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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 source] [-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.  Un‐      like 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 per‐              form 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 multi‐              ple 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 do any DNS or service lookups on any specified addresses, hostnames or ports.

     -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 authenti‐              cation 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 as‐              signs them.

     -S      Enable the RFC 2385 TCP MD5 signature option.

     -s source              Send packets from the interface with the source IP address.  For UNIX-domain datagram sockets, specifies the local tempo‐              rary 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.

     -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 op‐              tion, 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 re‐      quired and is the socket path to connect to (or listen on if the -l option is given).

     port can be a specified as a numeric port number, or as a service name.  Ports may be specified in a range 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 be‐      ing 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 DAN‐      GEROUS. 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 re‐      port 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 con‐      tained 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

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Justine le 22 juin 2022 à 11:57

Justine : Contenu remplacé par « https://www.thegeekstuff.com/2012/04/nc-command-examples/?utm_source=feedburner =Qu'est-ce que c'est ?= Netcat ou nc est un utilitaire servant à débuguer et investi... »

Justine : Page créée avec « NAME nc — arbitrary TCP and UDP connections and listens SYNOPSIS nc [-46bCDdFhklNnrStUuvZz] [-I length] [-i... »

Justine : Contenu remplacé par « https://www.thegeekstuff.com/2012/04/nc-command-examples/?utm_source=feedburner =Qu'est-ce que c'est ?= Netcat ou nc est un utilitaire servant à débuguer et investi... »

Justine : Page créée avec « NAME
nc — arbitrary TCP and UDP connections and listens SYNOPSIS
nc [-46bCDdFhklNnrStUuvZz] [-I length] [-i... »