Port 502: Modbus
Port 502 is Modbus, a simple industrial protocol from the 1970s that is still the most common way PLCs and other factory-floor equipment talk to each other. It has no built-in security, which is a well-known concern in operational technology networks.
| Port number | 502 |
|---|---|
| Service | Modbus Protocol |
| Protocol | TCP |
| Category | IoT & automation |
| Default encryption | No |
How do I check whether port 502 is open?
Port 502 carries TCP, so a connection either completes its handshake or it does not, which makes the check definitive.
| Question | Command | What to know |
|---|---|---|
| Is it open on a remote host? | nc -vz example.com 502 | A TCP handshake either completes or it does not, so netcat gives a definitive answer in one line. |
| Same check on Windows | Test-NetConnection example.com -Port 502 | Built into PowerShell. TcpTestSucceeded in the output is the answer; telnet is not installed by default on modern Windows. |
| What is listening locally? | ss -tlnp | grep ':502 ' | Lists the process bound to port 502 on Linux. On macOS use lsof -nP -iTCP:502 -sTCP:LISTEN, on Windows netstat -ano | findstr :502 |
Replace example.com with the host you are testing. A blocked port and a port with nothing listening on it look identical from the outside, so if a service should be running, check locally before blaming the firewall.
Frequently asked questions
What is port 502 used for?
Port 502 is Modbus, a simple industrial protocol from the 1970s that is still the most common way PLCs and other factory-floor equipment talk to each other. It has no built-in security, which is a well-known concern in operational technology networks.
Is port 502 TCP or UDP?
Port 502 (Modbus) uses TCP.
Is port 502 secure?
Port 502 is not encrypted by default. Where possible, use an encrypted alternative or tunnel it over TLS or a VPN.
Should I open port 502 on my firewall?
Only if you specifically need Modbus. Expose it to the smallest set of trusted sources, and never open database or Windows-service ports to the whole internet.