NETCAT(1)NETCAT(1)NAMEnetcat - TCP/IP swiss army knife
SYNOPSISnetcat [-options] hostname port[s] [ports] ...
netcat-l -p port [-options] [hostname] [port]
DESCRIPTIONnetcat is a simple unix utility which reads and writes data across net‐
work connections, using TCP or UDP protocol. It is designed to be a
reliable "back-end" tool that can be used directly or easily driven by
other programs and scripts. At the same time, it is a feature-rich
network debugging and exploration tool, since it can create almost any
kind of connection you would need and has several interesting built-in
capabilities. Netcat, or "netcat" as the actual program is named,
should have been supplied long ago as another one of those cryptic but
standard Unix tools.
In the simplest usage, "netcat host port" creates a TCP connection to
the given port on the given target host. Your standard input is then
sent to the host, and anything that comes back across the connection is
sent to your standard output. This continues indefinitely, until the
network side of the connection shuts down. Note that this behavior is
different from most other applications which shut everything down and
exit after an end-of-file on the standard input.
Netcat can also function as a server, by listening for inbound connec‐
tions on arbitrary ports and then doing the same reading and writing.
With minor limitations, netcat doesn't really care if it runs in
"client" or "server" mode -- it still shovels data back and forth until
there isn't any more left. In either mode, shutdown can be forced after
a configurable time of inactivity on the network side.
And it can do this via UDP too, so netcat is possibly the "udp telnet-
like" application you always wanted for testing your UDP-mode servers.
UDP, as the "U" implies, gives less reliable data transmission than TCP
connections and some systems may have trouble sending large amounts of
data that way, but it's still a useful capability to have.
You may be asking "why not just use telnet to connect to arbitrary
ports?" Valid question, and here are some reasons. Telnet has the
"standard input EOF" problem, so one must introduce calculated delays
in driving scripts to allow network output to finish. This is the main
reason netcat stays running until the *network* side closes. Telnet
also will not transfer arbitrary binary data, because certain charac‐
ters are interpreted as telnet options and are thus removed from the
data stream. Telnet also emits some of its diagnostic messages to
standard output, where netcat keeps such things religiously separated
from its *output* and will never modify any of the real data in transit
unless you *really* want it to. And of course telnet is incapable of
listening for inbound connections, or using UDP instead. Netcat
doesn't have any of these limitations, is much smaller and faster than
telnet, and has many other advantages.
OPTIONS-g gateway source-routing hop point[s], up to 8
-G num source-routing pointer: 4, 8, 12, ...
-h display help
-i secs delay interval for lines sent, ports scanned
-l listen mode, for inbound connects
-n numeric-only IP addresses, no DNS
-o file hex dump of traffic
-p port local port number (port numbers can be individual or
ranges: lo-hi [inclusive])
-r randomize local and remote ports
-s addr local source address
-t enable telnet negotiation
-u UDP mode
-v verbose [use twice to be more verbose]
-w secs timeout for connects and final net reads
-z zero-I/O mode [used for scanning]
COPYRIGHT
Netcat is entirely my own creation, although plenty of other code was
used as examples. It is freely given away to the Internet community in
the hope that it will be useful, with no restrictions except giving
credit where it is due. No GPLs, Berkeley copyrights or any of that
nonsense. The author assumes NO responsibility for how anyone uses it.
If netcat makes you rich somehow and you're feeling generous, mail me a
check. If you are affiliated in any way with Microsoft Network, get a
life. Always ski in control. Comments, questions, and patches to hob‐
bit@avian.org.
BUGS
Efforts have been made to have netcat "do the right thing" in all its
various modes. If you believe that it is doing the wrong thing under
whatever circumstances, please notify me and tell me how you think it
should behave. If netcat is not able to do some task you think up,
minor tweaks to the code will probably fix that. It provides a basic
and easily-modified template for writing other network applications,
and I certainly encourage people to make custom mods and send in any
improvements they make to it. Continued feedback from the Internet com‐
munity is always welcome!
Some port names in /etc/services contain hyphens --netcat currently
will not correctly parse those, so specify ranges using numbers if you
can.
SEE ALSO
/usr/share/doc/netcat/README
AUTHOR
This manual page was written by Joey Hess <joeyh@debian.org> and Robert
Woodcock <rcw@debian.org>, cribbing heavily from Netcat's README file.
Netcat was written by a guy we know as the Hobbit <hobbit@avian.org>.
NETCAT(1)
