Ncat 1 Ncat Ncat Reference Guide 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. connect mode (Ncat) client mode (Ncat)connect mode listen mode (Ncat) server mode (Ncat)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.default port of Ncat31337default port of Ncat 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. If port is omitted, it defaults to 31337. -4 (IPv4 only) -4 (Ncat option) Force the use of IPv4 only (default). -6 (IPv6 only) -6 (Ncat option) Force the use of IPv6 only. -u, --udp (Use UDP) -u (Ncat option)--udp --udp (Ncat option) Use UDP for the connection (the default is TCP). --sctp (Use SCTP) --sctp (Ncat option) Use SCTP for the connection (the default is TCP). SCTP support is implemented in TCP-compatible mode. -g hop1,hop2,... (Loose source routing) -g (Ncat option) 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) -G (Ncat option) 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) --source-port (Ncat option) -p (Ncat option)--source-port Set the port number for Ncat to bind to. -s host, --source host (Specify source address) --source (Ncat option) -s (Ncat option)--source Set the address for Ncat to bind to. See for information on limiting the hosts that may connect to the listening Ncat process. -l, --listen (Listen for connections) --listen (Ncat option) -l (Ncat option)--listen Listen for connections rather than connecting to a remote machine -m numconns, --max-conns numconns (Specify maximum number of connections) --max-conns (Ncat option) -m (Ncat option)--max-conns The maximum number of simultaneous connections accepted by an Ncat instance. 100 is the default. -k, --keep-open (Accept multiple connections) --keep-open (Ncat option) -k (Ncat option)--keep-open 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) --broker (Ncat option) 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) --chat (Ncat option) 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) --ssl (Ncat option) 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) --ssl-verify (Ncat option) 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.crtca-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. This option has no effect in server mode. --ssl-cert certfile.pem (Specify SSL certificate) --ssl-cert (Ncat option) 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) --ssl-key (Ncat option) 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) --ssl-trustfile (Ncat option) 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 PEMPEM (Privacy Enhanced Mail) 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) --proxy (Ncat option) 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) --proxy-type (Ncat option) 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) --proxy-auth (Ncat option) 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. -e command, --exec command (Execute command) --exec (Ncat option) -e (Ncat option)--exec 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) --sh-exec (Ncat option) -c (Ncat option)--sh-exec 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. --allow host,host,... (Allow connections) --allow (Ncat option) 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) --allowfile (Ncat option) 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) --deny (Ncat option) 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) --denyfile (Ncat option) 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) --delay (Ncat option) -d (Ncat option)--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) --idle-timeout (Ncat option) -i (Ncat option)--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) --wait (Ncat option) -w (Ncat option)--wait Set a fixed timeout for connection attempts. -o file, --output file (Save session data) --output (Ncat option) -o (Ncat option)--output Dump session data to a file -x file, --hex-dump file (Save session data in hex) --hex-dump (Ncat option) -x (Ncat option)--hex-dump Dump session data in hex to a file. This can be used to replay sessions. -v, --verbose (Be verbose) --verbose (Ncat option) -v (Ncat option)--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) --crlf (Ncat option) -C (Ncat option)--crlf This option tells Ncat to convert LFLF line ending line endings to CRLFCRLF line ending when taking input from standard input.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) --help (Ncat option) -h (Ncat option)--help Displays a short help screen with common options and parameters, and then exits. --recv-only (Only receive data) --recv-only (Ncat option) If this option is passed, Ncat will only receive data and will not try to send anything. --send-only (Only send data) --send-only (Ncat option) 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) -t (Ncat option) Handle DO/DONT WILL/WONT Telnet negotiations. This makes it possible to script Telnet sessions with Ncat. --version (Display version) --version (Ncat option) 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 . 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 . nmap-dev mailing list Read this full manual page as well. If nothing comes of this, mail a bug report to nmap-dev@insecure.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 nmap-dev@insecure.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 . Patches may be sent to nmap-dev (recommended) or to Fyodor directly. Chris Gibson chris@linuxops.net Kris Katterjohn katterjohn@gmail.com Mixter mixter@gmail.com Fyodor fyodor@insecure.org () The original Netcat was written by *Hobbit* hobbit@avian.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.