300-365 · Question #76
Which protocol/port number is used by mDNS for service advertisement?
The correct answer is C. UDP 5353. mDNS (Multicast DNS) uses UDP port 5353 to allow devices to advertise and discover services on a local network without a central DNS server.
Question
Which protocol/port number is used by mDNS for service advertisement?
Options
- ATCP 5353
- BUDP 53
- CUDP 5353
- DTCP 53
How the community answered
(56 responses)- A2% (1)
- B4% (2)
- C95% (53)
Why each option
mDNS (Multicast DNS) uses UDP port 5353 to allow devices to advertise and discover services on a local network without a central DNS server.
mDNS does not use TCP; the protocol relies on multicast UDP datagrams, and TCP cannot address multicast groups.
UDP 53 is the standard unicast DNS port used by traditional DNS resolvers, not by mDNS service advertisement.
mDNS is defined in RFC 6762 and operates over UDP because it sends multicast datagrams to the link-local multicast address 224.0.0.251 (IPv4) on port 5353. UDP is used because mDNS is connectionless and relies on multicast delivery, which TCP does not support.
TCP 53 is reserved for standard DNS zone transfers and large DNS responses, and is unrelated to multicast service discovery.
Concept tested: mDNS protocol and port for service discovery
Source: https://www.rfc-editor.org/rfc/rfc6762
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