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Port 4021 (nexus-portal)

IANA assigns port 4021 to nexus-portal: Nexus Portal.

Registered (user) port, 1024-49151 TCPUDP

IANA assignment

Service nameProtoDescriptionAssigneeReference
nexus-portal TCP Nexus Portal Damian Tarnawsky -
nexus-portal UDP Nexus Portal Damian Tarnawsky -

Check what is listening on port 4021

Linux: is anything listening on TCP 4021?
sudo ss -tlnp 'sport = :4021'
Linux: UDP 4021
sudo ss -ulnp 'sport = :4021'
Linux / macOS: which process owns the port
sudo lsof -nP -i :4021
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':4021 '
Windows (Command Prompt)
netstat -ano | findstr :4021
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 4021 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 4021
From Windows
Test-NetConnection HOST -Port 4021
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 4021

Block port 4021 in the firewall

ufw
sudo ufw deny 4021/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 4021 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 4021 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 4021' -Direction Inbound -Protocol TCP -LocalPort 4021 -Action Block
ufw
sudo ufw deny 4021/udp
iptables
sudo iptables -A INPUT -p udp --dport 4021 -j DROP
nftables
sudo nft add rule inet filter input udp dport 4021 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 4021' -Direction Inbound -Protocol UDP -LocalPort 4021 -Action Block

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All port numbers · Open port checker
Assignment data: IANA port number registry (public domain), retrieved 2026-11-10.