Nc

nc (netcat) — arbitrary TCP and UDP connections and listens

Beschreibung

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 sepa‐ rates 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

Syntax

$ 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]

Optionen

-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 con‐

  nected 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-nega‐
  tive 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 ab‐

  stract 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 can‐
  not 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.

Parameter

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 gen‐ eral, a destination port must be specified, unless the -U option is given.

Links

Siehe auch

1. cat(1)
2. ssh(1)

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