ncat - Concatenate and redirect sockets
ncat [OPTIONS...] [hostname]
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
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.
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.
Ncat 7.70 ( https://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
-z Zero-I/O mode, report connection status only
--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" or "socks5")
--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
--ssl-ciphers Cipherlist containing SSL ciphers to use
--ssl-alpn ALPN protocol list to use.
--version Display Ncat's version information and exit
See the ncat(1) manpage for full options, descriptions and usage examples
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.
-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
-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
-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
--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.
In UDP connect mode, this option enables Datagram TLS (DTLS). This
is not supported in server mode.
--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.
Some operating systems provide a default list of trusted certificates; these
will also be used if available. Use --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
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
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
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
--ssl-ciphers cipherlist (Specify SSL
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
--ssl-alpn ALPN list (Specify ALPN protocol
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.
--proxy host[:port] (Specify
Requests proxying through host
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-type proto (Specify proxy
In connect mode, this option requests the protocol
to connect through the proxy host specified by --proxy
listen mode, this option has Ncat act as a proxy server using the specified
The currently available protocols in connect mode are http
(CONNECT), socks4 (SOCKSv4), and socks5 (SOCKSv5). The only server currently
supported is http. If this option is not used, the default protocol is
(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
or --proxy-type socks5, the form should be username:password. For
--proxy-type socks4, it should be a username only.
-e command, --exec command (Execute
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.
All exec options add the following variables to the child's
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
The IP address and port number of the local end of the
The protocol in use: one of TCP, UDP, and SCTP.
--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
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
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
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
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 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
--no-shutdown (Do not shutdown into half-duplex mode)
If this option is passed, Ncat will not invoke shutdown
on a socket aftering seeing EOF on stdin. This is provided for
backward-compatibility with OpenBSD netcat, which exhibits this behavior when
executed with its '-d' option.
-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 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
ncat --sh-exec "ncat example.org 80" -l 8080
Bind to TCP port 8081 and attach /bin/bash for the world to access
ncat --exec "/bin/bash" -l 8081
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 joe smtphost 25
Connect to smtphost:25 through a SOCKS5 server on port 1080.
ncat --proxy socks5host --proxy-type socks5
--proxy-auth joe:secret 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
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
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 nmap-dev archives at
HOST1$ ncat -l 9899 < inputfile
HOST2$ ncat HOST1 9899 > outputfile
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.
•Chris Gibson <email@example.com>
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 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
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
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
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 Npcap 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.
- Creative Commons Attribution License
- Apache Software Foundation
- Libpcap portable packet capture library
- Npcap library
- OpenSSL cryptography toolkit