Port 5353 (mDNS (Bonjour))

Multicast DNS for .local name resolution and service discovery (Bonjour, Avahi, Chromecast, AirPlay, printers).

Registered (user) port, 1024-49151 UDP Network infrastructure Localhost or LAN only

Port 5353 is normally used over UDP; the checker tests TCP only, so a "filtered" result does not mean the UDP service is down.

What port 5353 is used for

Multicast DNS for .local name resolution and service discovery (Bonjour, Avahi, Chromecast, AirPlay, printers).

Security: should port 5353 be open?

Localhost or LAN only. Link-local only and should never answer unicast queries from the internet; exposed mDNS responders leak device info and are used for reflection.

IANA assignment

Service nameProtoDescriptionAssigneeReference

mdns TCP Multicast DNS IESG RFC 6762

mdns UDP Multicast DNS IESG RFC 6762

Check what is listening on port 5353

Linux: UDP 5353

sudo ss -ulnp 'sport = :5353'

Linux / macOS: which process owns the port

sudo lsof -nP -i :5353

Windows (Command Prompt)

netstat -ano | findstr :5353

Windows (PowerShell)

Get-NetUDPEndpoint -LocalPort 5353

Test it from outside

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

nc -vzu -w 5 HOST 5353

Block port 5353 in the firewall

ufw

sudo ufw deny 5353/udp

iptables

sudo iptables -A INPUT -p udp --dport 5353 -j DROP

nftables

sudo nft add rule inet filter input udp dport 5353 drop

Windows Firewall

New-NetFirewallRule -DisplayName 'Block UDP 5353' -Direction Inbound -Protocol UDP -LocalPort 5353 -Action Block

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