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 it.
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
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
--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
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
--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
--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
-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 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.
-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
--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 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 <email@example.com>.
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 firstname.lastname@example.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 https://svn.nmap.org/nmap/HACKING. Patches
may be sent to nmap-dev (recommended) or to Fyodor directly.
o Chris Gibson <email@example.com>
o Kris Katterjohn <firstname.lastname@example.org>
o Mixter <email@example.com>
o Fyodor <firstname.lastname@example.org> (http://insecure.org)
The original Netcat was written by *Hobbit* <email@example.com>. 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 <firstname.lastname@example.org> 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)