O
.	O
</s>
<s>
For	O
example	O
,	O
an	O
HTTP	B-Protocol
server	O
could	O
be	O
configured	O
to	O
just	O
run	O
httpd	O
and	O
have	O
no	O
other	O
ports	B-Protocol
open	O
.	O
</s>
<s>
systemd	B-Application
supports	O
inetd	B-Protocol
services	O
,	O
and	O
expands	O
socket	O
activation	O
beyond	O
IP	O
messaging	O
(	O
AF	O
INET+6	O
)	O
to	O
include	O
AF	B-Protocol
UNIX	I-Protocol
,	O
AF	B-Application
NETLINK	I-Application
and	O
more	O
.	O
</s>
<s>
While	O
the	O
inetd	B-Protocol
concept	O
as	O
a	O
service	B-Protocol
dispatcher	I-Protocol
is	O
not	O
inherently	O
insecure	O
,	O
the	O
long	O
list	O
of	O
services	O
that	O
inetd	B-Protocol
traditionally	O
provided	O
gave	O
computer	O
security	O
experts	O
pause	O
.	O
</s>
<s>
It	O
is	O
not	O
uncommon	O
to	O
find	O
an	O
/etc/inetd.conf	O
with	O
almost	O
all	O
the	O
services	O
commented	O
out	O
in	O
a	O
modern	O
Unix	B-Application
distribution	O
.	O
</s>
