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Port 6511 (dccp-udp)

IANA assigns port 6511 to dccp-udp: Datagram Congestion Control Protocol Encapsulation for NAT Traversal.

Registered (user) port, 1024-49151 UDP

Port 6511 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
dccp-udp UDP Datagram Congestion Control Protocol Encapsulation for NAT Traversal IESG RFC 6773

Check what is listening on port 6511

Linux: UDP 6511
sudo ss -ulnp 'sport = :6511'
Linux / macOS: which process owns the port
sudo lsof -nP -i :6511
Windows (Command Prompt)
netstat -ano | findstr :6511
Windows (PowerShell)
Get-NetUDPEndpoint -LocalPort 6511

Test it from outside

UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 6511

Block port 6511 in the firewall

ufw
sudo ufw deny 6511/udp
iptables
sudo iptables -A INPUT -p udp --dport 6511 -j DROP
nftables
sudo nft add rule inet filter input udp dport 6511 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 6511' -Direction Inbound -Protocol UDP -LocalPort 6511 -Action Block

Related ports

6509mgcs-mfp-port6510mcer-port6513netconf-tls6514Syslog over TLS

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