Provided by: netcat-openbsd_1.226-1ubuntu2_amd64 
NAME
nc — arbitrary TCP and UDP connections and listens
SYNOPSIS
nc [-46bCDdFhklNnrStUuvZz] [-I length] [-i interval] [-M ttl] [-m minttl] [-O length] [-P proxy_username]
[-p source_port] [-q seconds] [-s sourceaddr] [-T keyword] [-V rtable] [-W recvlimit] [-w timeout]
[-X proxy_protocol] [-x proxy_address[:port]] [destination] [port]
DESCRIPTION
The nc (or 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), 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 Use IPv4 addresses only.
-6 Use IPv6 addresses only.
-b Allow broadcast.
-C 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.
-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_config(5) ProxyUseFdpass option).
Cannot be used with -U.
-h Print out the nc help text and exit.
-I length
Specify the size of the TCP receive buffer.
-i interval
Sleep for interval seconds between lines of text sent and received. Also causes a delay time
between connections to multiple ports.
-k When a connection is completed, listen for another one. Requires -l. When used together with
the -u option, the server socket is not connected and it can receive UDP datagrams from multiple
hosts.
-l Listen for an incoming connection rather than initiating a connection to a remote host. The
destination and port to listen on can be specified either as non-optional arguments, or with
options -s and -p respectively. Cannot be used together with -x or -z. Additionally, any
timeouts specified with the -w 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 minttl.
-N shutdown(2) the network socket after EOF on the input. Some servers require this to finish their
work.
-n Do not perform domain name resolution. If a name cannot be resolved without DNS, an error will
be reported.
-O length
Specify 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
Specify 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 Choose source and/or destination ports randomly instead of sequentially within a range or in the
order that the system assigns them.
-S Enable the RFC 2385 TCP MD5 signature option.
-s sourceaddr
Set the source address to send packets from, which is useful on machines with multiple
interfaces. For Unix-domain datagram sockets, specifies the local temporary socket file to
create and use so that datagrams can be received. Cannot be used together with -x.
-T keyword
Change the IPv4 TOS/IPv6 traffic class value. keyword 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 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 Use Unix-domain sockets. Cannot be used together with -F or -x. On Linux, if the name starts
with an at symbol (`@') it is read as an abstract namespace socket: the leading `@' is replaced
with a NUL byte before binding or connecting. For details, see unix(7).
-u Use UDP instead of TCP. Cannot be used together with -x. 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 -s flag is given.
-V rtable
Set the routing table to be used.
-v Produce more verbose output.
-W recvlimit
Terminate after receiving recvlimit packets from the network.
-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
Use proxy_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]
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). An IPv6 address
can be specified unambiguously by enclosing proxy_address in square brackets. A proxy cannot be
used with any of the options -lsuU.
-Z DCCP mode.
-z Only scan for listening daemons, without sending any data to them. Cannot be used together with
-l.
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 specified as a numeric port number or as a service name. Port ranges may be specified as
numeric port numbers of 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 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 -N 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’), as the -N flag was given.
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
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 -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 “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
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 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
(unless the -r flag is set).
You can also specify a list of ports to scan, for example:
$ nc -zv host.example.com http 20 22-23
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!
nc: connect to host.example.com 23 (tcp) failed: Connection refused
The ports are scanned by the order you given (unless the -r flag is set).
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 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-domain 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_config(5) 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
SEE ALSO
cat(1), ssh(1)
AUTHORS
Original implementation by *Hobbit* <hobbit@avian.org>.
Rewritten with IPv6 support by
Eric Jackson <ericj@monkey.org>.
Modified for Debian port by Aron Xu ⟨aron@debian.org⟩.
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.
Debian September 11, 2022 NC(1)