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
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 6.01 ( 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
Executes the given command via /bin/sh
-e, --exec Executes the given command
-g hop1[,hop2,...] Loose source routing hop points (8 max)
-G Loose source routing hop pointer (4, 8, 12, ...)
-m, --max-conns Maximum simultaneous connections
-h, --help Display this help screen
-d, --delay 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 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 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 Specify address of host to proxy through
--proxy-type Specify proxy type ("http" or "socks4")
--proxy-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, --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
-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.
-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.
--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
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-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.
-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.
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. 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.
--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. -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.
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 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
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
Ncat 06/13/2012 NCAT(1)