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NC(1) General Commands Manual NC(1) NAME nc -- arbitrary TCP and UDP connections and listens SYNOPSIS nc [-46DdEFhklMNnrStUuvz] [-e IPsec_policy] [-I length] [-i interval] [--no-tcpopt] [--sctp] [-O length] [-P proxy_username] [-p source_port] [-s source] [-T toskeyword] [--tun tundev] [-V rtable] [-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 con- nections, 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 in- stead of sending them to standard output, as telnet(1) does with some. Common uses include: o simple TCP proxies o shell-script based HTTP clients and servers o network daemon testing o a SOCKS or HTTP ProxyCommand for ssh(1) o 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. -E Shortcut for "-e 'in ipsec esp/transport//require' -e 'out ipsec esp/transport//require'", which enables IPsec ESP trans- port 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. -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). -h Prints out nc 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 mul- tiple 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. 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 nc should listen for an incoming connec- tion 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. -M Collect per-connection TCP statistics using the stats(3) frame- work and print them in JSON format to stderr(4) after the con- nection is closed. -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 ad- dresses, hostnames or ports. --no-tcpopt Disables the use of TCP options on the socket, by setting the boolean TCP_NOOPT socket option. --sctp Use SCTP instead of the default option of TCP. -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 authentica- tion will not be attempted. Proxy authentication is only sup- ported for HTTP CONNECT proxies at present. -p source_port Specifies the source port nc should use, subject to privilege restrictions and availability. It is an error to use this op- tion 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 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. --tun tundev Causes nc to use the provided tun(4) for input and output rather than the default of stdin and stdout. -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 /tmp unless the -s flag is given. -V rtable Set the routing table ("FIB") to be used. -v Have nc give more verbose output. -w timeout Connections which cannot be established or are idle timeout af- ter 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 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 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). -z Specifies that nc should just scan for listening daemons, with- out sending any data to them. It is an error to use this op- tion in conjunction with the -l option. destination can be a numerical IP address or a symbolic hostname (un- less the -n option is given). In general, a destination must be speci- fied, 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 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 connec- tion will be output to the other end, and input and output can be eas- ily 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 automat- ically. 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 nec- essary 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 for- mat 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 to report open ports, rather than initiate a connection. For example: $ nc -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 run- ning, 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 Open a TCP connection to port 42 of host.example.com using IPsec ESP for incoming and outgoing traffic. $ nc -E host.example.com 42 Open a TCP connection to port 42 of host.example.com using IPsec ESP for outgoing traffic only. $ nc -e 'out ipsec esp/transport//require' 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 EXIT STATUS The nc utility exits 0 on success, and >0 if an error occurs. SEE ALSO cat(1), setfib(1), ssh(1), tcp(4) AUTHORS Original implementation by *Hobbit* <hobbit@avian.org>. Rewritten with IPv6 support by Eric Jackson <ericj@monkey.org>. CAVEATS UDP port scans using the -uz combination of flags will always report success irrespective of the target machine's state. However, in con- junction with a traffic sniffer either on the target machine or an in- termediary device, the -uz combination could be useful for communica- tions diagnostics. Note that the amount of UDP traffic generated may be limited either due to hardware resources and/or configuration set- tings. FreeBSD 13.2 January 17, 2023 NC(1)
NAME | SYNOPSIS | DESCRIPTION | CLIENT/SERVER MODEL | DATA TRANSFER | TALKING TO SERVERS | PORT SCANNING | EXAMPLES | EXIT STATUS | SEE ALSO | AUTHORS | CAVEATS
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