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Updated: July 2014

ncat (1)


ncat - Concatenate and redirect sockets


ncat [OPTIONS...] [hostname] [port]


Ncat Reference Guide                                      NCAT(1)

     ncat - Concatenate and redirect sockets

     ncat [OPTIONS...] [hostname] [port]

     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.

     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 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.

         Ncat 6.25 ( )
         Usage: ncat [options] [hostname] [port]

         Options taking a time assume seconds. Append 'ms' for milliseconds,
         's' for seconds, 'm' for minutes, or 'h' for hours (e.g. 500ms).
           -4                         Use IPv4 only
           -6                         Use IPv6 only
           -U, --unixsock             Use Unix domain sockets only
           -C, --crlf                 Use CRLF for EOL sequence
           -c, --sh-exec <command>    Executes the given command via /bin/sh
           -e, --exec <command>       Executes the given command
           -g hop1[,hop2,...]         Loose source routing hop points (8 max)
           -G <n>                     Loose source routing hop pointer (4, 8, 12, ...)
           -m, --max-conns <n>        Maximum <n> simultaneous connections
           -h, --help                 Display this help screen
           -d, --delay <time>         Wait between read/writes
           -o, --output <filename>    Dump session data to a file
           -x, --hex-dump <filename>  Dump session data as hex to a file
           -i, --idle-timeout <time>  Idle read/write timeout
           -p, --source-port port     Specify source port to use
           -s, --source addr          Specify source address to use (doesn't affect -l)
           -l, --listen               Bind and listen for incoming connections
           -k, --keep-open            Accept multiple connections in listen mode
           -n, --nodns                Do not resolve hostnames via DNS
           -t, --telnet               Answer Telnet negotiations
           -u, --udp                  Use UDP instead of default TCP

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               --sctp                 Use SCTP instead of default TCP
           -v, --verbose              Set verbosity level (can be used up to 3 times)
           -w, --wait <time>          Connect timeout
               --append-output        Append rather than clobber specified output files
               --send-only            Only send data, ignoring received; quit on EOF
               --recv-only            Only receive data, never send anything
               --allow                Allow only given hosts to connect to Ncat
               --allowfile            A file of hosts allowed to connect to Ncat
               --deny                 Deny given hosts from connecting to Ncat
               --denyfile             A file of hosts denied from connecting to Ncat
               --broker               Enable Ncat's connection brokering mode
               --chat                 Start a simple Ncat chat server
               --proxy <addr[:port]>  Specify address of host to proxy through
               --proxy-type <type>    Specify proxy type ("http" or "socks4")
               --proxy-auth <auth>    Authenticate with HTTP or SOCKS proxy server
               --ssl                  Connect or listen with SSL
               --ssl-cert             Specify SSL certificate file (PEM) for listening
               --ssl-key              Specify SSL private key (PEM) for listening
               --ssl-verify           Verify trust and domain name of certificates
               --ssl-trustfile        PEM file containing trusted SSL certificates
               --version              Display Ncat's version information and exit

         See the ncat(1) manpage for full options, descriptions and usage examples

     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.

     In connect mode, the hostname and port arguments tell what
     to connect to.  hostname is required, and may be a hostname
     or IP address. If port is supplied, it must be a decimal
     port number. If omitted, it defaults to 31337..

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

     -4 (IPv4 only) .
         Force the use of IPv4 only.

     -6 (IPv6 only) .
         Force the use of IPv6 only.

     -U, --unixsock (Use Unix domain sockets) .
         Use Unix domain sockets rather than network sockets.
         This option may be used on its own for stream sockets,

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         or combined with --udp for datagram sockets. A
         description of -U mode is in the section called "UNIX

     -u, --udp (Use UDP) .
         Use UDP for the connection (the default is TCP).

     --sctp (Use SCTP) .
         Use SCTP for the connection (the default is TCP). SCTP
         support is implemented in TCP-compatible mode.

     -g hop1[,hop2,...] (Loose source routing) .
         Sets hops for IPv4 loose source routing. You can use -g
         once with a comma-separated list of hops, use -g
         multiple times with single hops to build the list, or
         combine the two. Hops can be given as IP addresses or

     -G ptr (Set source routing pointer) .
         Sets the IPv4 source route "pointer" for use with -g.
         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.

     -p port, --source-port port (Specify source port) .
         Set the port number for Ncat to bind to.

     -s host, --source host (Specify source address) .
         Set the address for Ncat to bind to.

     See the section called "ACCESS CONTROL OPTIONS" for
     information on limiting the hosts that may connect to the
     listening Ncat process.

     -l, --listen (Listen for connections) .
         Listen for connections rather than connecting to a
         remote machine

     -m numconns, --max-conns numconns (Specify maximum number of
     connections) .
         The maximum number of simultaneous connections accepted
         by an Ncat instance. 100 is the default.

