Porting Netcat in Node.js. CLI util. 💻

To embed it in your Node.js app use the netcat package instead. This is meant to be used as a standalone tool, but it's not fully equal to the original implementation of netcat.

• TCP & UDP
• Backdoor (Reverse Shell)
• Honeypot
• File transfer
• Port forwarding
• Proxy
• Web Server & HTTP Client
• Port scanning

$ npm install -g nc

$ nc -l -p port [- options] [hostname] [port]

Available options:

-c shell commands    as `-e’; use /bin/sh to exec [dangerous!!]
-e filename          program to exec after connect [dangerous!!]
-b                   allow broadcasts
-h                   this cruft
-i secs              delay interval for lines sent, ports scanned
-k set               keepalive option on socket
-l                   listen mode, for inbound connects
-n                   numeric-only IP addresses, no DNS
-o file              hex dump of traffic
-p port              local port number
-r                   randomize local and remote ports
-s addr              local source address
-u                   UDP mode
-U                   Listen or connect to a UNIX domain socket
-v                   verbose
-w secs              timeout for connects and final net reads (client-side)
-z                   zero-I/O mode [used for scanning]

$ nc -l -p 2389

$ nc localhost 2389

Opening a raw connection to port 2389.

$ nc -w 10 localhost 2389

Connection above would be terminated after 10 seconds.

$ nc -kl 2389

In this way the server remains up even if the client got disconnected.

A far more exciting thing to do is to get a quick shell going on a remote machine by using the -l or listen option and the -e or execute option. When a connection is made, Netcat executes the program of your choice and connects the stdin and stdout of the program to the network connection.

$ nc -l -p 23 -e /bin/sh

Let's create a HTTP request file get.txt that contains the following line and then a blank line:

GET / HTTP/1.0

To use Netcat to retrieve the home page of a web site use:

$ nc -v www.website.com 80 < get.txt

You will see Netcat make a connection to port 80, send the text contained in the file get.txt, and then output the web server's response to stdout.

In a normal scenario, if the nc client disconnect, it will not retry the connection. With the --retry <secs> or -R <secs> param, it will retry the connection after tot seconds.

$ nc -R 5 localhost 2389

If you have docker, let's try to list our containers' images connecting to the docker unix socket file:

$ echo -e "GET /images/json HTTP/1.0\r\n" | nc -U /var/run/docker.sock

PS. for this example root permissions are required: sudo su.

$ mkfifo /tmp/fifo
$ nc -l -k -p 8080 </tmp/fifo | nc website.com 80 >/tmp/fifo

$ nc -z 192.168.1.100 1-255

If you use the -o option you can dump all hex traffic.

$ nc 127.0.0.1 4445 -o /tmp/log.txt

By default all the sockets that nc utility creates are TCP protocols but this utility also works with UDP protocol. To enable UDP protocol the -u flag is used.

$ echo 'message' | nc -w 1 -u 192.168.1.111 514

Pipe via UDP (-u) with a wait time (-w) of 1 second to 192.168.1.111 on port 514.

Debug matches the verbose mode. You can enable it with the -v param or the env var DEBUG=nc. This module uses the node implementation of netcat under the hood, to debug both use: DEBUG=netcat:*,nc.

Rocco Musolino (@roccomuso)