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Man page of NCAT


Section: Ncat Reference Guide (1)
Updated: 06/28/2011
Index of this MAN page

Back To MAN Pages From BackTrack 5 R1 Master List  


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 5.59BETA1 ( http://nmap.org/ncat )
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
  -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               Dump session data to a file
  -x, --hex-dump             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
      --sctp                 Use SCTP instead of default TCP
  -v, --verbose              Set verbosity level (can be used up to 3 times)
  -w, --wait <time>          Connect timeout
      --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. If port is omitted, it defaults to 31337.  


-4 (IPv4 only) .

Force the use of IPv4 only (default).

-6 (IPv6 only) .

Force the use of IPv6 only.

-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 hostnames.

-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 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.

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.

--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 certificates.


--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 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 100.

-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. be disconnected. 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 hours.

-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 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. This can be used to "replay" sessions.

-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.

-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.


ncat example.org 8080.RE

ncat -l 8080.RE

ncat --sh-exec "ncat example.org 80" -l 8080 --keep-open.RE

ncat --exec "/bin/bash" -l 8081 --keep-open.RE

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

ncat --proxy socks4host --proxy-type socks4 --proxy-auth user smtphost 25.RE

ncat -l --proxy-type http localhost 8888.RE

HOST1$ ncat -l 9899 > outputfile

HOST2$ ncat HOST1 9899 < inputfile

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 http://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 nmap-dev archives at http://seclists.org/. Read this full manual page as well. If nothing comes of this, mail a bug report to nmap-dev@insecure.org. 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 nmap-dev@insecure.org 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 http://nmap.org/data/HACKING. Patches may be sent to nmap-dev (recommended) or to Fyodor directly.  


• Chris Gibson chris@linuxops.net

• Kris Katterjohn katterjohn@gmail.com

• Mixter mixter@gmail.com

• Fyodor fyodor@insecure.org (http://insecure.org)

The original Netcat was written by *Hobbit* hobbit@avian.org. 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 functionality.




This document was created by man2html, using the manual pages.
Time: 07:34:21 GMT, September 13, 2011

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