# WHATPORT(5353)

## NAME

port 5353 — mDNS (Bonjour / Avahi) — Multicast DNS — how machines find each other by name on a LAN with no DNS server.

Protocol: UDP  
IANA registry

## DESCRIPTION

mDNS resolves .local names and carries DNS-SD service discovery over multicast to 224.0.0.251 (FF02::FB on IPv6). It is running on essentially every Mac, iPhone, Android device, Linux box with Avahi, printer, and Chromecast on the network — the daemon is mDNSResponder on Apple platforms and avahi-daemon elsewhere. It appears on a scan because the responder binds UDP 5353 and will also answer unicast queries sent to that port.

## EXPOSURE

**Do not expose.** Reachable from the internet it both leaks a device inventory (hostnames, service records, sometimes usernames) and works as a reflection/amplification source; on a trusted LAN it is normal and expected.

## CHECK

```sh
dig +short @224.0.0.251 -p 5353 -t ptr _services._dns-sd._udp.local
```

## SEE ALSO

- spec — [RFC 6762 — Multicast DNS](https://datatracker.ietf.org/doc/html/rfc6762) (datatracker.ietf.org)
- iana — [IANA port registry — 5353](https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.xhtml?search=5353) (iana.org)
- source — [apple-oss-distributions/mDNSResponder](https://github.com/apple-oss-distributions/mDNSResponder) (github.com)
- docs — [Apple — TCP and UDP ports used by Apple software products](https://support.apple.com/en-us/103229) (support.apple.com)

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HTML: <https://whatport.net/5353/> · JSON: <https://whatport.net/5353.json> · full dataset: <https://whatport.net/ports.json>