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Port 999 (applix)

IANA assigns port 999 to applix: Applix ac.

Well-known (system) port, 0-1023 UDP

Port 999 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
applix UDP Applix ac - -

Check what is listening on port 999

Linux: UDP 999
sudo ss -ulnp 'sport = :999'
Linux / macOS: which process owns the port
sudo lsof -nP -i :999
Windows (Command Prompt)
netstat -ano | findstr :999
Windows (PowerShell)
Get-NetUDPEndpoint -LocalPort 999

Test it from outside

UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 999

Block port 999 in the firewall

ufw
sudo ufw deny 999/udp
iptables
sudo iptables -A INPUT -p udp --dport 999 -j DROP
nftables
sudo nft add rule inet filter input udp dport 999 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 999' -Direction Inbound -Protocol UDP -LocalPort 999 -Action Block

Related ports

995POP3S996vsinet1001webpush1008Possibly used by Sun Solaris????

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