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Port 4755 (gre-udp-dtls)

IANA assigns port 4755 to gre-udp-dtls: GRE-in-UDP Encapsulation with DTLS.

Registered (user) port, 1024-49151 UDP

Port 4755 is normally used over UDP; the checker tests TCP only, so a "filtered" result does not mean the UDP service is down.

IANA assignment

Service nameProtoDescriptionAssigneeReference
gre-udp-dtls UDP GRE-in-UDP Encapsulation with DTLS IESG RFC 8086

Registry dates for this port: registered 2016-10-05.

Check what is listening on port 4755

Linux: UDP 4755
sudo ss -ulnp 'sport = :4755'
Linux / macOS: which process owns the port
sudo lsof -nP -i :4755
Windows (Command Prompt)
netstat -ano | findstr :4755
Windows (PowerShell)
Get-NetUDPEndpoint -LocalPort 4755

Test it from outside

UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 4755

Block port 4755 in the firewall

ufw
sudo ufw deny 4755/udp
iptables
sudo iptables -A INPUT -p udp --dport 4755 -j DROP
nftables
sudo nft add rule inet filter input udp dport 4755 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 4755' -Direction Inbound -Protocol UDP -LocalPort 4755 -Action Block

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Assignment data: IANA port number registry (public domain), retrieved 2026-11-10.