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Registered port (1024–10000) Standard/Low risk

Port 3933 PLBSERVE-PORT

Quick answer

Port 3933 is the IANA-assigned plbserve-port port for PL/B App Server User Port (TCP / UDP).

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

What is port 3933 used for?

Port 3933 is assigned to plbserve-port and uses TCP / UDP. The IANA registry describes the service as “PL/B App Server User Port”. 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.

Ports are the transport-layer address a service listens on. A client that wants to reach plbserve-port connects to port 3933. A TCP connection is refused when nothing is listening, while a UDP request simply goes unanswered. That is why a port conflict appears whenever two processes try to bind the same number.

Use the commands below to confirm whether port 3933 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: [Edward_R_Boedecker].

Check and block port 3933

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 = :3933' sudo ss -ulpn 'sport = :3933'

Check on Windows

Get-NetTCPConnection -LocalPort 3933 -State Listen -ErrorAction SilentlyContinue Get-NetUDPEndpoint -LocalPort 3933 -ErrorAction SilentlyContinue

Block with UFW

sudo ufw deny 3933/tcp sudo ufw deny 3933/udp

Block with iptables

sudo iptables -A INPUT -p tcp --dport 3933 -j DROP sudo iptables -A INPUT -p udp --dport 3933 -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 3933 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 3933 is usually used

Most traffic on port 3933 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 3933

Common problems with port 3933

Connection refused or timed out

Refused usually means the host is reachable but nothing is listening on port 3933. 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 = :3933' or sudo lsof -i :3933. 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 3933 at a glance

Port number3933
Service nameplbserve-port
Transport protocolTCP / UDP
CategoryApplication services
RangeRegistered port (1024–10000)
Risk levelStandard/Low
Registered assignee[Edward_R_Boedecker]
Registry descriptionPL/B App Server User Port
SourceIANA Service Name and Transport Protocol Port Number Registry

Frequently asked questions

What is port 3933 used for?

Port 3933 (PLBSERVE-PORT) uses TCP / UDP. PL/B App Server User Port.

How do I check if port 3933 is open?

On Linux run `sudo ss -tlpn 'sport = :3933'` and `sudo ss -ulpn 'sport = :3933'`. On Windows run `Get-NetTCPConnection -LocalPort 3933 -State Listen -ErrorAction SilentlyContinue` and `Get-NetUDPEndpoint -LocalPort 3933 -ErrorAction SilentlyContinue` in PowerShell. If the port is open, the output shows a process listening on port 3933; empty output usually means nothing is bound to it.

How do I block port 3933?

With UFW run `sudo ufw deny 3933/tcp` and `sudo ufw deny 3933/udp`. With iptables run `sudo iptables -A INPUT -p tcp --dport 3933 -j DROP` and `sudo iptables -A INPUT -p udp --dport 3933 -j DROP`. Persist the iptables rules with your distribution's netfilter-persistent or iptables-save tooling so they survive a reboot.

Is port 3933 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