# nc(1) - man - phpMan

[NC(1)](https://www.chedong.com/phpMan.php/man/NC/1/markdown)                     BSD General Commands Manual                    [NC(1)](https://www.chedong.com/phpMan.php/man/NC/1/markdown)

## 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)](https://www.chedong.com/phpMan.php/man/telnet/1/markdown), **nc**
     scripts nicely, and separates error messages onto standard error instead of sending them to
     standard output, as [telnet(1)](https://www.chedong.com/phpMan.php/man/telnet/1/markdown) 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)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown)
           ••   and much, much more

     The options are as follows:

### -4

### -6

### -b

### -C
             lated into CR+LF before being written to the socket.  Line feed characters that are al‐
             ready preceded with a carriage return (CR) are not translated.  Received data is not
             affected.

### -D

### -d

### -F
             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)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown) using the [ssh_config(5)](https://www.chedong.com/phpMan.php/man/sshconfig/5/markdown)
             **ProxyUseFdpass** option).  Cannot be used with **-U**.

### -h

### -I
             Specify the size of the TCP receive buffer.

### -i
             Sleep for _interval_ seconds between lines of text sent and received.  Also causes a de‐
             lay time between connections to multiple ports.

### -k -l
             gether with the **-u** option, the server socket is not connected and it can receive UDP
             datagrams from multiple hosts.

### -l
             The _destination_ and _port_ to listen on can be specified either as non-optional argu‐
             ments, 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

### -m
             Ask the kernel to drop incoming packets whose TTL / hop limit is under _minttl_.

### -N
             finish their work.

### -n
             error will be reported.

### -O
             Specify the size of the TCP send buffer.

### -P
             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
             Specify the source port **nc** should use, subject to privilege restrictions and availabil‐
             ity.

### -q
             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
             or in the order that the system assigns them.

### -S

### -s
             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
             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 dec‐
             imal.

### -t
             possible to use **nc** to script telnet sessions.

### -U -F -x

### -u -x
             a datagram socket instead of a stream socket.  If a UNIX-domain socket is used, a tem‐
             porary receiving socket is created in _/tmp_ unless the **-s** flag is given.

### -V
             Set the routing table to be used.

### -v

### -W
             Terminate after receiving _recvlimit_ packets from the network.

### -w
             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
             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
             Connect to _destination_ using a proxy at _proxy_address_ and _port_.  If _port_ is not speci‐
             fied, 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

### -z
             gether 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 speci‐
     fied as numeric port numbers of the form _nn_-_mm_.  In general, a destination port must be speci‐
     fied, 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 connec‐
     tion 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)](https://www.chedong.com/phpMan.php/man/sed/1/markdown), 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** op‐
     tion.

     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)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown); see the **ProxyCommand** directive in [ssh_config(5)](https://www.chedong.com/phpMan.php/man/sshconfig/5/markdown) for more informa‐
     tion.

           $ 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)](https://www.chedong.com/phpMan.php/man/cat/1/markdown), [ssh(1)](https://www.chedong.com/phpMan.php/man/ssh/1/markdown)

## 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 tar‐
     get 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 hard‐
     ware resources and/or configuration settings.

BSD                             March 31, 2021                             BSD