475 lines
12 KiB
Groff
475 lines
12 KiB
Groff
.\" $OpenBSD: nc.1,v 1.53 2010/02/23 23:00:52 schwarze Exp $
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.\"
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.\" Copyright (c) 1996 David Sacerdote
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.\" All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. The name of the author may not be used to endorse or promote products
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.\" derived from this software without specific prior written permission
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
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.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
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.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
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.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
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.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
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.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
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.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd February 23, 2010
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.Dt NC 1
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.Os
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.Sh NAME
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.Nm nc
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.Nd arbitrary TCP and UDP connections and listens
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.Sh SYNOPSIS
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.Nm nc
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.Bk -words
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.Op Fl 46DdEhklnorStUuvz
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.Op Fl e Ar IPsec_policy
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.Op Fl I Ar length
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.Op Fl i Ar interval
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.Op Fl -no-tcpopt
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.Op Fl O Ar length
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.Op Fl P Ar proxy_username
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.Op Fl p Ar source_port
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.Op Fl s Ar source_ip_address
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.Op Fl T Ar ToS
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.Op Fl V Ar fib
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.Op Fl w Ar timeout
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.Op Fl X Ar proxy_protocol
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.Oo Xo
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.Fl x Ar proxy_address Ns Oo : Ns
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.Ar port Oc
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.Xc Oc
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.Op Ar hostname
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.Op Ar port
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.Ek
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.Sh DESCRIPTION
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The
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.Nm
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(or
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.Nm netcat )
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utility is used for just about anything under the sun involving TCP
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or UDP.
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It can open TCP connections, send UDP packets, listen on arbitrary
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TCP and UDP ports, do port scanning, and deal with both IPv4 and
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IPv6.
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Unlike
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.Xr telnet 1 ,
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.Nm
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scripts nicely, and separates error messages onto standard error instead
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of sending them to standard output, as
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.Xr telnet 1
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does with some.
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.Pp
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Common uses include:
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.Pp
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.Bl -bullet -offset indent -compact
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.It
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simple TCP proxies
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.It
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shell-script based HTTP clients and servers
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.It
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network daemon testing
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.It
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a SOCKS or HTTP ProxyCommand for
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.Xr ssh 1
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.It
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and much, much more
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.El
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.Pp
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The options are as follows:
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.Bl -tag -width Ds
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.It Fl 4
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Forces
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.Nm
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to use IPv4 addresses only.
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.It Fl 6
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Forces
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.Nm
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to use IPv6 addresses only.
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.It Fl D
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Enable debugging on the socket.
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.It Fl d
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Do not attempt to read from stdin.
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.It Fl E
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Shortcut for
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.Qo
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.Li "-e 'in ipsec esp/transport//require'"
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.Li "-e 'out ipsec esp/transport//require'"
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.Qc ,
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which enables IPsec ESP transport mode in both
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directions.
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.It Fl e
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If IPsec support is available, then one can specify the IPsec policies
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to be used using the syntax described in
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.Xr ipsec_set_policy 3 .
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This flag can be specified up to two times, as typically one policy for
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each direction is needed.
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.It Fl h
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Prints out
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.Nm
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help.
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.It Fl I Ar length
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Specifies the size of the TCP receive buffer.
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.It Fl i Ar interval
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Specifies a delay time interval between lines of text sent and received.
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Also causes a delay time between connections to multiple ports.
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.It Fl k
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Forces
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.Nm
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to stay listening for another connection after its current connection
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is completed.
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It is an error to use this option without the
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.Fl l
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option.
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.It Fl l
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Used to specify that
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.Nm
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should listen for an incoming connection rather than initiate a
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connection to a remote host.
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It is an error to use this option in conjunction with the
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.Fl p ,
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.Fl s ,
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or
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.Fl z
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options.
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Additionally, any timeouts specified with the
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.Fl w
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option are ignored.
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.It Fl n
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Do not do any DNS or service lookups on any specified addresses,
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hostnames or ports.
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.It Fl -no-tcpopt
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Disables the use of TCP options on the socket, by setting the boolean
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TCP_NOOPT
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socket option.
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.It Fl O Ar length
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Specifies the size of the TCP send buffer.
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When
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.It Fl o
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.Dq Once-only mode .
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By default,
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.Nm
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does not terminate on EOF condition on input,
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but continues until the network side has been closed down.
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Specifying
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.Fl o
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will make it terminate on EOF as well.
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.It Fl P Ar proxy_username
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Specifies a username to present to a proxy server that requires authentication.
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If no username is specified then authentication will not be attempted.
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Proxy authentication is only supported for HTTP CONNECT proxies at present.
