nc - arbitrary TCP and UDP connections and listens
nc [-46DdhklnrStUuvz] [-i interval] [-p source_port]
[-s source_ip_address] [-w timeout] [-X socks_version]
[-x proxy_address[:port]] [hostname] [port[s]]
The nc (or netcat) utility is used for just about anything
under the sun
involving TCP or UDP. It can open TCP connections, send UDP
packets,
listen on arbitrary TCP and UDP ports, do port scanning, and
deal with
both IPv4 and IPv6. Unlike telnet(1), nc scripts nicely,
and separates
error messages onto standard error instead of sending them
to standard
output, as telnet(1) does with some.
Destination ports can be single integers or ranges. Ranges
are in the
form nn-mm.
Common uses include:
+o simple TCP proxies
+o shell-script based HTTP clients and servers
+o network daemon testing
+o and much, much more
The options are as follows:
-4 Forces nc to use IPv4 addresses only.
-6 Forces nc to use IPv6 addresses only.
-D Enable debugging on the socket.
-d Do not attempt to read from stdin.
-h Prints out nc help.
-i interval
Specifies a delay time interval between lines of
text sent and
received. Also causes a delay time between connections to multiple
ports.
-k Forces nc to stay listening for another connection
after its current
connection is completed. It is an error to use
this option
without the -l option.
-l Used to specify that nc should listen for an incoming connection
rather than initiate a connection to a remote host.
It is an error
to use this option in conjunction with the -p,
-s, or -z options.
-n Do not do any DNS or service lookups on any specified addresses,
hostnames or ports.
-p source_port
Specifies the source port nc should use, subject to
privilege restrictions
and availability. It is an error to use
this option
in conjunction with the -l option.
-r Specifies that source and/or destination ports
should be chosen
randomly instead of sequentially within a range or
in the order
that the system assigns them.
-S Enables the RFC 2385 TCP MD5 signature option.
-s source_ip_address
Specifies the IP of the interface which is used to
send the packets.
It is an error to use this option in conjunction with the
-l option.
-t Causes nc to send RFC 854 DON'T and WON'T responses
to RFC 854 DO
and WILL requests. This makes it possible to use nc
to script
telnet sessions.
-U Specifies to use Unix Domain Sockets.
-u Use UDP instead of the default option of TCP.
-v Have nc give more verbose output.
-w timeout
If a connection and stdin are idle for more than
timeout seconds,
then the connection is silently closed. The -w flag
has no effect
on the -l option, i.e. nc will listen forever
for a connection,
with or without the -w flag. The default is
no timeout.
-X socks_version
Requests that nc should use the specified version of
the SOCKS
protocol when talking to a SOCKS proxy. SOCKS versions 4 and 5
are currently supported. If the version is not
specified, SOCKS
version 5 is used.
-x proxy_address[:port]
Requests that nc should connect to hostname using a
SOCKS proxy
at proxy_address and port. If port is not specified, port 1080
is used.
-z Specifies that nc should just scan for listening
daemons, without
sending any data to them. It is an error to use
this option in
conjunction with the -l option.
CLIENT/SERVER MODEL
It is quite simple to build a very basic client/server model
using nc.
On one console, start nc listening on a specific port for a
connection.
For example:
$ nc -l 1234
nc is now listening on port 1234 for a connection. On a
second console
(or a second machine), connect to the machine and port being
listened on:
$ nc 127.0.0.1 1234
There should now be a connection between the ports. Anything typed at
the second console will be concatenated to the first, and
vice-versa.
After the connection has been set up, nc does not really
care which side
is being used as a `server' and which side is being used as
a `client'.
The connection may be terminated using an EOF (`^D').
The example in the previous section can be expanded to build
a basic data
transfer model. Any information input into one end of the
connection
will be output to the other end, and input and output can be
easily captured
in order to emulate file transfer.
Start by using nc to listen on a specific port, with output
captured into
a file:
$ nc -l 1234 > filename.out
Using a second machine, connect to the listening nc process,
feeding it
the file which is to be transferred:
$ nc host.example.com 1234 < filename.in
After the file has been transferred, the connection will
close automatically.
It is sometimes useful to talk to servers ``by hand'' rather
than through
a user interface. It can aid in troubleshooting, when it
might be necessary
to verify what data a server is sending in response to
commands issued
by the client. For example, to retrieve the home page
of a web
site:
$ echo "GET" | nc host.example.com 80
Note that this also displays the headers sent by the web
server. They
can be filtered, using a tool such as sed(1), if necessary.
More complicated examples can be built up when the user
knows the format
of requests required by the server. As another example, an
email may be
submitted to an SMTP server using:
$ nc localhost 25 << EOF
HELO host.example.com
MAIL FROM: <[email protected]>
RCPT TO: <[email protected]>
DATA
Body of email.
.
QUIT
EOF
It may be useful to know which ports are open and running
services on a
target machine. The -z flag can be used to tell nc not to
initiate a
connection, together with the -v (verbose) flag, to report
open ports.
For example:
$ nc -vz host.example.com 20-30
Connection to host.example.com 22 port [tcp/ssh] succeeded!
Connection to host.example.com 25 port [tcp/smtp] succeeded!
The port range was specified to limit the search to ports 20
- 30.
Alternatively, it might be useful to know which server software is running,
and which versions. This information is often contained within the
greeting banners. In order to retrieve these, it is necessary to first
make a connection, and then break the connection when the
banner has been
retrieved. This can be accomplished by specifying a small
timeout with
the -w flag, or perhaps by issuing a "QUIT" command to the
server:
$ echo "QUIT" | nc host.example.com 20-30
SSH-1.99-OpenSSH_3.6.1p2
Protocol mismatch.
220 host.example.com IMS SMTP Receiver Version 0.84
Ready
Open a TCP connection to port 42 of hostname, using port
31337 as the
source port, with a timeout of 5 seconds:
$ nc -p 31337 -w 5 hostname 42
Open a UDP connection to port 53 of hostname:
$ nc -u hostname 53
Open a TCP connection to port 42 of example.host using
10.1.2.3 as the IP
for the local end of the connection:
$ nc -s 10.1.2.3 example.host 42
Send UDP packets to ports 20-30 of example.host, and report
which ones
responded with an ICMP packet after three seconds:
$ nc -uvz -w 3 hostname 20-30
Create and listen on a Unix Domain Socket:
$ nc -lU /var/tmp/dsocket
cat(1)
Original implementation by *Hobbit* <[email protected]>.
Rewritten with IPv6 support by Eric Jackson <[email protected]>.
OpenBSD 3.6 June 25, 2001
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