.\"	$OpenBSD: nc.1,v 1.2 1998/03/11 18:42:23 deraadt Exp $
.\"
.\" Copyright (c) 1996 David Sacerdote
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.Dd August 1, 1996
.Dt nc 1
.Sh NAME
.Os
.Nm nc
.Nd
Arbitrary tcp and udp connections and listens.
.Pp
.Sh SYNOPSIS
.Nm nc
.Op Fl e Ar command
.Op Fl g Ar intermediates
.Op Fl G Ar hopcount
.Op Fl i Ar interval
.Op Fl lnrtuvz
.Op Fl o Ar filename
.Op Fl p Ar source port 
.Op Fl s Ar ip address
.Op Fl w Ar timeout 
.Op Ar hostname
.Op Ar port[s...]
.Pp
.Sh DESCRIPTION
The
.Nm nc
(or
.Nm netcat )
utility is used for just about anything under the sun
involving TCP or UDP.  It can open tcp connections, send udp packets,
listen on arbitrary tcp and udp ports, do port scanning, and source
routing.  Unlike
.Xr telnet 1 ,
.Nm nc
scripts nicely, and separates error messages onto standard error instead
of sending them to standard output, as 
.Xr telnet 1  
does with some.  
.Pp
Destination ports can be single integers, names as listed in
.Xr /etc/services 5 ,
or ranges.  Ranges are in the form nn-mm, and several separate ports and/or
ranges may be specified on the command line.
.Pp
Common uses include:
.Bl -bullet 
.It
simple tcp proxies
.It
shell\-script based http clients and servers
.It
network daemon testing
.It
source routing based connectivity testing
.It
and much, much more
.El
.Pp
The options are as follows:
.Bl -tag -width Ds
.It Fl e Ar command
Execute the specified command, using data from the network for stdin,
and sending stdout and stderr to the network.  This option is only present if
.Nm nc
was compiled with the GAPING_SECURITY_HOLE compile time option, since it
allows users to make arbitrary programs available to anyone on the network.
.It Fl g Ar intermediate-host
Specifies a hop along a loose source routed path.  Can be used more than
once to build a chain of hop points.
.It Fl G Ar pointer
Positions the "hop counter" within the list of machines in the path of
a source routed packet.  Must be a multiple of 4.
.It Fl i Ar seconds
Specifies a delay time interval between lines of text sent and received.
Also causes a delay time between connections to multiple ports.
.It Fl l
Is used to specify that
.Nm nc
should listen for an incoming connection, rather than initiate a
connection to a remote host.  Any hostname/ip address and port arguments
restrict the source of inbound connections to only that address and
source port.
.It Fl n
Do not do DNS lookups on any of the specified addresses or hostnames, or
names of port numbers from /etc/services.
.It Fl o Ar filename
Create a hexadecimal log of data transferred in the specified file.
Each line begins with < or >.  < means "from the net" and > means
"to the net."
.It Fl p Ar port
Specifies the source port
.Nm nc
should use, subject to privilege restrictions and availability.
.It Fl r
Specified that source and/or destination ports should be chosen semi-randomly
instead of sequentially within a range or in the order that the
system assigns.
.It Fl s Ar hostname/ip-address
Specifies the ip of the interface which is used to send the packets.
On some platforms, this can be used for udp spoofing by using ifconfig
to bring up a dummy interface with the desired source ip address.
.It Fl t
Causes
.Nm nc
to send RFC854 DON'T and WON'T responses to RFC854 DO
and WILL requests.  This makes it possible to use
.Nm nc
to script telnet sessions.  The presence of this option can be
enabled or disabled as a compile-time option.
.It Fl u
Use UDP instead of TCP. 
On most platforms,
.Nm nc
will behave as if a connection is established until it receives an
ICMP packet indicating that there is no program listening to what it
sends.
.It Fl v
Verbose.  Cause
.Nm nc
to display connection information.  Using \-v
more than once will cause
.Nm nc
to become even more verbose.
.It Fl w Ar timeout
Specifies the number of seconds
.Nm nc
should wait before deciding that
an attempt to establish a connection is hopeless.
Also used to specify how long to wait for more network data after standard
input closes.
.It Fl z
Specifies that
.Nm nc
should just scan for listening
daemons, without sending any data to them.  Diagnostic messages about refused
connections will not be
displayed unless \-v is specified twice.
.Sh EXAMPLES
.Pp
.Bl -tag -width x
.It Li "nc"
Wait for the user to type what would normally be command-line
arguments in at stdin.
.It Li "nc example.host 42"
Open a TCP connection to port 42 of example.host.  If the connection
fails, do not display any error messages, but simply exit.
.It Li "nc -p 31337 example.host 42"
Open a TCP connection to port 42 of example.host, and use port 31337
as the source port.
.It Li "nc -w 5 example.host 42"
Open a tcp connection to port 42 of example.host, and time out after
five seconds while attempting to connect.
.It Li "nc -u example.host 53"
Send any data from stdin
to UDP port 53 of example.host, and display any data returned.
.It Li "nc -s 10.1.2.3 example.host 42"
Open a tcp connection to port 42 of example.host using 10.1.2.3 as the
ip for the local end of the connection.
.It Li "nc -v example.host 42"
Open a tcp connection to port 42 of example.host, displaying some
diagnostic messages on stderr.
.It Li "nc -v -v example.host 42"
Open a tcp connection to port 42 of example.host, displaying all
diagnostic messages on stderr.
.It Li "nc -v -z example.host 20-30"
Attempt to open tcp connections to ports 20 through 30 of
example.host, and report which ones
.Nm nc
was able to connect to.
.It Li "nc -v -u -z -w 3 example.host 20-30"
Send udp packets to ports 20-30 of example.host, and report which ones
did not respond with an ICMP packet after three seconds.
.It Li "nc -l -p 3000"
Listen on TCP port 3000, and once there is a connection, send stdin to
the remote host, and send data from the remote host to stdout.
.It Li "echo foobar | nc example.host 1000"
Connect to port 1000 of example.host, send the string "foobar"
followed by a newline, and move data from port 1000 of example.host to
stdout until example.host closes the connection.
.El
.Sh SEE ALSO
.Xr telnet 1 ,
.Xr cat 1 ,
and the
.Nm netcat
.Pa README
.Sh AUTHOR
*Hobbit*  [hobbit@avian.org]