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Port 4086 (ftsync)

IANA assigns port 4086 to ftsync: Firewall/NAT state table synchronization.

Registered (user) port, 1024-49151 UDP

Port 4086 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
ftsync UDP Firewall/NAT state table synchronization Heiner Erne -

Registry dates for this port: registered 2008-06-06.

Check what is listening on port 4086

Linux: UDP 4086
sudo ss -ulnp 'sport = :4086'
Linux / macOS: which process owns the port
sudo lsof -nP -i :4086
Windows (Command Prompt)
netstat -ano | findstr :4086
Windows (PowerShell)
Get-NetUDPEndpoint -LocalPort 4086

Test it from outside

UDP probe (no reply does not prove it is closed)
nc -vzu -w 5 HOST 4086

Block port 4086 in the firewall

ufw
sudo ufw deny 4086/udp
iptables
sudo iptables -A INPUT -p udp --dport 4086 -j DROP
nftables
sudo nft add rule inet filter input udp dport 4086 drop
Windows Firewall
New-NetFirewallRule -DisplayName 'Block UDP 4086' -Direction Inbound -Protocol UDP -LocalPort 4086 -Action Block

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