Master Netcat (nc): Install, Test Ports, Transfer Files, and Measure Network Speed

What Is Netcat (nc)?

Netcat, often called the "Swiss army knife" of networking, is a command‑line utility for reading and writing data across network connections using TCP or UDP. It can listen on ports, scan ports, transfer files, and even measure network speed, making it indispensable for operations and troubleshooting.

Installing nc on CentOS 7

yum install -y nc

After installation, verify the version:

# nc --version
Ncat: Version 7.50 ( https://nmap.org/ncat )

Basic Syntax and Common Options

nc [options] hostname/IP port

Key options include: -h: display help -v: verbose output -u: use UDP (default is TCP) -z: zero‑I/O mode for scanning -l: listen mode (acts as a server) -w: set timeout in seconds

Environment Setup for Demonstrations

Two test servers are used:

Server 1 – IP 192.168.20.231

Server 2 – IP 192.168.20.232

Both firewalls are disabled for the examples.

1️⃣ Testing TCP/UDP Port Availability

TCP test

On Server 2 start a listener: nc -l 8888 On Server 1 probe the port: nc -vz 192.168.20.232 8888 If the connection succeeds you will see “1 bytes sent, 0 bytes received” and a return code of 0; otherwise “Connection refused” and a return code of 1.

UDP test

On Server 2 start a UDP listener: nc -lu 9999 On Server 1 probe the UDP port: nc -vuz 192.168.20.232 9999 Successful probes display the same “1 bytes sent, 0 bytes received” message.

2️⃣ Transferring Files and Directories with nc

File transfer

# On Server 2 (receiver)
nc -l 9898 > haodao_rece.txt

# On Server 1 (sender)
nc 192.168.20.232 9898 < haodao_send.txt

The received file matches the original.

Directory transfer

# On Server 2 (receiver)
nc -l 9898 | tar -xzvf -

# On Server 1 (sender)
tar czvf - haodao_test | nc 192.168.20.232 9898

The entire directory structure is reproduced on Server 2.

3️⃣ Measuring Network Throughput Between Two Servers

The method streams an infinite zero stream from Server 1 to Server 2 while dstat monitors bandwidth.

# Install dstat on both servers
yum install -y dstat

# On Server 2 (receiver)
nc -l 9696 > /dev/null

# On Server 1 (sender)
nc 192.168.20.232 9696 < /dev/zero

Run dstat on each server in separate terminals to observe the transfer rates (e.g., ~140 KB received vs. ~110 MB sent on Server 1, and the opposite on Server 2).

Conclusion

Netcat offers a versatile set of functions for port listening, scanning, file transfer, and network performance testing. By mastering its basic commands and options, engineers can streamline many routine operations and troubleshooting tasks.