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Port 604 (tunnel)

IANA assigns port 604 to tunnel: TUNNEL.

Well-known (system) port, 0-1023 TCPUDP

IANA assignment

Service nameProtoDescriptionAssigneeReference
tunnel TCP TUNNEL - RFC 3620
tunnel UDP TUNNEL - RFC 3620

Check what is listening on port 604

Linux: is anything listening on TCP 604?
sudo ss -tlnp 'sport = :604'
Linux: UDP 604
sudo ss -ulnp 'sport = :604'
Linux / macOS: which process owns the port
sudo lsof -nP -i :604
Older Linux / BSD (net-tools)
sudo netstat -tlnp | grep ':604 '
Windows (Command Prompt)
netstat -ano | findstr :604
Windows (PowerShell)
Get-NetTCPConnection -LocalPort 604 -State Listen | Select LocalAddress,OwningProcess

Test it from outside

From another machine
nc -vz -w 5 HOST 604
From Windows
Test-NetConnection HOST -Port 604
UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 604

Block port 604 in the firewall

ufw
sudo ufw deny 604/tcp
iptables
sudo iptables -A INPUT -p tcp --dport 604 -j DROP
nftables
sudo nft add rule inet filter input tcp dport 604 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block TCP 604' -Direction Inbound -Protocol TCP -LocalPort 604 -Action Block
ufw
sudo ufw deny 604/udp
iptables
sudo iptables -A INPUT -p udp --dport 604 -j DROP
nftables
sudo nft add rule inet filter input udp dport 604 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 604' -Direction Inbound -Protocol UDP -LocalPort 604 -Action Block

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