HTTPfy

Port 6699 (babel-dtls)

IANA assigns port 6699 to babel-dtls: Babel Routing Protocol over DTLS.

Registered (user) port, 1024-49151 UDP

Port 6699 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
babel-dtls UDP Babel Routing Protocol over DTLS IESG RFC 8968

Registry dates for this port: registered 2020-09-04.

Check what is listening on port 6699

Linux: UDP 6699
sudo ss -ulnp 'sport = :6699'
Linux / macOS: which process owns the port
sudo lsof -nP -i :6699
Windows (Command Prompt)
netstat -ano | findstr :6699
Windows (PowerShell)
Get-NetUDPEndpoint -LocalPort 6699

Test it from outside

UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 6699

Block port 6699 in the firewall

ufw
sudo ufw deny 6699/udp
iptables
sudo iptables -A INPUT -p udp --dport 6699 -j DROP
nftables
sudo nft add rule inet filter input udp dport 6699 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 6699' -Direction Inbound -Protocol UDP -LocalPort 6699 -Action Block

Related ports

6696babel6697IRC over TLS6701kti-icad-srvr6702e-design-net

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