     -k, --keep-open (Accept multiple connections) .
         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 --listen. In this mode
         there is no way for Ncat to know when its network input

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         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.

     --broker (Connection brokering) .
         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 --listen, which causes the
         --listen port to have broker mode enabled.

     --chat (Ad-hoc "chat server") .
         The --chat 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.

     --ssl (Use SSL) .
         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.

         In server mode, this option listens for incoming SSL
         connections, rather than plain untunneled traffic.

     --ssl-verify (Verify server certificates) .
         In client mode, --ssl-verify is like --ssl except that
         it also requires verification of the server certificate.
         Ncat comes with a default set of trusted certificates in
         the file ca-bundle.crt.  --ssl-trustfile to give a
         custom list. Use -v one or more times to get details
         about verification failures.  Ncat does not check for
         revoked certificates.

         This option has no effect in server mode.

     --ssl-cert certfile.pem (Specify SSL certificate) .
         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 --ssl-key.

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     --ssl-key keyfile.pem (Specify SSL private key) .
         This option gives the location of the PEM-encoded
         private key file that goes with the certificate named
         with --ssl-cert.

     --ssl-trustfile cert.pem (List trusted certificates) .
         This option sets a list of certificates that are trusted
         for purposes of certificate verification. It has no
         effect unless combined with --ssl-verify. The argument
         to this option is the name of a PEM.  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

     --proxy host[:port] (Specify proxy address) .
         Requests proxying through host:port, using the protocol
         specified by --proxy-type.

         If no port is specified, the proxy protocol's well-known
         port is used (1080 for SOCKS and 3128 for HTTP).
         However, when specifying an IPv6 HTTP proxy server using
         the IP address rather than the hostname, the port number
         MUST be specified as well. If the proxy requires
         authentication, use --proxy-auth.

     --proxy-type proto (Specify proxy protocol) .
         In connect mode, this option requests the protocol proto
         to connect through the proxy host specified by --proxy.
         In listen mode, this option has Ncat act as a proxy
         server using the specified protocol.

         The currently available protocols in connect mode are
         http (CONNECT) and socks4 (SOCKSv4). The only server
         currently supported is http. If this option is not used,
         the default protocol is http.

     --proxy-auth user[:pass] (Specify proxy credentials) .
         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 --proxy-type http, the form should
         be user:pass. For --proxy-type socks4, it should be a
         username only.

     -e command, --exec command (Execute command) .
         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

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         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 --keep-open, 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 -m option. By default this is set to

     -c command, --sh-exec command (Execute command via sh) .
         Same as -e, except it tries to execute the command via
         /bin/sh. This means you don't have to specify the full
         path for the command, and shell facilities like
         environment variables are available.

     --allow host[,host,...] (Allow connections) .
         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 --allow and --deny, --allow takes
         precedence. Host specifications follow the same syntax
         used by Nmap.

     --allowfile file (Allow connections from file) .
         This has the same functionality as --allow, except that
         the allowed hosts are provided in a new-line delimited
         allow file, rather than directly on the command line.

     --deny host[,host,...] (Deny connections) .
         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
         --allow and --deny, --allow takes precedence. Host
         specifications follow the same syntax used by Nmap.

     --denyfile file (Deny connections from file) .
         This is the same functionality as --deny, except that
         excluded hosts are provided in a new-line delimited deny
         file, rather than directly on the command line.

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

     -d time, --delay time (Specify line delay) .
         Set the delay interval for lines sent. This effectively
         limits the number of lines that Ncat will send in the

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         specified period. This may be useful for low-bandwidth
         sites, or have other uses such as coping with annoying
         iptables --limit options.

     -i time, --idle-timeout time (Specify idle timeout) .
         Set a fixed timeout for idle connections. If the idle
         timeout is reached, the connection is terminated.

     -w time, --wait time (Specify connect timeout) .
         Set a fixed timeout for connection attempts.

     -o file, --output file (Save session data) .
         Dump session data to a file

     -x file, --hex-dump file (Save session data in hex) .
         Dump session data in hex to a file.

     --append-output (Append output) .
         Issue Ncat with --append-ouput along with -o and/or -x
         and it will append the resulted output rather than
         truncating the specified output files.

     -v, --verbose (Be verbose) .
         Issue Ncat with -v and it will be verbose and display
         all kinds of useful connection based information. Use
         more than once (-vv, -vvv) for greater verbosity.  -vvv
         is the maximum level.

     -C, --crlf (Use CRLF as EOL) .
         This option tells Ncat to convert LF.  line endings to
         CRLF.  when taking input from standard input..  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.

     -h, --help (Help screen) .
         Displays a short help screen with common options and
         parameters, and then exits.

     --recv-only (Only receive data) .
         If this option is passed, Ncat will only receive data
         and will not try to send anything.

     --send-only (Only send data) .
         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.

