nc

Start a listener on the specified TCP port and send a file into it

$ nc -l -p [port] < [filename]

$ nc [host] [port] > [received_filename]

$ nc -v -z -w [timeout_in_seconds] [host] [start_port]-[end_port]

$ nc -l -p [port] -e [shell_executable]

$ nc [host] [port] -e [shell_executable]

$ nc -l -p [local_port] | nc [host] [remote_port]

$ echo -e "GET / HTTP/1.1\nHost: [host]\n\n" | nc [host] 80

nc [options] hostname port [port...]
nc -l [options] [hostname] port [port...]

-l
    Listen mode. For inbound connects.

-p port
    Specify the local port for outgoing connections.

-u
    Use UDP instead of the default TCP.

-v
    Verbose output. Provides more information about connections and errors.

-w timeout
    Connection timeout. Specifies how long to wait for a connection to establish.

-z
    Zero-I/O mode. Only scan for listening daemons, without sending any data. Useful for port scanning.

-n
    Do not do DNS lookups. Speeds up operations when dealing with IP addresses.

-k
    Force nc to stay listening for another connection after its current client disconnects. Only valid with -l.

-4
    Force nc to use IPv4 addresses only.

-6
    Force nc to use IPv6 addresses only.

The nc (or netcat) command is a powerful networking utility that reads and writes data across network connections, using the TCP/IP or UDP protocols. Often referred to as the "TCP/IP swiss army knife", it's incredibly versatile for various tasks.

It can be used for port scanning, file transfers, simple chat applications, network debugging, and even as a backdoor. It can act as both a client (connecting to a remote host and port) and a server (listening on a specified port for incoming connections). Its simplicity and raw data handling make it an essential tool for network administrators, developers, and security professionals.

While extremely powerful, nc sends data in plain text by default, making it unsuitable for transferring sensitive information without additional encryption (e.g., via a VPN or SSH tunnel). Its simplicity also means it lacks advanced features like authentication or robust error handling, which are present in more specialized tools. It's often misused in malicious contexts due to its ability to create simple backdoors or listener services.

• Port Scanning: nc -zv target port checks if a port is open.
• File Transfer: Server: nc -l -p port > file; Client: nc server_ip port < file.
• Simple Chat: Server: nc -l -p port; Client: nc server_ip port (type messages and press Enter).
• Debugging: Test connectivity to services or send raw HTTP requests.

The original Netcat program was written by Hobbit in 1995. It quickly gained popularity as a versatile command-line networking tool. Due to its utility, various implementations have emerged, including GNU Netcat, OpenBSD Netcat (which is often symlinked as nc on many Linux systems), and Ncat (part of the Nmap suite). While their core functionality remains similar, subtle differences in options and behavior exist between these versions.

ssh(1), telnet(1), socat(1), nmap(1), netcat(1)