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Registered port (1024–10000) Standard/Low risk

Port 4840 OPCUA-UDP

Quick answer

Port 4840 has a different IANA assignment for each transport protocol: TCP opcua-tcp; UDP opcua-udp.

Check and block Is it safe? Troubleshooting FAQ
TransportTCP / UDP
RiskStandard/Low
Typical useApplication services

What is port 4840 used for?

Port 4840 carries a different IANA service on each transport protocol: TCP opcua-tcp; UDP opcua-udp. It sits in the registered range, the block used by databases, application servers and developer tooling. Port conflicts here are common when several local services start at once. The same port is also registered under opcua-tcp.

Each transport reaches a different service on this number, so match the protocol to the service you are diagnosing. A TCP check says nothing about a UDP listener on the same port, and the reverse is also true. Both transports still bind to the same number, which is why two processes can collide even when they use different protocols.

Use the commands below to confirm whether port 4840 is actually in use before you change firewall rules. Blocking the port stops new inbound traffic at the firewall, but it does not stop a local process from listening, and it does not remove the service from the machine.

Source: IANA Service Name and Transport Protocol Port Number Registry. Registered assignee: [OPC_Foundation].

Check and block port 4840

Copy-ready commands for the protocols assigned to this port. Run the check first, then block only what should not be reachable.

Check on Linux

sudo ss -tlpn 'sport = :4840' sudo ss -ulpn 'sport = :4840'

Check on Windows

Get-NetTCPConnection -LocalPort 4840 -State Listen -ErrorAction SilentlyContinue Get-NetUDPEndpoint -LocalPort 4840 -ErrorAction SilentlyContinue

Block with UFW

sudo ufw deny 4840/tcp sudo ufw deny 4840/udp

Block with iptables

sudo iptables -A INPUT -p tcp --dport 4840 -j DROP sudo iptables -A INPUT -p udp --dport 4840 -j DROP

These commands are examples for a typical host. Check the listener before you change firewall rules, and persist iptables rules with iptables-save or netfilter-persistent. On Windows, use Windows Defender Firewall or New-NetFirewallRule for persistent blocks. The iptables rules here cover IPv4; mirror them with ip6tables if IPv6 is enabled.

Is port 4840 safe to expose?

Standard/Low

This port is not on our curated list of services with a widely exploited profile. Exposure is only safe when the listening service is intended, patched and protected by a firewall. Keep the listening service patched and remove it if unused. A default-deny firewall policy with explicit allow rules is still the safest baseline.

Where port 4840 is usually used

Most traffic on port 4840 falls into these scenarios. The demo below is a safe way to confirm the listener before you change anything.

Demo

Probe the port

Confirm something is listening before you debug the client. This checks a TCP handshake on the port.

nc -vz 127.0.0.1 4840

Common problems with port 4840

Connection refused or timed out

Refused usually means the host is reachable but nothing is listening on port 4840. A timeout more often means a firewall drop, a wrong host or a service bound to a different interface.

Port already in use

Find the process holding the port with sudo ss -tlpn 'sport = :4840' or sudo lsof -i :4840. Stop that process or configure it to bind another port; killing it without identifying it can take down a production service.

Firewall rule did not survive a reboot

UFW and Windows Firewall keep rules across restarts. Raw iptables rules do not: save them with iptables-save or the netfilter-persistent package, and re-apply after upgrades that reset the ruleset.

The port is blocked but still reachable

A firewall rule filters inbound traffic; it does not close the listener. Confirm with the check command above and bind the service to 127.0.0.1 when only local clients need it.

Port 4840 at a glance

Port number4840
Service nameopcua-udp
Aliasesopcua-tcp
Transport protocolTCP / UDP
Assignments by protocol
  • TCP opcua-tcp — OPC UA Connection Protocol
  • UDP opcua-udp — OPC UA Multicast Datagram Protocol
CategoryApplication services
RangeRegistered port (1024–10000)
Risk levelStandard/Low
Registered assignee[OPC_Foundation]
Registry descriptionOPC UA Multicast Datagram Protocol
SourceIANA Service Name and Transport Protocol Port Number Registry

Frequently asked questions

What is port 4840 used for?

Port 4840 (OPCUA-UDP). The IANA registry assigns a different service on each transport protocol: TCP opcua-tcp; UDP opcua-udp.

How do I check if port 4840 is open?

On Linux run `sudo ss -tlpn 'sport = :4840'` and `sudo ss -ulpn 'sport = :4840'`. On Windows run `Get-NetTCPConnection -LocalPort 4840 -State Listen -ErrorAction SilentlyContinue` and `Get-NetUDPEndpoint -LocalPort 4840 -ErrorAction SilentlyContinue` in PowerShell. If the port is open, the output shows a process listening on port 4840; empty output usually means nothing is bound to it.

How do I block port 4840?

With UFW run `sudo ufw deny 4840/tcp` and `sudo ufw deny 4840/udp`. With iptables run `sudo iptables -A INPUT -p tcp --dport 4840 -j DROP` and `sudo iptables -A INPUT -p udp --dport 4840 -j DROP`. Persist the iptables rules with your distribution's netfilter-persistent or iptables-save tooling so they survive a reboot.

Is port 4840 safe to expose?

This port is not on our curated list of services with a widely exploited profile. Exposure is only safe when the listening service is intended, patched and protected by a firewall. Nevertheless, keep the listening service patched and remove it if unused. A default-deny firewall policy with explicit allow rules is still the safest baseline.

Primary data: IANA Service Name and Transport Protocol Port Number Registry.

Last reviewed 2026-10-30 · Methodology · About · Privacy