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.It Fl p Ar source_port
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Specifies the source port
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.Nm
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should use, subject to privilege restrictions and availability.
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It is an error to use this option in conjunction with the
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.Fl l
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option.
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.It Fl r
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Specifies that source and/or destination ports should be chosen randomly
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instead of sequentially within a range or in the order that the system
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assigns them.
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.It Fl S
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Enables the RFC 2385 TCP MD5 signature option.
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.It Fl s Ar source_ip_address
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Specifies the IP of the interface which is used to send the packets.
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It is an error to use this option in conjunction with the
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.Fl l
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option.
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.It Fl T Ar ToS
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Specifies IP Type of Service (ToS) for the connection.
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Valid values are the tokens
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.Dq lowdelay ,
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.Dq throughput ,
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.Dq reliability ,
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or an 8-bit hexadecimal value preceded by
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.Dq 0x .
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.It Fl t
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Causes
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.Nm
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to send RFC 854 DON'T and WON'T responses to RFC 854 DO and WILL requests.
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This makes it possible to use
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.Nm
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to script telnet sessions.
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.It Fl U
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Specifies to use
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.Ux Ns -domain
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sockets.
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.It Fl u
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Use UDP instead of the default option of TCP.
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.It Fl V Ar fib
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Set the routing table (FIB).
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The default is 0.
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.It Fl v
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Have
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.Nm
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give more verbose output.
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.It Fl w Ar timeout
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If a connection and stdin are idle for more than
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.Ar timeout
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seconds, then the connection is silently closed.
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The
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.Fl w
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flag has no effect on the
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.Fl l
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option, i.e.\&
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.Nm
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will listen forever for a connection, with or without the
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.Fl w
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flag.
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The default is no timeout.
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.It Fl X Ar proxy_protocol
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Requests that
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.Nm
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should use the specified protocol when talking to the proxy server.
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Supported protocols are
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.Dq 4
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(SOCKS v.4),
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.Dq 5
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(SOCKS v.5)
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and
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.Dq connect
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(HTTPS proxy).
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If the protocol is not specified, SOCKS version 5 is used.
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.It Xo
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.Fl x Ar proxy_address Ns Oo : Ns
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.Ar port Oc
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.Xc
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Requests that
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.Nm
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should connect to
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.Ar hostname
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using a proxy at
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.Ar proxy_address
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and
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.Ar port .
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If
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.Ar port
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is not specified, the well-known port for the proxy protocol is used (1080
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for SOCKS, 3128 for HTTPS).
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.It Fl z
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Specifies that
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.Nm
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should just scan for listening daemons, without sending any data to them.
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It is an error to use this option in conjunction with the
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.Fl l
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option.
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.El
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.Pp
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.Ar hostname
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can be a numerical IP address or a symbolic hostname
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(unless the
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.Fl n
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option is given).
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In general, a hostname must be specified,
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unless the
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.Fl l
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option is given
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(in which case the local host is used).
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.Pp
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.Ar port
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can be a single integer or a range of ports.
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Ranges are in the form nn-mm.
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In general,
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a destination port must be specified,
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unless the
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.Fl U
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option is given
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(in which case a socket must be specified).
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.Sh CLIENT/SERVER MODEL
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It is quite simple to build a very basic client/server model using
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.Nm .
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On one console, start
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.Nm
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listening on a specific port for a connection.
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For example:
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.Pp
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.Dl $ nc -l 1234
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.Pp
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.Nm
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is now listening on port 1234 for a connection.
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On a second console
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.Pq or a second machine ,
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connect to the machine and port being listened on:
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.Pp
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.Dl $ nc 127.0.0.1 1234
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.Pp
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There should now be a connection between the ports.
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Anything typed at the second console will be concatenated to the first,
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and vice-versa.
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After the connection has been set up,
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.Nm
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does not really care which side is being used as a
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.Sq server
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and which side is being used as a
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.Sq client .
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The connection may be terminated using an
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.Dv EOF
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.Pq Sq ^D .
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.Sh DATA TRANSFER
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The example in the previous section can be expanded to build a
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basic data transfer model.
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Any information input into one end of the connection will be output
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to the other end, and input and output can be easily captured in order to
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emulate file transfer.
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.Pp
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Start by using
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.Nm
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to listen on a specific port, with output captured into a file:
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.Pp
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.Dl $ nc -l 1234 \*(Gt filename.out
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.Pp
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Using a second machine, connect to the listening
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.Nm
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process, feeding it the file which is to be transferred:
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.Pp
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.Dl $ nc host.example.com 1234 \*(Lt filename.in
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.Pp
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After the file has been transferred, the connection will close automatically.
