ncat/docs/ncat.xml - master - githubmirror/nmap

ncat/docs/ncat.xml
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dmiller
Document Ncat's -z option. Closes #2242
18 май 2026, 17:31
18 май 2026, 17:31b0293fe
<refentry id="ncatman">
  <refmeta>
    <refentrytitle>Ncat</refentrytitle>
    <manvolnum>1</manvolnum>
    <refmiscinfo class="source">Ncat</refmiscinfo>
    <refmiscinfo class="manual">Ncat Reference Guide</refmiscinfo>
  </refmeta>

  <refnamediv id="ncat-man-name">
    <refname>ncat</refname>
    <refpurpose>Concatenate and redirect sockets</refpurpose>
  </refnamediv>

  <refsynopsisdiv id="ncat-man-synopsis">
    <cmdsynopsis>
      <command>ncat</command>
      <arg choice="opt" rep="repeat">
        <replaceable>OPTIONS</replaceable>
      </arg>
      <arg choice="opt">
        <replaceable>hostname</replaceable>
      </arg>
      <arg choice="opt">
        <replaceable>port</replaceable>
      </arg>
    </cmdsynopsis>
  </refsynopsisdiv>

  <refsect1 id="ncat-man-description">
    <title>Description</title>

    <para>Ncat is a feature-packed networking utility which reads and writes
    data across networks from the command line.  Ncat was written for the Nmap
    Project and is the culmination of the currently splintered family of Netcat
    incarnations.  It is designed to
    be a reliable back-end tool to instantly provide network connectivity to other
    applications and users. Ncat will not only work with IPv4 and IPv6 but provides
    the user with a virtually limitless number of potential uses.</para>

    <para>Among Ncat's vast number of features there is the ability to chain Ncats
    together; redirection of TCP, UDP, and SCTP ports to other sites; SSL support; and proxy
    connections via SOCKS4, SOCKS5 or HTTP proxies (with optional proxy
    authentication as well).  Some general principles apply to most applications
    and thus give you the capability of instantly adding networking support to
    software that would normally never support it.</para>
  </refsect1>

  <refsect1 id="ncat-man-options-summary">
    <title>Options Summary</title>

    <para>
<screen><xi:include href="ncat.usage.txt" parse="text" xmlns:xi="http://www.w3.org/2001/XInclude" /></screen>
    </para>

  </refsect1>

  <refsect1 id="ncat-man-modes">
    <indexterm><primary>connect mode (Ncat)</primary></indexterm>
    <indexterm><primary>client mode (Ncat)</primary><see>connect mode</see></indexterm>
    <indexterm><primary>listen mode (Ncat)</primary></indexterm>
    <indexterm><primary>server mode (Ncat)</primary><see>listen mode</see></indexterm>
    <title>Connect Mode and Listen Mode</title>

    <para>
    Ncat operates in one of two primary modes: connect mode and listen
    mode. Other modes, such as the HTTP proxy server, act as special
    cases of these two. In connect mode, Ncat works as a client. In
    listen mode it is a server.
    </para>

    <para>
    In connect mode, the <option><replaceable>hostname</replaceable></option>
    and <option><replaceable>port</replaceable></option> arguments tell
    what to connect to.
    <option><replaceable>hostname</replaceable></option> is required,
    and may be a hostname or IP address. If
    <option><replaceable>port</replaceable></option> is supplied, it
    must be a decimal port number. If omitted, it defaults to
    31337.<indexterm><primary>default port of Ncat</primary></indexterm><indexterm><primary>31337</primary><see>default port of Ncat</see></indexterm>
    </para>

    <para>
    In listen mode, <option><replaceable>hostname</replaceable></option>
    and <option><replaceable>port</replaceable></option> control the
    address the server will bind to. Both arguments are optional in
    listen mode. If <option><replaceable>hostname</replaceable></option>
    is omitted, it defaults to listening on all available addresses over
    IPv4 and IPv6. If <option><replaceable>port</replaceable></option> is
    omitted, it defaults to 31337.
    </para>

  </refsect1>

  <refsect1 id="ncat-man-proto-options">
    <title>Protocol Options</title>

    <variablelist>
      <varlistentry>
        <term>
          <option>-4</option> (IPv4 only)
          <indexterm><primary><option>-4</option> (Ncat option)</primary></indexterm>
        </term>
        <listitem>
          <para>Force the use of IPv4 only.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>-6</option> (IPv6 only)
          <indexterm><primary><option>-6</option> (Ncat option)</primary></indexterm>
        </term>
        <listitem>
          <para>Force the use of IPv6 only.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>-U</option>,
          <option>--unixsock</option> (Use Unix domain sockets)
          <indexterm><primary><option>--unixsock</option> (Ncat option)</primary></indexterm>
          <indexterm><primary><option>-U</option> (Ncat option)</primary><see><option>--unixsock</option></see></indexterm>
        </term>
        <listitem>
          <para>Use Unix domain sockets rather than network sockets.
          This option may be used on its own for stream sockets, or
          combined with <option>--udp</option> for datagram sockets.
          A description of <option>-U</option> mode is in
          <xref linkend="ncat-man-unixsock"/>.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>-u</option>,
          <option>--udp</option> (Use UDP)
          <indexterm><primary><option>--udp</option> (Ncat option)</primary></indexterm>
          <indexterm><primary><option>-u</option> (Ncat option)</primary><see><option>--udp</option></see></indexterm>
        </term>
        <listitem>
          <para>Use UDP for the connection (the default is TCP).</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--sctp</option> (Use SCTP)
          <indexterm><primary><option>--sctp</option> (Ncat option)</primary></indexterm>
        </term>
        <listitem>
          <para>Use SCTP for the connection (the default is TCP).
          SCTP support is implemented in TCP-compatible mode.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--vsock</option> (Use AF_VSOCK sockets)
          <indexterm><primary><option>--vsock</option> (Ncat option)</primary></indexterm>
        </term>
        <listitem>
          <para>Use AF_VSOCK sockets rather than the default TCP sockets (Linux only).
          This option may be used on its own for stream sockets or combined with <option>--udp</option> for datagram sockets.
          A description of <option>--vsock</option> mode is in
          <xref linkend="ncat-man-vsock"/>.</para>
        </listitem>
      </varlistentry>
    </variablelist>

