Section: User Commands (1)
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- arbitrary TCP and UDP connections and listens
[-46bCDdFhklNnrStUuvZz [-I length
[-x proxy_address [: port
utility is used for just about anything under the sun involving TCP,
It can open TCP connections, send UDP packets, listen on arbitrary
TCP and UDP ports, do port scanning, and deal with both IPv4 and
scripts nicely, and separates error messages onto standard error instead
of sending them to standard output, as
does with some.
Common uses include:
simple TCP proxies
shell-script based HTTP clients and servers
network daemon testing
a SOCKS or HTTP ProxyCommand for
and much, much more
The options are as follows:
to use IPv4 addresses only.
to use IPv6 addresses only.
Send CRLF as line-ending. Each line feed (LF) character from the input
data is translated into CR+LF before being written to the socket. Line
feed characters that are already preceded with a carriage return (CR)
are not translated. Received data is not affected.
Enable debugging on the socket.
Do not attempt to read from stdin.
Pass the first connected socket using
to stdout and exit.
This is useful in conjunction with
perform connection setup with a proxy but then leave the rest of the
connection to another program (e.g.
- -I length
Specifies the size of the TCP receive buffer.
- -i interval
Specifies a delay time interval between lines of text sent and received.
Also causes a delay time between connections to multiple ports.
to stay listening for another connection after its current connection
It is an error to use this option without the
When used together with the
option, the server socket is not connected and it can receive UDP datagrams from
Used to specify that
should listen for an incoming connection rather than initiate a
connection to a remote host.
to listen on can be specified either as non-optional arguments, or with
It is an error to use
in conjunction with the
Additionally, any timeouts specified with the
option are ignored.
- -M ttl
Set the TTL / hop limit of outgoing packets.
- -m minttl
Ask the kernel to drop incoming packets whose TTL / hop limit is under
the network socket after EOF on the input.
Some servers require this to finish their work.
Do not do any DNS or service lookups on any specified addresses,
hostnames or ports.
- -O length
Specifies the size of the TCP send buffer.
- -P proxy_username
Specifies a username to present to a proxy server that requires authentication.
If no username is specified then authentication will not be attempted.
Proxy authentication is only supported for HTTP CONNECT proxies at present.
- -p source_port
Specifies the source port
should use, subject to privilege restrictions and availability.
- -q seconds
after EOF on stdin, wait the specified number of
and then quit. If
is negative, wait forever (default). Specifying a non-negative
Specifies that source and/or destination ports should be chosen randomly
instead of sequentially within a range or in the order that the system
Enables the RFC 2385 TCP MD5 signature option.
- -s source
Specifies the IP of the interface which is used to send the packets.
datagram sockets, specifies the local temporary socket file
to create and use so that datagrams can be received.
- -T keyword
Change IPv4 TOS value.
may be one of
or one of the DiffServ Code Points:
af11 ... af43
cs0 ... cs7
or a number in either hex or decimal.
to send RFC 854 DON'T and WON'T responses to RFC 854 DO and WILL requests.
This makes it possible to use
to script telnet sessions.
Specifies to use
Use UDP instead of the default option of TCP.
sockets, use a datagram socket instead of a stream socket.
socket is used, a temporary receiving socket is created in
flag is given.
- -V rtable
Set the routing table to be used.
give more verbose output.
- -W recvlimit
Terminate after receiving
packets from the network.
- -w timeout
Connections which cannot be established or are idle timeout after
flag has no effect on the
will listen forever for a connection, with or without the
The default is no timeout.
- -X proxy_protocol
should use the specified protocol when talking to the proxy server.
Supported protocols are
If the protocol is not specified, SOCKS version 5 is used.
- -x proxy_address [: port
should connect to
using a proxy at
is not specified, the well-known port for the proxy protocol is used (1080
for SOCKS, 3128 for HTTPS).
An IPv6 address can be specified unambiguously by enclosing
in square brackets.
should just scan for listening daemons, without sending any data to them.
It is an error to use this option in conjunction with the
can be a numerical IP address or a symbolic hostname
option is given).
