Port 9901 (enrp, enrp-sctp)

IANA assigns port 9901 to enrp, enrp-sctp: enrp server channel.

Registered (user) port, 1024-49151 UDP

Port 9901 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

enrp UDP enrp server channel - -

enrp-sctp SCTP enrp server channel - RFC 5353

Linux: UDP 9901

sudo ss -ulnp 'sport = :9901'

Linux / macOS: which process owns the port

sudo lsof -nP -i :9901

Windows (Command Prompt)

netstat -ano | findstr :9901

Windows (PowerShell)

Get-NetUDPEndpoint -LocalPort 9901

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

nc -vzu -w 5 HOST 9901

ufw

sudo ufw deny 9901/udp

iptables

sudo iptables -A INPUT -p udp --dport 9901 -j DROP

nftables

sudo nft add rule inet filter input udp dport 9901 drop

Windows Firewall

New-NetFirewallRule -DisplayName 'Block UDP 9901' -Direction Inbound -Protocol UDP -LocalPort 9901 -Action Block

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