  </refsect1>

  <refsect1 id="ncat-man-connect-options">
    <title>Connect Mode Options</title>

    <variablelist>
      <varlistentry>
        <term>
          <option>-g <replaceable>hop1</replaceable><optional>,<replaceable>hop2</replaceable>,...</optional></option> (Loose source routing)
          <indexterm><primary><option>-g</option> (Ncat option)</primary></indexterm>
        </term>
        <listitem>
          <para>Sets hops for IPv4 loose source routing.  You can use <option>-g</option>
          once with a comma-separated list of hops, use <option>-g</option> multiple
          times with single hops to build the list, or combine the two.  Hops can be
          given as IP addresses or hostnames.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>-G <replaceable>ptr</replaceable></option> (Set source routing pointer)
          <indexterm><primary><option>-G</option> (Ncat option)</primary></indexterm>
        </term>
        <listitem>
          <para>Sets the IPv4 source route <quote>pointer</quote> for use with <option>-g</option>.
          The argument must be a multiple of 4 and no more than 28.  Not all operating
          systems support setting this pointer to anything other than four.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>-p <replaceable>port</replaceable></option>,
          <option>--source-port <replaceable>port</replaceable></option> (Specify source port)
          <indexterm><primary><option>--source-port</option> (Ncat option)</primary></indexterm>
          <indexterm><primary><option>-p</option> (Ncat option)</primary><see><option>--source-port</option></see></indexterm>
        </term>
        <listitem>
          <para>Set the port number for Ncat to bind to.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>-s <replaceable>host</replaceable></option>,
          <option>--source <replaceable>host</replaceable></option> (Specify source address)
          <indexterm><primary><option>--source</option> (Ncat option)</primary></indexterm>
          <indexterm><primary><option>-s</option> (Ncat option)</primary><see><option>--source</option></see></indexterm>
        </term>
        <listitem>
          <para>Set the address for Ncat to bind to.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>-z</option> (Zero-I/O mode)
          <indexterm><primary><option>-z</option> (Ncat option)</primary></indexterm>
        </term>
        <listitem>
          <!-- We do not promote Ncat as a port scanner. That is what Nmap is for. -->
          <para>Causes Ncat to quit immediately after establishing a connection. This is provided
            for compatibility with other netcat implementations which use it as a test for
            connectivity or for port scanning. Ncat does not allow specifying multiple target
            ports.</para>
        </listitem>
      </varlistentry>
    </variablelist>

  </refsect1>

  <refsect1 id="ncat-man-listen-options">
    <title>Listen Mode Options</title>

    <para>See <xref linkend="ncat-man-access-options"/> for information on limiting the
    hosts that may connect to the listening Ncat process.</para>

    <variablelist>
      <varlistentry>
        <term>
          <option>-l</option>,
          <option>--listen</option> (Listen for connections)
          <indexterm><primary><option>--listen</option> (Ncat option)</primary></indexterm>
          <indexterm><primary><option>-l</option> (Ncat option)</primary><see><option>--listen</option></see></indexterm>
        </term>
        <listitem>
          <para>Listen for connections rather than connecting to a remote
          machine</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>-m <replaceable>numconns</replaceable></option>,
          <option>--max-conns <replaceable>numconns</replaceable></option> (Specify maximum number of connections)
          <indexterm><primary><option>--max-conns</option> (Ncat option)</primary></indexterm>
          <indexterm><primary><option>-m</option> (Ncat option)</primary><see><option>--max-conns</option></see></indexterm>
        </term>
        <listitem>
          <para>The maximum number of simultaneous connections accepted by an Ncat
          instance. 100 is the default (60 on Windows).</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>-k</option>,
          <option>--keep-open</option> (Accept multiple connections, do not quit on socket EOF)
          <indexterm><primary><option>--keep-open</option> (Ncat option)</primary></indexterm>
          <indexterm><primary><option>-k</option> (Ncat option)</primary><see><option>--keep-open</option></see></indexterm>
        </term>
        <listitem>
          <para>Normally a listening server accepts only one connection and
          then quits when the connection is closed. This option makes it accept
          multiple simultaneous connections and wait for more connections after
          they have all been closed. It must be combined with
          <option>--listen</option>. In this mode there is no way for Ncat to
          know when its network input is finished, so it will keep running
          until interrupted. This also means that it will never close its
          output stream, so any program reading from Ncat and looking for
          end-of-file will also hang.</para>
        <para>In connect mode, the <option>-k</option> option has a different
          meaning: when the remote end of a TCP connection shuts down its side
          of the connection, Ncat will keep running and sending any available
          data. This was the default behavior in Ncat 7.95 and earlier, but it
          was changed to better align with existing netcat implementations. TCP
          is the only transport that supports this option in connect mode.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--broker</option> (Connection brokering)
          <indexterm><primary><option>--broker</option> (Ncat option)</primary></indexterm>
        </term>
        <listitem>
          <para>Allow multiple parties to connect to a centralised Ncat server
          and communicate with each other. Ncat can broker communication between
          systems that are behind a NAT or otherwise unable to directly connect.
          This option is used in conjunction with <option>--listen</option>, which
          causes the <option>--listen</option> port to have broker mode enabled.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--chat</option> (Ad-hoc <quote>chat server</quote>)
          <indexterm><primary><option>--chat</option> (Ncat option)</primary></indexterm>
        </term>
        <listitem>
          <para>The <option>--chat</option> option enables chat mode, intended
          for the exchange of text between several users. In chat mode,
          connection brokering is turned on. Ncat prefixes each message received
          with an ID before relaying it to the other connections. The ID is
          unique for each connected client. This helps distinguish who sent
          what. Additionally, non-printing characters such as control characters
          are escaped to keep them from doing damage to a terminal.</para>
        </listitem>
      </varlistentry>
    </variablelist>