In general, a destination must be specified,
option is given
(in which case the local host is used).
sockets, a destination is required and is the socket path to connect to
(or listen on if the
option is given).
can be a specified as a numeric port number, or as a service name.
Ports may be specified in a range of the form nn-mm.
a destination port must be specified,
option is given.
It is quite simple to build a very basic client/server model using
On one console, start
listening on a specific port for a connection.
$ nc -l 1234
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,
After the connection has been set up,
does not really care which side is being used as a
and which side is being used as a
The connection may be terminated using an
There is no
option in this netcat, but you still can execute a command after connection
being established by redirecting file descriptors. Be cautious here because
opening a port and let anyone connected execute arbitrary command on your
site is DANGEROUS. If you really need to do this, here is an example:
$ rm -f /tmp/f; mkfifo /tmp/f
$ cat /tmp/f | /bin/sh -i 2>&1 | nc -l 127.0.0.1 1234 > /tmp/f
$ nc host.example.com 1234
$ (shell prompt from host.example.com)
By doing this, you create a fifo at /tmp/f and make nc listen at port 1234
of address 127.0.0.1 on
side, when a
establishes a connection successfully to that port, /bin/sh gets executed
side and the shell prompt is given to
When connection is terminated,
quits as well. Use
if you want it keep listening, but if the command quits this option won't
restart it or keep
running. Also don't forget to remove the file descriptor once you don't need
$ rm -f /tmp/f
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
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
process, feeding it the file which is to be transferred:
$ nc -N host.example.com 1234 < filename.in
After the file has been transferred, the connection will close automatically.
TALKING TO SERVERS
It is sometimes useful to talk to servers
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:
$ printf "GET / HTTP/1.0\r\n\r\n" | 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
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 [-C] localhost 25 << EOF
Body of email.
It may be useful to know which ports are open and running services on
a target machine.
flag can be used to tell
to report open ports,
rather than initiate a connection. Usually it's useful to turn on verbose
output to stderr by use this option in conjunction with
$ nc -zv 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, and is
scanned by increasing order.
You can also specify a list of ports to scan, for example:
$ nc -zv host.example.com 80 20 22
nc: connect to host.example.com 80 (tcp) failed: Connection refused
nc: connect to host.example.com 20 (tcp) failed: Connection refused
Connection to host.example.com port [tcp/ssh] succeeded!
The ports are scanned by the order you given.
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
flag, or perhaps by issuing a
Qq Dv QUIT
command to the server:
$ echo "QUIT" | nc host.example.com 20-30
220 host.example.com IMS SMTP Receiver Version 0.84 Ready
Open a TCP connection to port 42 of host.example.com, using port 31337 as
the source port, with a timeout of 5 seconds:
$ nc -p 31337 -w 5 host.example.com 42
Open a UDP connection to port 53 of host.example.com:
$ nc -u host.example.com 53
Open a TCP connection to port 42 of host.example.com using 10.1.2.3 as the
IP for the local end of the connection:
$ nc -s 10.1.2.3 host.example.com 42
Create and listen on a
$ nc -lU /var/tmp/dsocket
Connect to port 42 of host.example.com via an HTTP proxy at 10.2.3.4,
This example could also be used by
for more information.
$ nc -x10.2.3.4:8080 -Xconnect host.example.com 42
The same example again, this time enabling proxy authentication with username
if the proxy requires it:
$ nc -x10.2.3.4:8080 -Xconnect -Pruser host.example.com 42
Original implementation by *Hobbit*
Aq Mt email@example.com .
Rewritten with IPv6 support by
An Eric Jackson Aq Mt firstname.lastname@example.org .
Modified for Debian port by Aron Xu
Aq email@example.com .
UDP port scans using the
combination of flags will always report success irrespective of
the target machine's state.
in conjunction with a traffic sniffer either on the target machine
or an intermediary device,
combination could be useful for communications diagnostics.
Note that the amount of UDP traffic generated may be limited either
due to hardware resources and/or configuration settings.
- CLIENT/SERVER MODEL
- DATA TRANSFER
- TALKING TO SERVERS
- PORT SCANNING
- SEE ALSO
This document was created by
using the manual pages.
Time: 06:09:40 GMT, September 23, 2018