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// Other services

Port 4021: Nexus Portal

Port 4021 is assigned by IANA to nexus-portal (Nexus Portal).

4021

How nexus-portal works

Other services
client:4021nexus-portalresponse client:4021nexus-portalresponse

The client sends a request to port 4021 and the nexus-portal service answers.

Transport
TCP · UDP
Exposure risk
not rated depends on the program listening
Encryption
Unknownnot stated by the registry
IANA status
Assigned1 registered service
Range
Registered (1024-49151)
Category
Other services
01 Security

Is port 4021 dangerous?

We have no specific security note for port 4021. Its risk depends on the program actually listening on it. General advice for other services services:

  • Close the port if nothing on the host needs it.
  • Identify the process that listens on it (commands below) before opening it in a firewall.
  • Expose a service only to the networks that use it.
02 Registry

IANA registry entries for port 4021

ServicePortTransportDescriptionReferenceUpdated
nexus-portal 4021 TCP, UDP Nexus Portal - -

// source: IANA service name and port number registry

03 Check it

Which program uses port 4021, and is it reachable?

Linux

sudo ss -ltnup 'sport = :4021'      # process listening on 4021
sudo lsof -nP -i :4021

macOS

sudo lsof -nP -i :4021

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 4021 -State Listen | Select LocalAddress, OwningProcess
Get-NetUDPEndpoint -LocalPort 4021
netstat -ano | findstr :4021

From another machine

nmap -p 4021 -sV <host>
sudo nmap -sU -p 4021 <host>
nc -zv <host> 4021

Block it in the firewall

sudo ufw deny 4021/tcp                                         # Linux (ufw)
New-NetFirewallRule -DisplayName "Block 4021" -Direction Inbound -LocalPort 4021 -Protocol TCP -Action Block  # Windows
04 FAQ
What is port 4021 used for?

Port 4021 is assigned by IANA to nexus-portal (Nexus Portal).

Is port 4021 TCP or UDP?

Port 4021 is registered for TCP and UDP.

Is it safe to open port 4021?

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. Close the port if nothing on the host needs it.

How do I check if port 4021 is open?

On Linux run "ss -ltnup 'sport = :4021'", on macOS "lsof -nP -i :4021", and on Windows "netstat -ano | findstr :4021" to see the local process. From another machine, "nmap -p 4021 <host>" shows whether it is reachable.

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