Chapter 17. Ncat Reference Guide

Name

ncat — Concatenate and redirect sockets

Synopsis

ncat [ <OPTIONS> ...] [ <hostname> ] [ <port> ]

Description

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.

Options Summary

Ncat 7.92SVN ( 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 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.

Protocol Options

Connect 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 (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 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 revoked certificates.

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 ALL:!aNULL:!eNULL:!LOW:!EXP:!RC4:!MD5:@STRENGTH

--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 by --proxy-type.

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 takes precedence.)

--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 with IPv6.

All exec options add the following variables to the child's environment:

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.

-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.

--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 option --proxy-dns.)

-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.

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.

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 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 https://seclists.org/. 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 dev@nmap.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.

Authors

The original Netcat was written by *Hobbit* . 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.