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// IoT & industrial (ICS)

Port 4840: OPC UA Connection Protocol

Port 4840 is assigned by IANA to opcua-tcp (OPC UA Connection Protocol), opcua-udp (OPC UA Multicast Datagram Protocol).

4840

How opcua-tcp works

IoT & industrial (ICS)
controllerPLCopcua-tcpno authentication controllerPLCopcua-tcpno authentication

A controller sends commands to an industrial device on port 4840, which acts on the physical process.

Transport
TCP · UDP
Exposure risk
not rated depends on the program listening
Encryption
Unknownnot stated by the registry
IANA status
Assigned2 registered services
Range
Registered (1024-49151)
Category
IoT & industrial (ICS)
01 Security

Is port 4840 dangerous?

We have no specific security note for port 4840. Its risk depends on the program actually listening on it. General advice for iot & industrial (ics) services:

  • Industrial and IoT protocols often have no authentication: never expose them to the Internet.
  • Segment OT networks from IT and use protocol-aware firewalls.
  • Monitor with an ICS-aware intrusion detection system.
02 Registry

IANA registry entries for port 4840

ServicePortTransportDescriptionReferenceUpdated
opcua-tcp 4840 TCP OPC UA Connection Protocol - 2018-01-04
opcua-udp 4840 UDP OPC UA Multicast Datagram Protocol - 2018-01-04

// source: IANA service name and port number registry

03 Check it

Which program uses port 4840, and is it reachable?

Linux

sudo ss -ltnup 'sport = :4840'      # process listening on 4840
sudo lsof -nP -i :4840

macOS

sudo lsof -nP -i :4840

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 4840 -State Listen | Select LocalAddress, OwningProcess
Get-NetUDPEndpoint -LocalPort 4840
netstat -ano | findstr :4840

From another machine

nmap -p 4840 -sV <host>
sudo nmap -sU -p 4840 <host>
nc -zv <host> 4840

Block it in the firewall

sudo ufw deny 4840/tcp                                         # Linux (ufw)
New-NetFirewallRule -DisplayName "Block 4840" -Direction Inbound -LocalPort 4840 -Protocol TCP -Action Block  # Windows
04 FAQ
What is port 4840 used for?

Port 4840 is assigned by IANA to opcua-tcp (OPC UA Connection Protocol), opcua-udp (OPC UA Multicast Datagram Protocol).

Is port 4840 TCP or UDP?

Port 4840 is registered for TCP and UDP.

Is it safe to open port 4840?

It depends on the program that listens on it. Open it only for a service you know and only to the networks that need it. Industrial and IoT protocols often have no authentication: never expose them to the Internet.

How do I check if port 4840 is open?

On Linux run "ss -ltnup 'sport = :4840'", on macOS "lsof -nP -i :4840", and on Windows "netstat -ano | findstr :4840" to see the local process. From another machine, "nmap -p 4840 <host>" shows whether it is reachable.

05 Related
:1883 MQTT tcp · udp · high risk :8883 MQTT over TLS tcp · udp · moderate risk :502 Modbus TCP tcp · udp · critical risk :102 Siemens S7 (ISO-TSAP) tcp · udp · critical risk :47808 BACnet udp · tcp · critical risk :20000 DNP3 tcp · udp · sctp · critical risk :44818 EtherNet/IP tcp · udp · critical risk :802 Modbus Application Protocol Secure tcp · udp

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