  </refsect1>

  <refsect1 id="ncat-man-ssl-options">
    <title>SSL Options</title>

    <variablelist>
      <varlistentry>
        <term>
          <option>--ssl</option> (Use SSL)
          <indexterm><primary><option>--ssl</option> (Ncat option)</primary></indexterm>
        </term>
        <listitem>
          <para>In connect mode, this option transparently negotiates an SSL
          session with an SSL server to securely encrypt the connection. This is
          particularly handy for talking to SSL enabled HTTP servers, etc.</para>
          <para>In server mode, this option listens for incoming SSL connections,
          rather than plain untunneled traffic.</para>
          <para>In UDP mode, this option enables Datagram TLS (DTLS).
          </para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--ssl-verify</option> (Verify server certificates)
          <indexterm><primary><option>--ssl-verify</option> (Ncat option)</primary></indexterm>
        </term>
        <listitem>
          <para>In client mode, <option>--ssl-verify</option> is like
          <option>--ssl</option> except that it also requires verification of
          the server certificate. Ncat comes with a default set of trusted
          certificates in the file
          <filename>ca-bundle.crt</filename>.<indexterm><primary><filename>ca-bundle.crt</filename></primary></indexterm>
          Some operating systems provide a default list of
          trusted certificates; these will also be used if available. Use
          <option>--ssl-trustfile</option> to give a custom list. Use
          <option>-v</option> one or more times to get details about
          verification failures.</para>
	  <indexterm><primary>revoked certificates</primary><see>certificate revocation</see></indexterm>
	  <para>Ncat does not check for revoked
	  certificates.<indexterm><primary>certification revocation</primary></indexterm></para>
          <para>This option has no effect in server mode.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--ssl-cert <replaceable>certfile.pem</replaceable></option> (Specify SSL certificate)
          <indexterm><primary><option>--ssl-cert</option> (Ncat option)</primary></indexterm>
        </term>
        <listitem>
          <para>This option gives the location of a PEM-encoded
          certificate files used to authenticate the server (in listen
          mode) or the client (in connect mode). Use it in combination
          with <option>--ssl-key</option>.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--ssl-key <replaceable>keyfile.pem</replaceable></option> (Specify SSL private key)
          <indexterm><primary><option>--ssl-key</option> (Ncat option)</primary></indexterm>
        </term>
        <listitem>
          <para>This option gives the location of the PEM-encoded
          private key file that goes with the certificate named with
          <option>--ssl-cert</option>.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--ssl-trustfile <replaceable>cert.pem</replaceable></option> (List trusted certificates)
          <indexterm><primary><option>--ssl-trustfile</option> (Ncat option)</primary></indexterm>
        </term>
        <listitem>
          <para>This option sets a list of certificates that are trusted for
          purposes of certificate verification. It has no effect unless combined
          with <option>--ssl-verify</option>. The argument to this option is the
          name of a PEM<indexterm><primary>PEM (Privacy Enhanced Mail)</primary></indexterm>
          file containing trusted certificates. Typically, the file will contain
          certificates of certification authorities, though it may also contain
          server certificates directly. When this option is used, Ncat does not
          use its default certificates.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--ssl-ciphers <replaceable>cipherlist</replaceable></option> (Specify SSL ciphersuites)
          <indexterm><primary><option>--ssl-ciphers</option> (Ncat option)</primary></indexterm>
        </term>
        <listitem>
          <para>This option sets the list of ciphersuites that Ncat will use
            when connecting to servers or when accepting SSL connections from
            clients. The syntax is described in the OpenSSL ciphers(1) man
            page, and defaults to
            <literal>ALL:!aNULL:!eNULL:!LOW:!EXP:!RC4:!MD5:@STRENGTH</literal></para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--ssl-servername <replaceable>name</replaceable></option> (Request distinct server name)
          <indexterm><primary><option>--ssl-servername</option> (Ncat option)</primary></indexterm>
        </term>
        <listitem>
          <para>In client mode, this option sets the TLS SNI (Server Name
            Indication) extension, which tells the server the name of the
            logical server Ncat is contacting. This is important when the
            target server hosts multiple virtual servers at a single underlying
            network address. If the option is not provided, the TLS SNI
            extension will be populated with the target server hostname.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--ssl-alpn <replaceable>ALPN list</replaceable></option> (Specify ALPN protocol list)
          <indexterm><primary><option>--ssl-alpn</option> (Ncat option)</primary></indexterm>
        </term>
        <listitem>
          <para>This option allows you to specify a comma-separated list of
            protocols to send via the Application-Layer Protocol Negotiation
            (ALPN) TLS extension. Not supported by all versions of OpenSSL.
          </para>
        </listitem>
      </varlistentry>

    </variablelist>

  </refsect1>

  <refsect1 id="ncat-man-proxy-options">
    <title>Proxy Options</title>

    <variablelist>
      <varlistentry>
        <term>
          <option>--proxy <replaceable>host</replaceable><optional>:<replaceable>port</replaceable></optional></option> (Specify proxy address)
          <indexterm><primary><option>--proxy</option> (Ncat option)</primary></indexterm>
        </term>
        <listitem>
          <para>Requests proxying through <replaceable>host</replaceable>:<replaceable>port</replaceable>,
          using the protocol specified by <option>--proxy-type</option>.</para>

