netcat, nc

netcat - arbitrary TCP and UDP connections and listens 

Command


SYNOPSIS

netcat -46DdhklNnrStUuvz [-I length] [-i interval] [-O length] [-P proxy_username] [-p source_port] [-s source] [-T toskeyword] [-V rtable] [-w timeout] [-X proxy_protocol] [-x proxy_address[:port]] [destination] [port]

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


DESCRIPTION

netcat The netcat (or nc) 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 bsdtelnet, netcat scripts nicely, and separates error messages onto standard error instead of sending them to standard output, as bsdtelnet 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
— and much, much more

Options

-4 

Forces netcat to use IPv4 addresses only.

-6 

Forces netcat to use IPv6 addresses only.

-D 

Enable debugging on the socket.

-d 

Do not attempt to read from stdin.

-h 

Prints out netcat 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 netcat 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 netcat 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()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 netcat 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 

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 -l option.

-T toskeyword 

Change IPv4 TOS value. toskeyword may be one of critical, inetcontrol, 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 netcat to send RFC 854 DON'T and WON'T responses to RFC 854 DO and WILL requests. This makes it possible to use netcat to script bsdtelnet sessions.

-U 

Specifies to use UNIX-domain sockets.

-u 

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 $TMPDIR unless the -s flag is given.

-V rtable 

Set the routing table to be used. The default is 0.

-v 

Have netcat 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. netcat will listen forever for a connection, with or without the -w flag. The default is no timeout.

-x proxy_address[:port] 

Requests that netcat 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).

-X proxy_protocol 

Requests that netcat 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.

-z 

Specifies that netcat 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-domain 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.


CLIENT/SERVER MODEL

It is quite simple to build a very basic client/server model using netcat. On one console, start netcat listening on a specific port for a connection. For example:

$ netcat -l 1234

netcat 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:

$ netcat 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, netcat 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').


DATA TRANSFERS

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 netcat to listen on a specific port, with output captured into a file:

$ netcat -l 1234 > filename.out

Using a second machine, connect to the listening netcat process, feeding it the file which is to be transferred:

$ netcat 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 ``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" | netcat 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, 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:

$ netcat 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

PORT SCANNING

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 netcat to report open ports, rather than initiate a connection. For example:

$ netcat -z 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" | netcat 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:

$ netcat -p 31337 -w 5 host.example.com 42

Open a UDP connection to port 53 of host.example.com:

$ netcat -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:

$ netcat -s 10.1.2.3 host.example.com 42

Create and listen on a UNIX-domain stream socket:

$ netcat -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; see the ProxyCommand directive in ssh_config(5) for more information.

$ netcat -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:

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

ENVIRONMENT VARIABLES

TMPDIR 

The MKS Toolkit port will use TMPDIR in place of a hard coded /tmp.


DIAGNOSTICS

Possible exit status values are:

0 

Successful completion.

1 

Failure due to any of the following:

— unknown command line option


CAVEATS

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.


PORTABILITY

All UNIX systems. Windows 7. Windows Server 2008 R2. Windows 8. Windows Server 2012. Windows 10. Windows Server 2016. Windows Server 2019.


AVAILABILITY

PTC MKS Toolkit for System Administrators
PTC MKS Toolkit for Developers
PTC MKS Toolkit for Interoperability
PTC MKS Toolkit for Professional Developers
PTC MKS Toolkit for Professional Developers 64-Bit Edition
PTC MKS Toolkit for Enterprise Developers
PTC MKS Toolkit for Enterprise Developers 64-Bit Edition


AUTHORS

Original implementation by *Hobbit* <hobbit@avian.org>. Rewritten with IPv6 support by Eric Jackson <ericj@monkey.org>.


SEE ALSO

Commands:
cat, ssh


PTC MKS Toolkit 10.2 Documentation Build 28.