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     -t, --telnet (Answer Telnet negotiations) .
         Handle DO/DONT WILL/WONT Telnet negotiations. This makes
         it possible to script Telnet sessions with Ncat.

     --version (Display version) .
         Displays the Ncat version number and exits.

     The -U option (same as --unixsock) 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,

     ncat -U ~/unixsock

     To listen on a socket:

     ncat -l -U ~/unixsock

     Listen mode will create the socket if it doesn't exist. The
     socket will continue to exist after the program ends.

     Both stream and datagram domain sockets are supported. Use
     -U on its own for stream sockets, or combine it with --udp
     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 --source with a path to use a
     source socket with a specific name.

     Connect to on TCP port 8080.
         ncat 8080

     Listen for connections on TCP port 8080.
         ncat -l 8080

     Redirect TCP port 8080 on the local machine to host on port
         ncat --sh-exec "ncat 80" -l 8080 --keep-open

     Bind to TCP port 8081 and attach /bin/bash for the world to
     access freely.
         ncat --exec "/bin/bash" -l 8081 --keep-open

     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.
         ncat --exec "/bin/bash" --max-conns 3 --allow -l 8081 --keep-open

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     Connect to smtphost:25 through a SOCKS4 server on port 1080.
         ncat --proxy socks4host --proxy-type socks4 --proxy-auth
         user smtphost 25

     Create an HTTP proxy server on localhost port 8888.
         ncat -l --proxy-type http localhost 8888

     Send a file over TCP port 9899 from host2 (client) to host1
         HOST1$ ncat -l 9899 > outputfile

         HOST2$ ncat HOST1 9899 < inputfile

     Transfer in the other direction, turning Ncat into a "one
     file" server.
         HOST1$ ncat -l 9899 < inputfile

         HOST2$ ncat HOST1 9899 > outputfile

     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
     "Connection refused" or "Connection reset". 2 is reserved
     for all other errors, like an invalid option or a
     nonexistent file.

     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 blue]]. 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 nmap-dev archives at blue]-].  Read this full manual page as well.
     If nothing comes of this, mail a bug report to 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 are far more likely to be answered
     than those sent to Fyodor directly.

     Code patches to fix bugs are even better than bug reports.
     Basic instructions for creating patch files with your
     changes are available at blue]-]. Patches may be sent to
     nmap-dev (recommended) or to Fyodor directly.

     o   Chris Gibson

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     o   Kris Katterjohn

     o   Mixter

     o   Fyodor (blue]])

     The original Netcat was written by *Hobbit* While Ncat isn't built on any code from
     the "traditional" Netcat (or any other implementation), Ncat
     is most definitely based on Netcat in spirit and

  Ncat Copyright and Licensing
     Ncat is (C) 2005-2012 Insecure.Com 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 from blue]].

  Creative Commons License for this Ncat Guide
     This Ncat Reference Guide is (C) 2005-2012 Insecure.Com LLC.
     It is hereby placed under version 3.0 of the blue]Creative
     Commons Attribution License][1]. 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 Ncap itself (discussed previously).

  Source Code Availability and Community Contributions
     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).

     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 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 (Insecure.Com LLC)
     the unlimited, non-exclusive right to reuse, modify, and
     relicense the code. Nmap will always be available open
     source,.  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.

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  No Warranty.
     This program is distributed in the hope that it will be
     useful, but WITHOUT ANY WARRANTY; without even the implied
     PURPOSE. See the GNU General Public License v2.0 for more
     details at blue]],
     or in the COPYING file included with Nmap.

  Inappropriate Usage
     Ncat should never be installed with special privileges (e.g.
     suid root)..  That would open up a major security
     vulnerability as other users on the system (or attackers)
     could use it for privilege escalation.

  Third-Party Software
     This product includes software developed by the blue]Apache
     Software Foundation][2]. A modified version of the
     blue]Libpcap portable packet capture library][3].  is
     distributed along with Ncat. The Windows version of Ncat
     utilized the Libpcap-derived blue]WinPcap library][4].
     instead. Certain raw networking functions use the
     blue]Libdnet][5].  networking library, which was written by
     Dug Song..  A modified version is distributed with Ncat.
     Ncat can optionally link with the blue]OpenSSL cryptography
     toolkit][6].  for SSL version detection support. All of the
     third-party software described in this paragraph is freely
     redistributable under BSD-style software licenses.

     See attributes(5) for descriptions of the following

     |Availability   | diagnostic/nmap  |
     |Stability      | Volatile         |
      1. Creative Commons Attribution License

      2. Apache Software Foundation

      3. Libpcap portable packet capture library

      4. WinPcap library

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      5. Libdnet

      6. OpenSSL cryptography toolkit

     This software was built from source available at  The original
     community source was downloaded from

     Further information about this software can be found on the
     open source community website at

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