          <para>If no port is specified, the proxy protocol's well-known port is used (1080 for
          SOCKS and 3128 for HTTP).  When specifying an IPv6 HTTP proxy server
          using the IP address rather than the hostname, the square-bracket
          notation (for example [2001:db8::1]:8080) MUST be used to separate
          the port from the IPv6 address.
          If the proxy requires authentication, use <option>--proxy-auth</option>.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--proxy-type <replaceable>proto</replaceable></option> (Specify proxy protocol)
          <indexterm><primary><option>--proxy-type</option> (Ncat option)</primary></indexterm>
        </term>
        <listitem>
          <para>In connect mode, this option requests the protocol <replaceable>proto</replaceable>
          to connect through the proxy host specified by <option>--proxy</option>.  In listen mode,
          this option has Ncat act as a proxy server using the specified protocol.</para>

          <para>The currently available protocols in connect mode are <literal>http</literal>
          (CONNECT), <literal>socks4</literal> (SOCKSv4), and
          <literal>socks5</literal> (SOCKSv5).
          The only server currently supported is <literal>http</literal>.
          If this option is not used, the default protocol is <literal>http</literal>.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--proxy-auth <replaceable>user</replaceable><optional>:<replaceable>pass</replaceable></optional></option> (Specify proxy credentials)
          <indexterm><primary><option>--proxy-auth</option> (Ncat option)</primary></indexterm>
        </term>
        <listitem>
          <para>In connect mode, gives the credentials that will be used to
          connect to the proxy server. In listen mode, gives the credentials
          that will be required of connecting clients. For use with
          <option>--proxy-type http</option> or
          <option>--proxy-type socks5</option>, the form should be
          username:password.  For
          <option>--proxy-type socks4</option>, it should be a username only.</para>
          <para>These credentials can be alternatively passed onto Ncat by
          setting environment variable
          <envar>NCAT_PROXY_AUTH</envar><indexterm><primary><envar>NCAT_PROXY_AUTH</envar> environment variable</primary></indexterm>,
          which reduces the risk of the credentials being captured in process
          logs.  (Option <option>--proxy-auth</option> takes precedence.)</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--proxy-dns <replaceable>type</replaceable></option> (Specify where to resolve proxy destination)
          <indexterm><primary><option>--proxy-dns</option> (Ncat option)</primary></indexterm>
        </term>
        <listitem>
          <para>In connect mode, it provides control over whether proxy
          destination hostnames are resolved by the remote proxy server or
          locally, by Ncat itself.
          Possible values for <replaceable>type</replaceable> are:</para>

          <para><literal>local</literal> - Hostnames are resolved locally on
          the Ncat host. Ncat exits with error if the hostname cannot be
          resolved.</para>

          <para><literal>remote</literal> - Hostnames are passed directly onto
          the remote proxy server. This is the default behavior.</para>

          <para><literal>both</literal> - Hostname resolution is first
          attempted on the Ncat host. Unresolvable hostnames are passed onto
          the remote proxy server.</para>

          <para><literal>none</literal> - Hostname resolution is completely
          disabled. Only a literal IPv4 or IPv6 address can be used as
          the proxy destination.</para>

          <para>Local hostname resolution generally respects IP version
          specified with options <option>-4</option> or <option>-6</option>,
          except for SOCKS4, which is incompatible with IPv6.</para>
        </listitem>
      </varlistentry>
    </variablelist>

  </refsect1>

  <refsect1 id="ncat-man-command-options">
    <title>Command Execution Options</title>

    <variablelist>
      <varlistentry>
        <term>
          <option>-e <replaceable>command</replaceable></option>,
          <option>--exec <replaceable>command</replaceable></option> (Execute command)
          <indexterm><primary><option>--exec</option> (Ncat option)</primary></indexterm>
          <indexterm><primary><option>-e</option> (Ncat option)</primary><see><option>--exec</option></see></indexterm>
        </term>
        <listitem>
          <para>Execute the specified command after a connection has been
          established. The command must be specified as a full pathname. All
          input from the remote client will be sent to the application and
          responses sent back to the remote client over the socket, thus
          making your command-line application interactive over a
          socket. Combined with <option>--keep-open</option>,
          Ncat will handle multiple simultaneous connections to your
          specified port/application like inetd. Ncat will only
          accept a maximum, definable, number of simultaneous connections
          controlled by the <option>-m</option> option. By default this is set
          to 100 (60 on Windows).</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>-c <replaceable>command</replaceable></option>,
          <option>--sh-exec <replaceable>command</replaceable></option> (Execute command via sh)
          <indexterm><primary><option>--sh-exec</option> (Ncat option)</primary></indexterm>
          <indexterm><primary><option>-c</option> (Ncat option)</primary><see><option>--sh-exec</option></see></indexterm>
        </term>
        <listitem>
          <para>Same as <option>-e</option>, except it tries to execute
          the command via <filename>/bin/sh</filename>. This means you don't
          have to specify the full path for the command, and shell facilities
          like environment variables are available.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--lua-exec <replaceable>file</replaceable></option> (Execute a .lua script)
          <indexterm><primary><option>--lua-exec</option> (Ncat option)</primary></indexterm>
        </term>
        <listitem>
          <para>Runs the specified file as a Lua script after a connection has been
          established, using a built-in interpreter. Both the script's standard input and
          the standard output are redirected to the connection data streams.</para>
        </listitem>
      </varlistentry>