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.Sh TALKING TO SERVERS
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It is sometimes useful to talk to servers
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.Dq by hand
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rather than through a user interface.
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It can aid in troubleshooting,
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when it might be necessary to verify what data a server is sending
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in response to commands issued by the client.
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For example, to retrieve the home page of a web site:
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.Bd -literal -offset indent
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$ echo -n "GET / HTTP/1.0\er\en\er\en" | nc host.example.com 80
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.Ed
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.Pp
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Note that this also displays the headers sent by the web server.
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They can be filtered, using a tool such as
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.Xr sed 1 ,
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if necessary.
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.Pp
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More complicated examples can be built up when the user knows the format
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of requests required by the server.
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As another example, an email may be submitted to an SMTP server using:
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.Bd -literal -offset indent
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$ nc localhost 25 \*(Lt\*(Lt EOF
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HELO host.example.com
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MAIL FROM:\*(Ltuser@host.example.com\*(Gt
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RCPT TO:\*(Ltuser2@host.example.com\*(Gt
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DATA
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Body of email.
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\&.
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QUIT
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EOF
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.Ed
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.Sh PORT SCANNING
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It may be useful to know which ports are open and running services on
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a target machine.
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The
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.Fl z
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flag can be used to tell
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.Nm
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to report open ports,
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rather than initiate a connection.
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For example:
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.Bd -literal -offset indent
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$ nc -z host.example.com 20-30
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Connection to host.example.com 22 port [tcp/ssh] succeeded!
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Connection to host.example.com 25 port [tcp/smtp] succeeded!
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.Ed
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.Pp
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The port range was specified to limit the search to ports 20 \- 30.
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.Pp
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Alternatively, it might be useful to know which server software
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is running, and which versions.
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This information is often contained within the greeting banners.
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In order to retrieve these, it is necessary to first make a connection,
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and then break the connection when the banner has been retrieved.
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This can be accomplished by specifying a small timeout with the
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.Fl w
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flag, or perhaps by issuing a
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.Qq Dv QUIT
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command to the server:
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.Bd -literal -offset indent
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$ echo "QUIT" | nc host.example.com 20-30
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SSH-1.99-OpenSSH_3.6.1p2
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Protocol mismatch.
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220 host.example.com IMS SMTP Receiver Version 0.84 Ready
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.Ed
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.Sh EXAMPLES
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Open a TCP connection to port 42 of host.example.com, using port 31337 as
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the source port, with a timeout of 5 seconds:
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.Pp
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.Dl $ nc -p 31337 -w 5 host.example.com 42
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.Pp
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Open a UDP connection to port 53 of host.example.com:
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.Pp
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.Dl $ nc -u host.example.com 53
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.Pp
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Open a TCP connection to port 42 of host.example.com using 10.1.2.3 as the
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IP for the local end of the connection:
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.Pp
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.Dl $ nc -s 10.1.2.3 host.example.com 42
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.Pp
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Open a TCP connection to port 42 of host.example.com using IPsec ESP for
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incoming and outgoing traffic.
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.Pp
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.Dl $ nc -E host.example.com 42
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.Pp
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Open a TCP connection to port 42 of host.example.com using IPsec ESP for
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outgoing traffic only.
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.Pp
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.Dl $ nc -e 'out ipsec esp/transport//require' host.example.com 42
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.Pp
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Create and listen on a
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.Ux Ns -domain
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socket:
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.Pp
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.Dl $ nc -lU /var/tmp/dsocket
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.Pp
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Connect to port 42 of host.example.com via an HTTP proxy at 10.2.3.4,
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port 8080.
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This example could also be used by
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.Xr ssh 1 ;
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see the
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.Cm ProxyCommand
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directive in
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.Xr ssh_config 5
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for more information.
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.Pp
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.Dl $ nc -x10.2.3.4:8080 -Xconnect host.example.com 42
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.Pp
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The same example again, this time enabling proxy authentication with username
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.Dq ruser
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if the proxy requires it:
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.Pp
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.Dl $ nc -x10.2.3.4:8080 -Xconnect -Pruser host.example.com 42
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.Sh EXIT STATUS
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.Ex -std
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.Sh SEE ALSO
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.Xr cat 1 ,
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.Xr setfib 1 ,
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.Xr ssh 1 ,
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.Xr tcp 4
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.Sh AUTHORS
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Original implementation by *Hobbit*
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.Aq hobbit@avian.org .
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.br
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Rewritten with IPv6 support by
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.An Eric Jackson Aq ericj@monkey.org .
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.Sh CAVEATS
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UDP port scans will always succeed
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(i.e. report the port as open),
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rendering the
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.Fl uz
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combination of flags relatively useless.
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