|NC(1)||General Commands Manual||NC(1)|
arbitrary TCP and UDP connections and listens
netcat) utility is used for just about anything under the sun involving TCP, UDP, or UNIX-domain sockets. 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),
ncscripts nicely, and separates error messages onto standard error instead of sending them to standard output, as telnet(1) 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 ssh(1)
- and much, much more
ncto use IPv4 addresses only.
ncto use IPv6 addresses only.
- Allow broadcast.
- Send CRLF as line-ending.
- Enable debugging on the socket.
- Do not attempt to read from stdin.
- Pass the first connected socket using
sendmsg(2) to stdout and exit. This is useful
in conjunction with
ncperform connection setup with a proxy but then leave the rest of the connection to another program (e.g. ssh(1) using the ssh_config(5)
- Prints out
- Specifies the size of the TCP receive buffer.
- Specifies a delay time interval between lines of text sent and received. Also causes a delay time between connections to multiple ports.
ncto stay listening for another connection after its current connection is completed. It is an error to use this option without the
-loption. When used together with the
-uoption, the server socket is not connected and it can receive UDP datagrams from multiple hosts.
- Used to specify that
ncshould 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
-zoptions. Additionally, any timeouts specified with the
-woption are ignored.
- shutdown(2) 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.
- Specifies the size of the TCP send buffer.
- 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.
- Specifies the source port
ncshould use, subject to privilege restrictions and availability.
- after EOF on stdin, wait the specified number of
seconds and then quit. If
seconds 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 assigns them.
- Enables the RFC 2385 TCP MD5 signature option.
- Specifies the IP of the interface which is used to send the packets. For
UNIX-domain datagram sockets, specifies the local
temporary socket file to create and use so that datagrams can be received.
It is an error to use this option in conjunction with the
- Change IPv4 TOS value. toskeyword may be one of critical, inetcontrol, lowcost, lowdelay, netcontrol, throughput, reliability, or one of the DiffServ Code Points: ef, af11 ... af43, cs0 ... cs7; or a number in either hex or decimal.
ncto send RFC 854 DON'T and WON'T responses to RFC 854 DO and WILL requests. This makes it possible to use
ncto script telnet sessions.
- Specifies to use UNIX-domain sockets.
- Use UDP instead of the default option of TCP. For
UNIX-domain sockets, use a datagram socket instead
of a stream socket. If a UNIX-domain socket is
used, a temporary receiving socket is created in
/tmp unless the
-sflag is given.
- Set the routing table to be used.
ncgive more verbose output.
- Connections which cannot be established or are idle timeout after
timeout seconds. The
-wflag has no effect on the
ncwill listen forever for a connection, with or without the
-wflag. The default is no timeout.
- Requests that
ncshould use the specified protocol when talking to the proxy server. Supported protocols are “4” (SOCKS v.4), “5” (SOCKS v.5) and “connect” (HTTPS proxy). If the protocol is not specified, SOCKS version 5 is used.
- Requests that
ncshould connect to destination using a proxy at proxy_address and port. If port is not specified, the well-known port for the proxy protocol is used (1080 for SOCKS, 3128 for HTTPS).
- DCCP mode.
- Specifies that
ncshould just scan for listening daemons, without sending any data to them. It is an error to use this option in conjunction with the
-noption is given). In general, a destination must be specified, unless the
-loption is given (in which case the local host is used). For UNIX-domain sockets, a destination is required and is the socket path to connect to (or listen on if the
-loption is given). port can be a single integer or a range of ports. Ranges are in the form nn-mm. In general, a destination port must be specified, unless the
-Uoption is given.
CLIENT/SERVER MODEL¶It is quite simple to build a very basic client/server model using
nc. On one console, start
nclistening on a specific port for a connection. For example:
$ nc -l 1234
ncis 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:
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 127.0.0.1 1234
ncdoes 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’). There is no
-eoption 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: On ‘server’ side:
$ rm -f /tmp/f; mkfifo /tmp/f
On ‘client’ side:
$ cat /tmp/f | /bin/sh -i 2>&1 | nc -l 127.0.0.1 1234 > /tmp/f
$ nc host.example.com 1234
By doing this, you create a fifo at /tmp/f and make nc listen at port 1234 of address 127.0.0.1 on ‘server’ side, when a ‘client’ establishes a connection successfully to that port, /bin/sh gets executed on ‘server’ side and the shell prompt is given to ‘client’ side. When connection is terminated,
$ (shell prompt from host.example.com)
ncquits as well. Use
-kif you want it keep listening, but if the command quits this option won't restart it or keep
ncrunning. Also don't forget to remove the file descriptor once you don't need it anymore:
$ rm -f /tmp/f
DATA TRANSFER¶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
ncto listen on a specific port, with output captured into a file:
Using a second machine, connect to the listening
$ nc -l 1234 > filename.out
ncprocess, feeding it the file which is to be transferred:
After the file has been transferred, the connection will close automatically.
$ nc -N host.example.com 1234 < filename.in
TALKING TO SERVERS¶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:
$ printf "GET / HTTP/1.0\r\n\r\n" | nc host.example.com 80
$ nc [-C] localhost 25 << EOF HELO host.example.com MAIL FROM:<email@example.com> RCPT TO:<firstname.lastname@example.org> DATA Body of email. . QUIT EOF
PORT SCANNING¶It may be useful to know which ports are open and running services on a target machine. The
-zflag can be used to tell
ncto 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
-voption. For example:
$ 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!
$ 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!
-wflag, 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
EXAMPLES¶Open a TCP connection to port 42 of host.example.com, using port 31337 as the source port, with a timeout of 5 seconds:
Open a UDP connection to port 53 of host.example.com:
$ nc -p 31337 -w 5 host.example.com 42
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 -u host.example.com 53
Create and listen on a UNIX-domain stream socket:
$ nc -s 10.1.2.3 host.example.com 42
Connect to port 42 of host.example.com via an HTTP proxy at 10.2.3.4, port 8080. This example could also be used by ssh(1); see the
$ nc -lU /var/tmp/dsocket
ProxyCommanddirective in ssh_config(5) for more information.
The same example again, this time enabling proxy authentication with username “ruser” if the proxy requires it:
$ nc -x10.2.3.4:8080 -Xconnect host.example.com 42
$ nc -x10.2.3.4:8080 -Xconnect -Pruser host.example.com 42
SEE ALSO¶cat(1), ssh(1)
AUTHORS¶Original implementation by *Hobbit* <email@example.com>.
CAVEATS¶UDP port scans using the
-uzcombination of flags will always report success irrespective of the target machine's state. However, in conjunction with a traffic sniffer either on the target machine or an intermediary device, the
-uzcombination 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.
|March 26, 2015||Debian|