NCAT(1) 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
Ncat 6.40 ( 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
-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
--lua-exec <filename> Executes the given Lua script
-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
--sctp Use SCTP instead of default TCP
-v, --verbose Set verbosity level (can be used several 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
CONNECT MODE AND LISTEN MODE
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, or combined with --udp for
datagram sockets. A description of -U mode is in the section called "UNIX DOMAIN SOCKETS".
-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.
CONNECT MODE OPTIONS
-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.
LISTEN MODE OPTIONS
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 (60 on Windows).
-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. 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.
--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.
COMMAND EXECUTION OPTIONS
-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 (60 on Windows).
-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.
--lua-exec file (Execute a .lua script) .
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.
ACCESS CONTROL OPTIONS
--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 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.
--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
-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.
-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.
UNIX DOMAIN SOCKETS
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 example.org on TCP port 8080.
ncat example.org 8080
Listen for connections on TCP port 8080.
ncat -l 8080
Redirect TCP port 8080 on the local machine to host on port 80.
ncat --sh-exec "ncat example.org 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 192.168.0.0/24 -l 8081 --keep-open
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 (server).
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
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 <firstname.lastname@example.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 email@example.com are far more likely to be answered than those sent to Fyodor
Code patches to fix bugs are even better than bug reports. Basic instructions for creating patch files with your changes are available at
https://svn.nmap.org/nmap/HACKING. Patches may be sent to nmap-dev (recommended) or to Fyodor directly.
o Chris Gibson <firstname.lastname@example.org>
o Kris Katterjohn <email@example.com>
o Mixter <firstname.lastname@example.org>
o Fyodor <email@example.com> (http://insecure.org)
The original Netcat was written by *Hobbit* <firstname.lastname@example.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.
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 http://nmap.org/man/man-legal.html.
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 Creative Commons Attribution
License. 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 <email@example.com> 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.
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 GNU General Public License v2.0 for more details at
http://www.gnu.org/licenses/gpl-2.0.html, or in the COPYING file included with Nmap.
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.
This product includes software developed by the Apache Software Foundation. A modified version of the Libpcap portable packet capture
library. is distributed along with Ncat. The Windows version of Ncat utilized the Libpcap-derived WinPcap library. instead. Certain
raw networking functions use the Libdnet. networking library, which was written by Dug Song.. A modified version is distributed with
Ncat. Ncat can optionally link with the OpenSSL cryptography toolkit. for SSL version detection support. All of the third-party
software described in this paragraph is freely redistributable under BSD-style software licenses.
1. Creative Commons Attribution License
2. Apache Software Foundation
3. Libpcap portable packet capture library
4. WinPcap library
6. OpenSSL cryptography toolkit
Ncat 07/28/2013 NCAT(1)