NAME

nc — arbitrary TCP and UDP connections and listens

SYNOPSIS

[-46cDdFhklNnrStUuvz] [-C certfile] [-e name] [-H hash] [-I length] [-i interval] [-K keyfile] [-M ttl] [-m minttl] [-O length] [-o staplefile] [-P proxy_username] [-p source_port] [-R CAfile] [-s source] [-T keyword] [-V rtable] [-W recvlimit] [-w timeout] [-X proxy_protocol] [-x proxy_address[:port]] [-Z peercertfile] [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: Forces nc to use IPv4 addresses only.

-6: Forces nc to use IPv6 addresses only.

-C certfile: Specifies the filename from which the public key part of the TLS certificate is loaded, in PEM format. May only be used with TLS.

-c: If using a TCP socket to connect or listen, use TLS. Illegal if not using TCP sockets.

-D: Enable debugging on the socket.

-d: Do not attempt to read from stdin.

-e name: Specify the name that must be present in the peer certificate when using TLS. Illegal if not using TLS.

-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 hash: Specifies the required hash string of the peer certificate when using TLS. The string format required is that used by tls_peer_cert_hash(3). Illegal if not using TLS, and may not be used with -T noverify.

-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 multiple ports.

-K keyfile: Specifies the filename from which the private key is loaded in PEM format. May only be used with TLS.

-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 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.

-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: Specifies the size of the TCP send buffer.

-o staplefile: Specifies the filename from which to load data to be stapled during the TLS handshake. The file is expected to contain an OCSP response from an OCSP server in DER format. May only be used with TLS and when a certificate is being used.

-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: 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 CAfile: Specifies the filename from which the root CA bundle for certificate verification is loaded, in PEM format. Illegal if not using TLS. The default is /etc/ssl/cert.pem.

-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 keyword: Change IPv4 TOS value or TLS options. For TLS options keyword may be one of: noverify, which disables certificate verification; noname, which disables certificate name checking; clientcert, which requires a client certificate on incoming connections; or muststaple, which requires the peer to provide a valid stapled OCSP response with the handshake. The following TLS options specify a value in the form of a key=value pair: ciphers, which allows the supported TLS ciphers to be specified (see tls_config_set_ciphers(3) for further details); protocols, which allows the supported TLS protocols to be specified (see tls_config_parse_protocols(3) for further details). It is illegal to specify TLS options if not using TLS.
For IPv4 TOS value keyword 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.

-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 to be used.

-v: Have nc give 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: 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). An IPv6 address can be specified unambiguously by enclosing proxy_address in square brackets.

-Z peercertfile: Specifies the filename in which the peer supplied certificates will be saved in PEM format. May only be used with TLS.

-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.

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 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 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 -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 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 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 TCP connection to port 443 of www.example.com, and negotiate TLS with any supported TLS protocol version and "compat" ciphers:

$ nc -cv -T protocols=all -T ciphers=compat
  www.example.com 443

Open a TCP connection to port 443 of www.google.ca, and negotiate TLS. Check for a different name in the certificate for validation:

$ nc -cv -e adsf.au.doubleclick.net
  www.google.ca 443

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

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 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.