Netcat Command Reference

Netcat is a versatile networking utility used for reading from and writing to network connections using TCP or UDP. It can be used for port scanning, transferring files, creating simple chat programs, and testing network connectivity. This reference provides common netcat commands and examples to help you utilize its features effectively.

Basic Syntax

The general syntax for using netcat is:

nc [options] hostname port

Or to listen for incoming connections:

nc -l [options] [port]

Common Options

Basic Usage Examples

1. Testing Network Connectivity

Check if a specific port is open on a remote host:

# Check if port 80 is open on example.com
nc -v example.com 80

2. Port Scanning

Scan a range of ports on a remote host:

# Scan ports 20 to 25 on example.com
nc -v -z example.com 20-25

The -z option tells netcat to use zero-I/O mode, suitable for scanning.

3. Simple Chat between Two Computers

On the listening machine (Machine A):

# Listen on port 5000
nc -l 5000

On the connecting machine (Machine B):

# Connect to Machine A's IP on port 5000
nc [Machine_A_IP] 5000

Now, both users can type messages and communicate.

4. Transferring Files

Send a File

On the receiving machine:

# Listen on port 6000 and redirect output to received_file.txt
nc -l 6000 > received_file.txt

On the sending machine:

# Send file.txt to the receiver
nc [Receiver_IP] 6000 < file.txt

Receive a File

The roles can be reversed depending on your preference.

5. HTTP Requests

Manually send an HTTP GET request to a web server:

nc example.com 80
GET / HTTP/1.1
Host: example.com

[Press Enter Twice]

The server's response will be displayed.

6. Creating a Simple Web Server

Serve a directory over HTTP:

# Use ncat (from Nmap) to serve files
ncat -l 8080 --exec "/bin/cat index.html" --keep-open

Note: This example uses ncat from Nmap, which has additional features.

7. Redirecting Ports

Forward connections from one port to another:

# Forward port 8080 to 80
mkfifo backpipe
nc -l 8080 0<backpipe | nc localhost 80 1>backpipe

Note: This is a simplified example. For more robust port forwarding, consider using ssh or dedicated tools.

Advanced Usage Examples

1. Monitoring Network Traffic

Listen on a UDP port to monitor incoming messages:

# Listen on UDP port 9999
nc -u -l 9999

2. Sending Broadcast Messages

Send a UDP broadcast message:

# Send "Hello Network" to UDP port 9999
echo "Hello Network" | nc -u -b 192.168.1.255 9999

Note: Replace 192.168.1.255 with your network's broadcast address.

3. Checking Open Ports Locally

Check which ports are open on your local machine:

# Scan local ports from 1 to 1024
nc -zv localhost 1-1024

4. Setting a Connection Timeout

Set a timeout for connections:

# Try to connect to example.com on port 80 with a 5-second timeout
nc -w 5 example.com 80

Using Netcat as a Server and Client

Listening for Incoming Connections

Start a simple TCP server that echoes received data:

# Listen on port 7000
nc -l 7000

Connecting to a Netcat Server

Connect to the server and send messages:

# Connect to the server on port 7000
nc [Server_IP] 7000

Troubleshooting Tips

• Verbose Output: Use -v or -vv to get more detailed information during connections.
• DNS Resolution: If experiencing delays, use -n to skip DNS lookups.
• Firewall Settings: Ensure that firewalls on both ends allow the desired traffic.
• Compatibility: Be aware of differences between netcat versions. Check the man pages for version-specific options.

Commonly Used Commands Summary

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