    </variablelist>

    <para>All exec options add the following variables to the child's environment:</para>
    <variablelist>
    <varlistentry>
      <term><envar>NCAT_REMOTE_ADDR</envar></term><indexterm><primary><envar>NCAT_REMOTE_ADDR></envar> environment variable</primary></indexterm>
      <term><envar>NCAT_REMOTE_PORT</envar></term><indexterm><primary><envar>NCAT_REMOTE_PORT></envar> environment variable</primary></indexterm>
      <listitem>
      <para>
      The IP address and port number of the remote host. In connect mode, it's
      the target's address; in listen mode, it's the client's address.
      </para>
      </listitem>
    </varlistentry>
    <varlistentry>
      <term><envar>NCAT_LOCAL_ADDR</envar></term><indexterm><primary><envar>NCAT_LOCAL_ADDR></envar> environment variable</primary></indexterm>
      <term><envar>NCAT_LOCAL_PORT</envar></term><indexterm><primary><envar>NCAT_LOCAL_PORT></envar> environment variable</primary></indexterm>
      <listitem>
      <para>
      The IP address and port number of the local end of the connection.
      </para>
      </listitem>
    </varlistentry>
    <varlistentry>
      <term><envar>NCAT_PROTO</envar></term><indexterm><primary><envar>NCAT_PROTO></envar> environment variable</primary></indexterm>
      <listitem>
      <para>
      The protocol in use: one of <code>TCP</code>, <code>UDP</code>, and <code>SCTP</code>.
      </para>
      </listitem>
    </varlistentry>
    </variablelist>
  </refsect1>

  <refsect1 id="ncat-man-access-options">
    <title>Access Control Options</title>

    <variablelist>
      <varlistentry>
        <term>
          <option>--allow <replaceable>host</replaceable><optional>,<replaceable>host</replaceable>,...</optional></option> (Allow connections)
          <indexterm><primary><option>--allow</option> (Ncat option)</primary></indexterm>
        </term>
        <listitem>
          <para>The list of hosts specified will be the only hosts allowed
          to connect to the Ncat process. All other connection attempts will
          be disconnected. In case of a conflict between
          <option>--allow</option> and <option>--deny</option>,
          <option>--allow</option> takes precedence. Host
          specifications follow the same syntax used
          by Nmap.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--allowfile <replaceable>file</replaceable></option> (Allow connections from file)
          <indexterm><primary><option>--allowfile</option> (Ncat option)</primary></indexterm>
        </term>
        <listitem>
          <para>This has the same functionality as <option>--allow</option>,
          except that the allowed hosts are provided in a new-line delimited allow
          file, rather than directly on the command line.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--deny <replaceable>host</replaceable><optional>,<replaceable>host</replaceable>,...</optional></option> (Deny connections)
          <indexterm><primary><option>--deny</option> (Ncat option)</primary></indexterm>
        </term>
        <listitem>
          <para>Issue Ncat with a list of hosts that will not be allowed to connect
          to the listening Ncat process. Specified hosts will have their session
          silently terminated if they try to connect.
          In case of a conflict between
          <option>--allow</option> and <option>--deny</option>,
          <option>--allow</option> takes precedence. Host
          specifications follow the same syntax used by Nmap.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--denyfile <replaceable>file</replaceable></option> (Deny connections from file)
          <indexterm><primary><option>--denyfile</option> (Ncat option)</primary></indexterm>
        </term>
        <listitem>
          <para>This is the same functionality as <option>--deny</option>,
          except that excluded hosts are provided in a new-line delimited deny
          file, rather than directly on the command line.</para>
        </listitem>
      </varlistentry>
    </variablelist>

  </refsect1>

  <refsect1 id="ncat-man-timing-options">
    <title>Timing Options</title>

    <para>These options accept a <literal>time</literal> parameter.  This is specified
    in seconds by default, though you can append <literal>ms</literal>, <literal>s</literal>, <literal>m</literal>,
    or <literal>h</literal> to the value to specify milliseconds, seconds, minutes, or hours.</para>

    <variablelist>
      <varlistentry>
        <term>
          <option>-d <replaceable>time</replaceable></option>,
          <option>--delay <replaceable>time</replaceable></option> (Specify line delay)
          <indexterm><primary><option>--delay</option> (Ncat option)</primary></indexterm>
          <indexterm><primary><option>-d</option> (Ncat option)</primary><see><option>--delay</option></see></indexterm>
        </term>
        <listitem>
          <para>Set the delay interval for lines sent. This effectively limits
          the number of lines that Ncat will send in the specified period. This
          may be useful for low-bandwidth sites, or have other uses such as
          coping with
          annoying <command>iptables --limit</command> options.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>-i <replaceable>time</replaceable></option>,
          <option>--idle-timeout <replaceable>time</replaceable></option> (Specify idle timeout)
          <indexterm><primary><option>--idle-timeout</option> (Ncat option)</primary></indexterm>
          <indexterm><primary><option>-i</option> (Ncat option)</primary><see><option>--idle-timeout</option></see></indexterm>
        </term>
        <listitem>
          <para>Set a fixed timeout for idle connections. If the idle timeout
          is reached, the connection is terminated.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>-q <replaceable>time</replaceable></option> (Delay quit after EOF on stdin)
          <indexterm><primary><option>-q</option> (Ncat option)</primary></indexterm>
        </term>
        <listitem>
          <para>After EOF on stdin, wait the specified amount of time, then quit. If <replaceable>time</replaceable> is negative, wait forever. Ncat may still quit based on idle timeout or EOF on the socket.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>-w <replaceable>time</replaceable></option>,
          <option>--wait <replaceable>time</replaceable></option> (Specify connect timeout)
          <indexterm><primary><option>--wait</option> (Ncat option)</primary></indexterm>
          <indexterm><primary><option>-w</option> (Ncat option)</primary><see><option>--wait</option></see></indexterm>
        </term>
        <listitem>
          <para>Set a fixed timeout for connection attempts.</para>
        </listitem>
      </varlistentry>
    </variablelist>

