.\"     $OpenBSD: nc.1,v 1.32 2004/07/15 15:07:52 markus Exp $
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.\" Copyright (c) 1996 David Sacerdote
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.Dd June 25, 2001
.Dt NC 1
.Os
.Sh NAME
.Nm nc
.Nd arbitrary TCP and UDP connections and listens
.Sh SYNOPSIS
.Nm nc
.Bk -words
.Op Fl 46DdhklnrStUuvz
.Op Fl i Ar interval
.Op Fl p Ar source_port
.Op Fl s Ar source_ip_address
.Op Fl w Ar timeout
.Op Fl X Ar socks_version
.Oo Xo
.Fl x Ar proxy_address Ns Oo : Ns
.Ar port Oc Oc
.Xc
.Op Ar hostname
.Op Ar port Ns Bq Ar s
.Ek
.Sh DESCRIPTION
The
.Nm
(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 deal with both IPv4 and
IPv6.
Unlike
.Xr telnet 1 ,
.Nm
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 or ranges.
Ranges are in the form nn-mm.
.Pp
Common uses include:
.Pp
.Bl -bullet -offset indent -compact
.It
simple TCP proxies
.It
shell-script based HTTP clients and servers
.It
network daemon testing
.It
and much, much more
.El
.Pp
The options are as follows:
.Bl -tag -width Ds
.It Fl 4
Forces
.Nm
to use IPv4 addresses only.
.It Fl 6
Forces
.Nm
to use IPv6 addresses only.
.It Fl D
Enable debugging on the socket.
.It Fl d
Do not attempt to read from stdin.
.It Fl h
Prints out
.Nm
help.
.It Fl i Ar interval
Specifies a delay time interval between lines of text sent and received.
Also causes a delay time between connections to multiple ports.
.It Fl k
Forces
.Nm
to stay listening for another connection after its current connection
is completed.
It is an error to use this option without the
.Fl l
option.
.It Fl l
Used to specify that
.Nm
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
.Fl p ,
.Fl s ,
or
.Fl z
options.
.It Fl n
Do not do any DNS or service lookups on any specified addresses,
hostnames or ports.
.It Fl p Ar source_port
Specifies the source port
.Nm
should use, subject to privilege restrictions and availability.
It is an error to use this option in conjunction with the
.Fl l
option.
.It Fl 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.
.It Fl S
Enables the RFC 2385 TCP MD5 signature option.
.It Fl s Ar source_ip_address
Specifies the IP of the interface which is used to send the packets.
It is an error to use this option in conjunction with the
.Fl l
option.
.It Fl t
Causes
.Nm
to send RFC 854 DON'T and WON'T responses to RFC 854 DO and WILL requests.
This makes it possible to use
.Nm
to script telnet sessions.
.It Fl U
Specifies to use Unix Domain Sockets.
.It Fl u
Use UDP instead of the default option of TCP.
.It Fl v
Have
.Nm
give more verbose output.
.It Fl w Ar timeout
If a connection and stdin are idle for more than
.Ar timeout
seconds, then the connection is silently closed.
The
.Fl w
flag has no effect on the
.Fl l
option, i.e.\&
.Nm
will listen forever for a connection, with or without the
.Fl w
flag.
The default is no timeout.
.It Fl X Ar socks_version
Requests that
.Nm
should use the specified version of the SOCKS protocol when talking to
a SOCKS proxy.
SOCKS versions 4 and 5 are currently supported.
If the version is not specified, SOCKS version 5 is used.
.It Xo
.Fl x Ar proxy_address Ns Oo : Ns
.Ar port Oc
.Xc
Requests that
.Nm
should connect to
.Ar hostname
using a SOCKS proxy at
.Ar proxy_address
and
.Ar port .
If
.Ar port
is not specified, port 1080 is used.
.It Fl z
Specifies that
.Nm
should just scan for listening daemons, without sending any data to them.
It is an error to use this option in conjunction with the
.Fl l
option.
.El
.Sh CLIENT/SERVER MODEL
It is quite simple to build a very basic client/server model using
.Nm .
On one console, start
.Nm
listening on a specific port for a connection.
For example:
.Pp
.Dl $ nc -l 1234
.Pp
.Nm
is now listening on port 1234 for a connection.
On a second console
.Pq or a second machine ,
connect to the machine and port being listened on:
.Pp
.Dl $ nc 127.0.0.1 1234
.Pp
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,
.Nm
does not really care which side is being used as a
.Sq server
and which side is being used as a
.Sq client .
The connection may be terminated using an
.Dv EOF
.Pq Sq ^D .
.Sh 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.
.Pp
Start by using
.Nm
to listen on a specific port, with output captured into a file:
.Pp
.Dl $ nc -l 1234 \*(Gt filename.out
.Pp
Using a second machine, connect to the listening
.Nm
process, feeding it the file which is to be transferred:
.Pp
.Dl $ nc host.example.com 1234 \*(Lt filename.in
.Pp
After the file has been transferred, the connection will close automatically.
.Sh TALKING TO SERVERS
It is sometimes useful to talk to servers
.Dq 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:
.Pp
.Dl $ echo \&"GET\&" | nc host.example.com 80
.Pp
Note that this also displays the headers sent by the web server.
They can be filtered, using a tool such as
.Xr sed 1 ,
if necessary.
.Pp
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:
.Bd -literal -offset indent
$ nc localhost 25 \*(Lt\*(Lt EOF
HELO host.example.com
MAIL FROM: \*(Ltuser@host.example.com\*(Gt
RCPT TO: \*(Ltuser2@host.example.com\*(Gt
DATA
Body of email.
\&.
QUIT
EOF
.Ed
.Sh PORT SCANNING
It may be useful to know which ports are open and running services on
a target machine.
The
.Fl z
flag can be used to tell
.Nm
not to initiate a connection,
together with the
.Fl v
.Pq verbose
flag,
to report open ports.
For example:
.Bd -literal -offset indent
$ nc -vz 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!
.Ed
.Pp
The port range was specified to limit the search to ports 20 \- 30.
.Pp
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
.Fl w
flag, or perhaps by issuing a
.Qq Dv QUIT
command to the server:
.Bd -literal -offset indent
$ 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
.Ed
.Sh EXAMPLES
Open a TCP connection to port 42 of hostname, using port 31337 as
the source port, with a timeout of 5 seconds:
.Pp
.Dl $ nc -p 31337 -w 5 hostname 42
.Pp
Open a UDP connection to port 53 of hostname:
.Pp
.Dl $ nc -u hostname 53
.Pp
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:
.Pp
.Dl $ nc -s 10.1.2.3 example.host 42
.Pp
Send UDP packets to ports 20-30 of example.host, and report which ones
responded with an ICMP packet after three seconds:
.Pp
.Dl $ nc -uvz -w 3 hostname 20-30
.Pp
Create and listen on a Unix Domain Socket:
.Pp
.Dl $ nc -lU /var/tmp/dsocket
.Sh SEE ALSO
.Xr cat 1
.Sh AUTHORS
Original implementation by *Hobbit*
.Aq hobbit@avian.org .
.br
Rewritten with IPv6 support by
.An Eric Jackson Aq ericj@monkey.org .