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NAME

nc - arbitrary TCP and UDP connections and listens

CONTENTS

Synopsis
Description
Client/server Model
Data Transfer
Talking To Servers
Port Scanning
Examples
See Also
Authors

SYNOPSIS

nc
.Bk -words [-46DEdhklnorStUuvz] [-e IPsec_policy] [-i interval] [-p source_port] [-s source_ip_address] [-w timeout] [-X proxy_protocol] [Xo -x proxy_address[: port]] [hostname] [port[s]]
.Ek

DESCRIPTION

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.

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.
-D Enable debugging on the socket.
-d Do not attempt to read from stdin.
-h Prints out nc help.
-E Shortcut for " "-e ’in ipsec esp/transport//require’" "-e ’out ipsec esp/transport//require’" ", which enables IPsec ESP transport mode in both directions.
-e If IPsec support is available, then one can specify the IPsec policies to be used using the syntax described in ipsec_set_policy(3). This flag can be specified up to two times, as typically one policy for each direction is needed.
-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. Additionally, any timeouts specified with the -w option are ignored.
-n Do not do any DNS or service lookups on any specified addresses, hostnames or ports.
-o "Once-only mode". By default, nc does not terminate on EOF condition on input, but continues until the network side has been closed down. Specifying -o will make it terminate on EOF as well.
-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 proxy_version
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 hostname 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 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.

hostname can be a numerical IP address or a symbolic hostname (unless the -n option is given). In general, a hostname must be specified, unless the -l option is given (in which case the local host is used).

port [s] can be single integers or ranges. Ranges are in the form nn-mm. In general, a destination port must be specified, unless the -U option is given (in which case a socket must be specified).

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').

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 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.

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:

$ 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: <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 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

EXAMPLES

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 -E example.host 42"

Open a TCP connection to port 42 of example.host using IPsec ESP for incoming and outgoing traffic.

"nc -e ’out ipsec esp/transport//require’ example.host 42"

Open a TCP connection to port 42 of example.host using IPsec ESP for outgoing traffic only.

$ 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

Connect to port 42 of hostname via an HTTP proxy at 10.2.3.4, port 8080:

$ nc -x10.2.3.4:8080 -Xconnect hostname 42

SEE ALSO

cat(1)

AUTHORS

 
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