  </refsect1>

  <refsect1 id="ncat-man-output-options">
    <title>Output Options</title>

    <variablelist>
      <varlistentry>
        <term>
          <option>-o <replaceable>file</replaceable></option>,
          <option>--output <replaceable>file</replaceable></option> (Save session data)
          <indexterm><primary><option>--output</option> (Ncat option)</primary></indexterm>
          <indexterm><primary><option>-o</option> (Ncat option)</primary><see><option>--output</option></see></indexterm>
        </term>
        <listitem>
          <para>Dump session data to a file</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>-x <replaceable>file</replaceable></option>,
          <option>--hex-dump <replaceable>file</replaceable></option> (Save session data in hex)
          <indexterm><primary><option>--hex-dump</option> (Ncat option)</primary></indexterm>
          <indexterm><primary><option>-x</option> (Ncat option)</primary><see><option>--hex-dump</option></see></indexterm>
        </term>
        <listitem>
          <para>Dump session data in hex to a file.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--append-output</option> (Append output)
          <indexterm><primary><option>--append-output</option> (Ncat option)</primary></indexterm>
        </term>
        <listitem>
          <para>Issue Ncat with <option>--append-ouput</option> along with
          <option>-o</option> and/or <option>-x</option> and it will append
          the resulted output rather than truncating the specified output files.
          </para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>-v</option>,
          <option>--verbose</option> (Be verbose)
          <indexterm><primary><option>--verbose</option> (Ncat option)</primary></indexterm>
          <indexterm><primary><option>-v</option> (Ncat option)</primary><see><option>--verbose</option></see></indexterm>
        </term>
        <listitem>
          <para>Issue Ncat with <option>-v</option> and it will be verbose and
          display all kinds of useful connection based information. Use more
          than once (<option>-vv</option>, <option>-vvv</option>...) for greater
          verbosity.</para>
        </listitem>
      </varlistentry>
    </variablelist>

  </refsect1>

  <refsect1 id="ncat-man-misc-options">
    <title>Misc Options</title>

    <variablelist>
      <varlistentry>
        <term>
          <option>-C</option>,
          <option>--crlf</option> (Use CRLF as EOL)
          <indexterm><primary><option>--crlf</option> (Ncat option)</primary></indexterm>
          <indexterm><primary><option>-C</option> (Ncat option)</primary><see><option>--crlf</option></see></indexterm>
        </term>
        <listitem>
          <para>This option tells Ncat to convert
          LF<indexterm><primary>LF line ending</primary></indexterm>
          line endings to
          CRLF<indexterm><primary>CRLF line ending</primary></indexterm>
          when taking input from
          standard input.<indexterm><primary>standard input</primary></indexterm>
          This is useful for talking to some stringent
          servers directly from a terminal in one of the many common plain-text
          protocols that use CRLF for end-of-line.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>-h</option>,
          <option>--help</option> (Help screen)
          <indexterm><primary><option>--help</option> (Ncat option)</primary></indexterm>
          <indexterm><primary><option>-h</option> (Ncat option)</primary><see><option>--help</option></see></indexterm>
        </term>
        <listitem>
          <para>Displays a short help screen with common options and parameters,
          and then exits.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--recv-only</option> (Only receive data)
          <indexterm><primary><option>--recv-only</option> (Ncat option)</primary></indexterm>
        </term>
        <listitem>
          <para>If this option is passed, Ncat will only receive data and will
          not try to send anything.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--send-only</option> (Only send data)
          <indexterm><primary><option>--send-only</option> (Ncat option)</primary></indexterm>
        </term>
        <listitem>
          <para>If this option is passed, then Ncat will only send data and will
          ignore anything received.  This option also causes Ncat to close the
          network connection and terminate after EOF is received on standard
          input.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--no-shutdown</option> (Do not shutdown into half-duplex mode)
          <indexterm><primary><option>--no-shutdown</option> (Ncat option)</primary></indexterm>
        </term>
        <listitem>
          <para>If this option is passed, Ncat will not invoke shutdown on a
          socket after seeing EOF on stdin. This is provided for
          backward-compatibility with OpenBSD netcat, which exhibits this
          behavior when executed with its '-d' option.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>-n</option>,
          <option>--nodns</option> (Do not resolve hostnames)
          <indexterm><primary><option>--nodns</option> (Ncat option)</primary></indexterm>
          <indexterm><primary><option>-n</option> (Ncat option)</primary><see><option>--nodns</option></see></indexterm>
        </term>
        <listitem>
          <para>Completely disable hostname resolution across all Ncat options,
          such as the destination, source address, source routing hops, and
          the proxy. All addresses must be specified numerically.
          (Note that resolution of proxy destinations is controlled separately
          via option <option>--proxy-dns</option>.)</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>-t</option>,
          <option>--telnet</option> (Answer Telnet negotiations)
          <indexterm><primary><option>--telnet</option> (Ncat option)</primary></indexterm>
          <indexterm><primary><option>-t</option> (Ncat option)</primary><see><option>--telnet</option></see></indexterm>
        </term>
        <listitem>
          <para>Handle DO/DONT WILL/WONT Telnet negotiations.  This makes it
          possible to script Telnet sessions with Ncat.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>
          <option>--version</option> (Display version)
          <indexterm><primary><option>--version</option> (Ncat option)</primary></indexterm>
        </term>
        <listitem>
          <para>Displays the Ncat version number and exits.</para>
        </listitem>
      </varlistentry>

