ncat - Concatenate and redirect sockets


ncat [OPTIONS...] [hostname] [port]


Ncat is a feature-packed networking utility which will read and write data across a network from the command line. Ncat was written for the Nmap Project and is the culmination of the currently splintered family of Netcat incarnations. It uses both TCP and UDP for communication and 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, redirect both TCP and UDP ports to other sites, SSL support, and proxy connections via SOCKS4 or HTTP (CONNECT method) 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.

Options Summary

Ncat 5.00 ( ) Usage: ncat [options] [hostname] [port]

Options taking a time assume milliseconds, unless you append an 's' (seconds), 'm' (minutes), or 'h' (hours) to the value (e.g. 30s) -4 Use IPv4 only -6 Use IPv6 only -C, --crlf Use CRLF for EOL sequence -c, --sh-exec <command> Executes specified command via /bin/sh -e, --exec <command> Executes specified command -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 Dump session data to a file -x, --hex-dump 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 (doesn't affect -l) -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 -v, --verbose Set verbosity level (can be used up to 3 times) -w, --wait <time> Connect timeout --send-only Only send data, ignoring received; quit on EOF --recv-only Only receive data, never send anything --allow Allow specific hosts to connect to Ncat --allowfile A file of hosts allowed to connect to Ncat --deny Hosts to be denied 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

Host Specification

When passing a host parameter to Ncat, the simplest case is just to list a single hostname or IP address. If you are supplying a range of hosts, such as with --deny or --allow options, you can denote the chosen range of IP addresses by appending the CIDR-style '/mask' to the IP address. The mask must be between zero (select the whole subnet) and 32 (scan the single host specified). For example, you may use /24 to scan a class C subnet and /16 for a class B.

Protocol Options

-4 (IPv4 only) .

Force the use of IPv4 only (default).

-6 (IPv6 only) .

Force the use of IPv6 only.

-u, --udp (Use UDP) .

Use UDP for the connection (the default is TCP)

Client-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 \(lqpointer\(rq for use with -g. The argument must be a multiple of four 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.

Server-Mode Options

See the Access Control section for information on limiting which hosts can 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 max number of connections) .

The maximum number of simultaneous connections accepted for 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 \(lqchat server\(rq) .

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 Options

--ssl (Use SSL) .

In client-mode Ncat, 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 Ncat, 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. 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.

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 Options

--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, --proxy-auth is available.

--proxy-type proto (Specify proxy protocol) .

In client-mode, this option requests using proxy protocol proto to connect through the proxy host specified by --proxy. In server-mode, this option requests Ncat to actually act as a proxy server using the specified protocol.

The currently available protocols in client-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) .

Used to specify proxy authentication credentials for client-mode. For use with --proxy-type http, the form should be user:pass. For --proxy-type socks4, it should just be a username.

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, effectively instantly making your application interactive over a socket. Ncat will handle multiple simultaneous connections to your specified port/application rather like inetd does. Ncat will only accept a maximum, definable, number of simultaneous connections. 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 (so you don't have to specify the full path for the command).

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 silently dropped. 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. The syntax for hosts is the same as for --allow.

--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.

Timing Options

These options accept a time parameter. This is specified in milliseconds by default, though you can append s, m, or h to the value to specify 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 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.

Output Options

-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 \(lqreplay\(rq sessions, etc.

-v, --verbose (Verbosity) .

Issue Ncat with -v and it will be verbose and display all kinds of useful connection based information. If you issue this twice (-vv) then you will get all the code debugging information. Issue it three times (-vvv) and you get the connection information and the code debugging information.

Misc Options

-C, --crlf (Use CRLF as EOL) .

This option tells Ncat to try to use CRLF for line-endings if only an LF is found. This doesn't convert all LFs to CRLFs, only if it's at the end of the read buffer. This is useful for talking to some stringent servers directly from a terminal in one of the many common plain-text protocols which specify CRLF as the required EOL sequence.

-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) .

This displays the Ncat version, release information and any additional build information and exits.


Connect to on TCP port 8080

ncat 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 80" -l 8080

Bind to TCP port 8081 and attach /bin/bash for the world to access freely

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 three

ncat --exec "/bin/bash" --max-conns 3 --allow -l 8081

Connect to 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 \(lqone file\(rq server

HOST1$ ncat -l 9899 <inputfile

HOST2$ ncat HOST1 9899 >outputfile


Like its author, 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 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 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 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 Patches may be sent to nmap-dev (recommended) or to Fyodor directly.


Chris Gibson

Kris Katterjohn



The original Netcat was written by *Hobbit* While Ncat isn't built on any code from the Netcat (or any other implementation), Ncat is most definitely based on Netcat in spirit and functionality.