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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, 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 7.93 ( 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
--vsock Use vsock 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
--no-shutdown Continue half-duplex when receiving EOF on stdin
--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", "socks4", "socks5")
--proxy-auth <auth> Authenticate with HTTP or SOCKS proxy server
--proxy-dns <type> Specify where to resolve proxy destination
--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-servername Request distinct server name (SNI)
--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
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
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.
--vsock (Use AF_VSOCK sockets)
Use AF_VSOCK sockets rather than the default TCP sockets (Linux
only). This option may be used on its own for stream sockets or
combined with --udp for datagram sockets. A description of --vsock
mode is in the section called "AF_VSOCK SOCKETS".
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
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
--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.
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
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.
--ssl-ciphers cipherlist (Specify SSL ciphersuites)
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-servername name (Request distinct server name)
In client mode, this option sets the TLS SNI (Server Name
Indication) extension, which tells the server the name of the
logical server Ncat is contacting. This is important when the
target server hosts multiple virtual servers at a single underlying
network address. If the option is not provided, the TLS SNI
extension will be populated with the target server hostname.
--ssl-alpn ALPN list (Specify ALPN protocol list)
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 proxy address)
Requests proxying through host:port, using the protocol specified
If no port is specified, the proxy protocol's well-known port is
used (1080 for SOCKS and 3128 for HTTP). When specifying an IPv6
HTTP proxy server using the IP address rather than the hostname,
the square-bracket notation (for example [2001:db8::1]:8080) MUST
be used to separate the port from the IPv6 address. 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), socks4 (SOCKSv4), and socks5 (SOCKSv5). 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 or --proxy-type socks5, the form should be username:password.
For --proxy-type socks4, it should be a username only.
These credentials can be alternatively passed onto Ncat by setting
environment variable NCAT_PROXY_AUTH, which reduces the risk of the
credentials being captured in process logs. (Option --proxy-auth
--proxy-dns type (Specify where to resolve proxy destination)
In connect mode, it provides control over whether proxy destination
hostnames are resolved by the remote proxy server or locally, by
Ncat itself. Possible values for type are:
local - Hostnames are resolved locally on the Ncat host. Ncat exits
with error if the hostname cannot be resolved.
remote - Hostnames are passed directly onto the remote proxy
server. This is the default behavior.
both - Hostname resolution is first attempted on the Ncat host.
Unresolvable hostnames are passed onto the remote proxy server.
none - Hostname resolution is completely disabled. Only a literal
IPv4 or IPv6 address can be used as the proxy destination.
Local hostname resolution generally respects IP version specified
with options -4 or -6, except for SOCKS4, which is incompatible
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.
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 connection.
The protocol in use: one of TCP, UDP, and SCTP.
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
--no-shutdown (Do not shutdown into half-duplex mode)
If this option is passed, Ncat will not invoke shutdown on a socket
after seeing EOF on stdin. This is provided for
backward-compatibility with OpenBSD netcat, which exhibits this
behavior when executed with its '-d' option.
-n, --nodns (Do not resolve hostnames)
Completely disable hostname resolution across all Ncat options,
such as the destination, source address, source routing hops, and
the proxy. All addresses must be specified numerically. (Note that
resolution of proxy destinations is controlled separately via
-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.
The --vsock option causes Ncat to use AF_VSOCK sockets rather than
network sockets. A CID must be given instead of a hostname or IP
address. For example, to make a connection to the host,
ncat --vsock 2 1234
To listen on a socket:
ncat -l --vsock 1234
Both stream and datagram domain sockets are supported, but socket type
availability depends on the hypervisor. Use --vsock on its own for
stream sockets, or combine it with --udp for datagram sockets.
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
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
Connect to smtphost:25 through a SOCKS4 server on port 1080.
ncat --proxy socks4host --proxy-type socks4 --proxy-auth joe
Connect to smtphost:25 through a SOCKS5 server on port 1080.
ncat --proxy socks5host --proxy-type socks5 --proxy-auth joe:secret
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
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
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 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 <firstname.lastname@example.org>
o Gordon Lyon (Fyodor)<email@example.com> (http://insecure.org)
o Kris Katterjohn <firstname.lastname@example.org>
o Mixter <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 Copyright and Licensing
Ncat is (C) 2005-2022 Nmap Software 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
Creative Commons License for this Ncat Guide
This Ncat Reference Guide is (C) 2005-2022 Nmap Software 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
Ncat 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 (Nmap Software 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 Nmap
Public Source License for more details at https://nmap.org/npsl/, or in
the LICENSE 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
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
1. Creative Commons Attribution License
2. Apache Software Foundation
3. Libpcap portable packet capture library
4. Npcap library
6. OpenSSL cryptography toolkit
Ncat 08/31/2022 NCAT(1)