Port 5353: mDNS
Port 5353 is Multicast DNS, used by protocols like Apple's Bonjour and various IoT devices to resolve hostnames on the local network without a DNS server. It only works within a single broadcast domain.
| Port number | 5353 |
|---|---|
| Service | Multicast DNS |
| Protocol | UDP |
| Category | Core services |
| Default encryption | No |
How do I check whether port 5353 is open?
Port 5353 is UDP, which changes the answer: there is no handshake to complete, so nothing outside the host can prove the port is open. Asking the host itself is the reliable route.
| Question | Command | What to know |
|---|---|---|
| Is it open on a remote host? | sudo nmap -sU -p 5353 example.com | UDP has no handshake, so there is nothing to complete. nmap infers the state from whether an ICMP port-unreachable comes back, needs root, and is slow and easy to misread. A silent port may be open, filtered, or simply not replying to a probe it does not recognise. |
| What is listening locally? | ss -ulnp | grep ':5353 ' | Shows the UDP socket bound to port 5353. This is the reliable way to answer the question, because you are asking the host itself rather than guessing from outside. |
Replace example.com with the host you are testing. A blocked port and a port with nothing listening on it look identical from the outside, so if a service should be running, check locally before blaming the firewall.
Frequently asked questions
What is port 5353 used for?
Port 5353 is Multicast DNS, used by protocols like Apple's Bonjour and various IoT devices to resolve hostnames on the local network without a DNS server. It only works within a single broadcast domain.
Is port 5353 TCP or UDP?
Port 5353 (mDNS) uses UDP.
Is port 5353 secure?
Port 5353 is not encrypted by default. Where possible, use an encrypted alternative or tunnel it over TLS or a VPN.
Should I open port 5353 on my firewall?
Only if you specifically need mDNS. Expose it to the smallest set of trusted sources, and never open database or Windows-service ports to the whole internet.