    </variablelist>
  </refsect1>

  <refsect1 id="ncat-man-unixsock">
    <title>Unix Domain Sockets</title>

    <para>
    The <option>-U</option> option (same as <option>--unixsock</option>)
    causes Ncat to use Unix domain sockets rather than network sockets.
    Unix domain sockets exist as an entry in the filesystem. You must
    give the name of a socket to connect to or to listen on. For
    example, to make a connection,
    </para>
    <para><command>ncat -U ~/unixsock</command></para>
    <para>
    To listen on a socket:
    </para>
    <para><command>ncat -l -U ~/unixsock</command></para>
    <para>
    Listen mode will create the socket if it doesn't exist. The socket
    will continue to exist after the program ends.
    </para>

    <para>
    Both stream and datagram domain sockets are supported. Use
    <option>-U</option> on its own for stream sockets, or
    combine it with <option>--udp</option> for datagram sockets.
    Datagram sockets require a source socket to connect from. By
    default, a source socket with a random filename will be created as
    needed, and deleted when the program ends. Use the
    <option>--source</option> with a path to use a source socket with a
    specific name.
    </para>

  </refsect1>

  <refsect1 id="ncat-man-vsock">
    <title>AF_VSOCK Sockets</title>

    <para>
      The <option>--vsock</option> option causes Ncat to use AF_VSOCK
      sockets rather than network sockets. A CID must be given instead of a
      hostname or IP address. For example, to make a connection to the host,
    </para>
    <para><command>ncat --vsock 2 1234</command></para>
    <para>
    To listen on a socket:
    </para>
    <para><command>ncat -l --vsock 1234</command></para>
    <para>
    Both stream and datagram domain sockets are supported, but socket type
    availability depends on the hypervisor. Use
    <option>--vsock</option> on its own for stream sockets, or
    combine it with <option>--udp</option> for datagram sockets.
    </para>
  </refsect1>
  <refsect1 id="ncat-man-examples">
    <title>Examples</title>

    <variablelist>
      <varlistentry>
        <term>
        Connect to example.org on TCP port 8080.
        </term>
        <listitem>
        <para><command>ncat example.org 8080</command></para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term>
        Listen for connections on TCP port 8080.
        </term>
        <listitem>
        <para><command>ncat -l 8080</command></para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term>
        Redirect TCP port 8080 on the local machine to host on port 80.
        </term>
        <listitem>
        <para><command>ncat --sh-exec "ncat example.org 80" -l 8080 --keep-open</command></para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term>
        Bind to TCP port 8081 and attach <filename>/bin/bash</filename>
        for the world to access freely.
        </term>
        <listitem>
        <para><command>ncat --exec "/bin/bash" -l 8081 --keep-open</command></para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term>
        Bind a shell to TCP port 8081, limit access to hosts on a local
        network, and limit the maximum number of simultaneous connections to 3.
        </term>
        <listitem>
        <para><command>ncat --exec "/bin/bash" --max-conns 3 --allow 192.168.0.0/24 -l 8081 --keep-open</command></para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term>
        Connect to smtphost:25 through a SOCKS4 server on port 1080.
        </term>
        <listitem>
        <para><command>ncat --proxy socks4host --proxy-type socks4 --proxy-auth joe smtphost 25</command></para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term>
        Connect to smtphost:25 through a SOCKS5 server on port 1080.
        </term>
        <listitem>
        <para><command>ncat --proxy socks5host --proxy-type socks5 --proxy-auth joe:secret smtphost 25</command></para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term>
        Create an HTTP proxy server on localhost port 8888.
        </term>
        <listitem>
        <para><command>ncat -l --proxy-type http localhost 8888</command></para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term>
        Send a file over TCP port 9899 from host2 (client) to host1
        (server).
        </term>
        <listitem>
<para>HOST1$ <command>ncat -l 9899 &gt; outputfile</command></para>
<para>HOST2$ <command>ncat HOST1 9899 &lt; inputfile</command></para>
        </listitem>
      </varlistentry>
      <varlistentry>
        <term>
        Transfer in the other direction, turning Ncat into a <quote>one
        file</quote> server.
        </term>
        <listitem>
<para>HOST1$ <command>ncat -l 9899 &lt; inputfile</command></para>
<para>HOST2$ <command>ncat HOST1 9899 &gt; outputfile</command></para>
        </listitem>
      </varlistentry>
    </variablelist>
  </refsect1>

  <refsect1 id="ncat-man-exit-code">
    <title>Exit Code</title>

    <para>The exit code reflects whether a connection was made and
    completed successfully. 0 means there was no error. 1 means there
    was a network error of some kind, for example <quote>Connection
    refused</quote> or <quote>Connection reset</quote>. 2 is reserved
    for all other errors, like an invalid option or a nonexistent
    file.</para>
  </refsect1>

  <refsect1 id="ncat-man-bugs">
    <title>Bugs</title>

    <para>Like its authors, Ncat isn't perfect.  But you can help make
    it better by sending bug reports or even writing patches.  If Ncat
    doesn't behave the way you expect, first upgrade to the latest
    version available from <ulink
    url="https://nmap.org"/>.  If the problem persists,
    do some research to determine whether it has already been
    discovered and addressed.  Try Googling the error message or
    browsing the <citetitle>nmap-dev</citetitle> archives at <ulink
    url="https://seclists.org/" />.
    <indexterm><primary><citetitle>nmap-dev</citetitle> mailing list</primary></indexterm>
    Read this full manual page as
    well.  If nothing comes of this, mail a bug report to
    <email>dev@nmap.org</email>.  Please include everything
    you have learned about the problem, as well as what version of
    Ncat you are running and what operating system version it is
    running on.  Problem reports and Ncat usage questions sent to
    dev@nmap.org are far more likely to be answered than
    those sent to Fyodor directly.</para>

