Port 9903 (multicast-ping)

IANA assigns port 9903 to multicast-ping: Multicast Ping Protocol.

Registered (user) port, 1024-49151 UDP

Port 9903 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

multicast-ping UDP Multicast Ping Protocol - RFC 6450

Registry dates for this port: last modified 2011-12-09.

Linux: UDP 9903

sudo ss -ulnp 'sport = :9903'

Linux / macOS: which process owns the port

sudo lsof -nP -i :9903

Windows (Command Prompt)

netstat -ano | findstr :9903

Windows (PowerShell)

Get-NetUDPEndpoint -LocalPort 9903

UDP probe (no reply does not prove it is closed)

nc -vzu -w 5 HOST 9903

ufw

sudo ufw deny 9903/udp

iptables

sudo iptables -A INPUT -p udp --dport 9903 -j DROP

nftables

sudo nft add rule inet filter input udp dport 9903 drop

Windows Firewall

New-NetFirewallRule -DisplayName 'Block UDP 9903' -Direction Inbound -Protocol UDP -LocalPort 9903 -Action Block

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