<s>
tcpdump	B-Protocol
is	O
a	O
data-network	O
packet	B-Protocol
analyzer	I-Protocol
computer	O
program	O
that	O
runs	O
under	O
a	O
command	B-Application
line	I-Application
interface	I-Application
.	O
</s>
<s>
It	O
allows	O
the	O
user	O
to	O
display	O
TCP/IP	B-Protocol
and	O
other	O
packets	O
being	O
transmitted	O
or	O
received	O
over	O
a	O
network	B-Architecture
to	O
which	O
the	O
computer	O
is	O
attached	O
.	O
</s>
<s>
Distributed	O
under	O
the	O
BSD	B-Operating_System
license	I-Operating_System
,	O
tcpdump	B-Protocol
is	O
free	B-Application
software	I-Application
.	O
</s>
<s>
Tcpdump	B-Protocol
works	O
on	O
most	O
Unix-like	B-Operating_System
operating	I-Operating_System
systems	I-Operating_System
:	O
Linux	B-Application
,	O
Solaris	B-Application
,	O
FreeBSD	B-Operating_System
,	O
DragonFly	B-Application
BSD	I-Application
,	O
NetBSD	B-Device
,	O
OpenBSD	B-Operating_System
,	O
OpenWrt	B-Application
,	O
macOS	B-Application
,	O
HP-UX	B-Application
11i	I-Application
,	O
and	O
AIX	B-Application
.	O
</s>
<s>
In	O
those	O
systems	O
,	O
tcpdump	B-Protocol
uses	O
the	O
libpcap	B-Language
library	O
to	O
capture	O
packets	O
.	O
</s>
<s>
The	O
port	O
of	O
tcpdump	B-Protocol
for	O
Windows	B-Application
is	O
called	O
WinDump	O
;	O
it	O
uses	O
WinPcap	B-Language
,	O
the	O
Windows	B-Application
version	O
of	O
libpcap	B-Language
.	O
</s>
<s>
tcpdump	B-Protocol
was	O
originally	O
written	O
in	O
1988	O
by	O
Van	O
Jacobson	O
,	O
Sally	O
Floyd	O
,	O
Vern	O
Paxson	O
and	O
Steven	O
McCanne	O
who	O
were	O
,	O
at	O
the	O
time	O
,	O
working	O
in	O
the	O
Lawrence	O
Berkeley	O
Laboratory	O
Network	B-Architecture
Research	O
Group	O
.	O
</s>
<s>
By	O
the	O
late	O
1990s	O
there	O
were	O
numerous	O
versions	O
of	O
tcpdump	B-Protocol
distributed	O
as	O
part	O
of	O
various	O
operating	B-General_Concept
systems	I-General_Concept
,	O
and	O
numerous	O
patches	O
that	O
were	O
not	O
well	O
coordinated	O
.	O
</s>
<s>
tcpdump	B-Protocol
prints	O
the	O
contents	O
of	O
network	B-Architecture
packets	O
.	O
</s>
<s>
It	O
can	O
read	O
packets	O
from	O
a	O
network	B-Architecture
interface	O
card	O
or	O
from	O
a	O
previously	O
created	O
saved	O
packet	O
file	O
.	O
</s>
<s>
tcpdump	B-Protocol
can	O
write	O
packets	O
to	O
standard	O
output	O
or	O
a	O
file	O
.	O
</s>
<s>
It	O
is	O
also	O
possible	O
to	O
use	O
tcpdump	B-Protocol
for	O
the	O
specific	O
purpose	O
of	O
intercepting	O
and	O
displaying	O
the	O
communications	O
of	O
another	O
user	O
or	O
computer	O
.	O
</s>
<s>
A	O
user	O
with	O
the	O
necessary	O
privileges	O
on	O
a	O
system	O
acting	O
as	O
a	O
router	B-Protocol
or	O
gateway	B-Application
through	O
which	O
unencrypted	O
traffic	O
such	O
as	O
Telnet	B-Protocol
or	O
HTTP	B-Protocol
passes	O
can	O
use	O
tcpdump	B-Protocol
to	O
view	O
login	O
IDs	O
,	O
passwords	O
,	O
the	O
URLs	O
and	O
content	O
of	O
websites	O
being	O
viewed	O
,	O
or	O
any	O
other	O
unencrypted	O
information	O
.	O
</s>
<s>
The	O
user	O
may	O
optionally	O
apply	O
a	O
BPF-based	O
filter	O
to	O
limit	O
the	O
number	O
of	O
packets	O
seen	O
by	O
tcpdump	B-Protocol
;	O
this	O
renders	O
the	O
output	O
more	O
usable	O
on	O
networks	O
with	O
a	O
high	O
volume	O
of	O
traffic	O
.	O
</s>
<s>
Example	O
of	O
available	O
capture	O
interfaces	O
on	O
a	O
Linux	B-Application
system	O
:	O
</s>
<s>
In	O
some	O
Unix-like	B-Operating_System
operating	I-Operating_System
systems	I-Operating_System
,	O
a	O
user	O
must	O
have	O
superuser	B-Application
privileges	O
to	O
use	O
tcpdump	B-Protocol
because	O
the	O
packet	B-Protocol
capturing	I-Protocol
mechanisms	O
on	O
those	O
systems	O
require	O
elevated	O
privileges	O
.	O
</s>
<s>
In	O
other	O
Unix-like	B-Operating_System
operating	I-Operating_System
systems	I-Operating_System
,	O
the	O
packet	B-Protocol
capturing	I-Protocol
mechanism	O
can	O
be	O
configured	O
to	O
allow	O
non-privileged	O
users	O
to	O
use	O
it	O
;	O
if	O
that	O
is	O
done	O
,	O
superuser	B-Application
privileges	O
are	O
not	O
required	O
.	O
</s>
