nc_openbsd

nc -words [-46bCDdFhklNnrStUuvZz [-I length ] ] [-i interval ] [-O length ] [-P proxy_username ] [-p source_port ] [-q seconds ] [-s source ] [-T toskeyword ] [-V rtable ] [-w timeout ] [-X proxy_protocol ] [ -x proxy_address [: port ] Oc [destination ] [port ]

The nc (or netcat utility is used for just about anything under the sun involving TCP, UDP, or UNIX 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), nc scripts 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

The options are as follows:

-4

Forces nc to use IPv4 addresses only.

-6

Forces nc to use IPv6 addresses only.

-b

Allow broadcast.

-C

Send CRLF as line-ending.

-D

Enable debugging on the socket.

-d

Do not attempt to read from stdin.

-F

Pass the first connected socket using sendmsg(2) to stdout and exit. This is useful in conjunction with -X to have nc perform connection setup with a proxy but then leave the rest of the connection to another program (e.g. ssh(1) using the ssh_config5 ProxyUseFdpass option).

-h

Prints out nc help.

-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.

-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. When used together with the -u option, the server socket is not connected and it can receive UDP datagrams from multiple hosts.

-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. Additionally, any timeouts specified with the -w option are ignored.

-N

shutdown(2) the network socket after EOF on the input. Some servers require this to finish their work.

-n

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 nc should use, subject to privilege restrictions and availability.

-q seconds

after EOF on stdin, wait the specified number of seconds and then quit. If seconds is negative, wait forever (default). Specifying a non-negative seconds implies -N

-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

Specifies the IP of the interface which is used to send the packets. For UNIX 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 -l option.

-T toskeyword

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.

-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 sockets.

-u

Use UDP instead of the default option of TCP. For UNIX sockets, use a datagram socket instead of a stream socket. If a UNIX socket is used, a temporary receiving socket is created in /tmp unless the -s flag is given.

-V rtable

Set the routing table to be used.

-v

Have nc give more verbose output.

-w timeout

Connections which cannot be established or are idle timeout after timeout seconds. 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 proxy_protocol

Requests that nc should 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.

-x proxy_address [: port ]

Requests that nc should 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).

-Z

DCCP mode.

-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.

destination can be a numerical IP address or a symbolic hostname (unless the -n option is given). In general, a destination must be specified, unless the -l option is given (in which case the local host is used). For UNIX sockets, a destination is required and is the socket path to connect to (or listen on if the -l option 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 -U option is given.

It is quite simple to build a very basic client/server model using . 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' )

There is no -c or -e 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:

On `server' side:

$ rm -f /tmp/f; mkfifo /tmp/f

$ cat /tmp/f | /bin/sh -i 2>&1 | nc -l 127.0.0.1 1234 > /tmp/f

On `client' side:

$ 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 `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, nc quits as well. Use -k if you want it keep listening, but if the command quits this option won't restart it or keep nc running. Also don't forget to remove the file descriptor once you don't need it anymore:

$ 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 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 -N 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:

$ 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 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 [-C] localhost 25 << EOF HELO host.example.com MAIL FROM:<user@host.example.com> RCPT TO:<user2@host.example.com> 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 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 -v option.

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! 

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 -w flag, or perhaps by issuing a Qq Dv 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 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 UNIX stream socket:

$ nc -lU /var/tmp/dsocket

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 ProxyCommand directive in ssh_config5 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 ``ruser'' if the proxy requires it:

$ nc -x10.2.3.4:8080 -Xconnect -Pruser host.example.com 42

Original implementation by *Hobbit* Aq Mt hobbit@avian.org . Rewritten with IPv6 support by An Eric Jackson Aq Mt ericj@monkey.org . Modified for Debian port by Aron Xu Aq aron@debian.org .

UDP port scans using the -uz combination 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 -uz 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.

cat(1), ssh(1)