    <para>Code patches to fix bugs are even better than bug reports.
    Basic instructions for creating patch files with your changes are
    available at <ulink
    url="https://svn.nmap.org/nmap/HACKING" />.  Patches may
    be sent to <citetitle>nmap-dev</citetitle> (recommended) or to Fyodor directly.</para>
  </refsect1>

  <refsect1 id="ncat-man-author">
    <title>Authors</title>

    <itemizedlist>
      <listitem>
        <para>Chris Gibson <email>chris@linuxops.net</email></para>
      </listitem>
      <listitem>
        <para>Gordon Lyon (Fyodor)<email>fyodor@nmap.org</email>
        (<ulink url="http://insecure.org" />)</para>
      </listitem>
      <listitem>
        <para>Kris Katterjohn <email>katterjohn@gmail.com</email></para>
      </listitem>
      <listitem>
        <para>Mixter <email>mixter@gmail.com</email></para>
      </listitem>
    </itemizedlist>

    <para>The original Netcat was written by *Hobbit* <email>hobbit@avian.org</email>.
    While Ncat isn't built on any code from the <quote>traditional</quote> Netcat (or any
    other implementation), Ncat is most definitely based on Netcat in spirit
    and functionality.</para>
  </refsect1>

  <refsect1 id='ncat-man-legal'>
  <title>Legal Notices</title>

<refsect2 id="ncat-copyright">
  <title>Ncat Copyright and Licensing</title>
  <indexterm><primary>copyright</primary></indexterm>

  <para>Ncat is (C) 2005&ndash;2022 Nmap Software LLC. It is distributed
  as free and open source software under the same license terms as our
  Nmap software.  Precise terms and further details are available
  <man>from <ulink url="https://nmap.org/man/man-legal.html"/>.</man>
  <notman>in <xref linkend="nmap-copyright"/>.</notman></para>
</refsect2>

<refsect2 id="ncat-man-copyright">
  <title>Creative Commons License for this Ncat Guide</title>
  <para>This <citetitle>Ncat Reference Guide</citetitle> is (C)
  2005&ndash;2022 Nmap Software LLC. It is
   hereby placed under version 3.0 of the <ulink
   url="http://creativecommons.org/licenses/by/3.0/">Creative Commons
   Attribution License</ulink>.  This allows you redistribute and modify
   the work as you desire, as long as you credit the original source.
   Alternatively, you may choose to treat this document as falling under
   the same license as Ncat itself (discussed previously).</para>
</refsect2>

<refsect2 id="ncat-source-contrib">
  <title>Source Code Availability and Community Contributions</title>

<para>Source is provided to this software because we believe users
have a right to know exactly what a program is going to do before they
run it. This also allows you to audit the software for security holes
(none have been found so far).</para>

<para>Source code also allows you to port Nmap (which includes Ncat)
to new platforms, fix bugs, and add new features.  You are highly
encouraged to send your changes to
<email>dev@nmap.org</email> for possible incorporation into
the main distribution.  By sending these changes to Fyodor or one of
the Insecure.Org development mailing lists, it is assumed that you are
offering the Nmap Project (Nmap Software LLC) the unlimited,
non-exclusive right to reuse, modify, and relicense the code.  Nmap
will always be available open source,<indexterm><primary>open
source</primary></indexterm> but this is important because the
inability to relicense code has caused devastating problems for other
Free Software projects (such as KDE and NASM).  We also occasionally
relicense the code to third parties as discussed in the Nmap man page.
If you wish to specify special license conditions of your
contributions, just say so when you send them.</para>

</refsect2>

<refsect2 id="ncat-no-warranty"><title>No Warranty<indexterm><primary>warranty (lack of)</primary></indexterm></title>

<para>This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the Nmap Public Source
License for more details at <ulink url="https://nmap.org/npsl/" />,
or in the <filename>LICENSE</filename> file
included with Nmap.</para>
</refsect2>

<refsect2 id="ncat-inappropriate-usage"><title>Inappropriate Usage</title>
<para>Ncat should never be installed with special privileges
(e.g. suid root).<indexterm><primary>suid</primary><see>setuid</see></indexterm>
That would open up a major security vulnerability as other users on the
system (or attackers) could use it for privilege escalation.
</para>
</refsect2>

<refsect2 id="ncat-third-party-soft"><title>Third-Party Software</title>

<para>This product includes software developed by
the <ulink role="hidepdf" url="http://www.apache.org">Apache Software
Foundation</ulink>.  A modified version of the <ulink role="hidepdf"
url="http://www.tcpdump.org">Libpcap portable packet capture
library</ulink><indexterm><primary>libpcap</primary></indexterm>
is distributed along with Ncat.
The Windows version of Ncat utilized the Libpcap-derived
<ulink role="hidepdf" url="https://npcap.com">Npcap library</ulink><indexterm><primary>Npcap</primary></indexterm>
instead.
Certain raw networking functions use the
<ulink role="hidepdf" url="http://libdnet.sourceforge.net">Libdnet</ulink><indexterm><primary>libdnet</primary></indexterm>
networking library, which was written by Dug Song.<indexterm><primary>Song, Dug</primary></indexterm>
A modified version is distributed with Ncat.
Ncat can optionally link with the
<ulink role="hidepdf" url="http://www.openssl.org">OpenSSL
cryptography toolkit</ulink><indexterm><primary>OpenSSL</primary></indexterm>
for SSL version detection support.
All of the third-party software described in this paragraph is freely
redistributable under BSD-style software licenses.</para>
</refsect2>
</refsect1>

</refentry>