CODEWITHSUNDEEP
dnsnetwork-programmingpacketnetwork-protocolsmdns
David Haim
David Haim

Reputation: 26486

Questions about mDns response packets

I have a few questions about the mdns protocol.

1) mdns additional records add additional data about the services and domains specified in the Answers section. Can the Answer section contain more than one service, and each additional record "point" to a differnt service, depending on its offset flag? in other words, when querying an mdns packet, should we assume additional records refer to different answers, if more than one answer exists? is this scenario possible?

2) mdns provide an "A" type which is the IP address of the service. can this IP mismatch the IP address the response packet was sent from? usually, the service response on its behalf, providing its IP address when responding. but the IP address is anyway known to the receiver because this is the source IP. can the mdns responder provide an IP different than the IP it currently using?

thanks!

Upvotes: 0

Views: 1044

Answers (1)

Bergi
Bergi

Reputation: 664513

Can the Answer section contain more than one service, and each additional record "point" to a different service, depending on its offset flag?

Yes, a single response can contain multiple answers for the same name, e.g. multiple services being announced by the same mdns responder, and so can the additional answers.

In other words, when querying an mdns packet, should we assume additional records refer to different answers?

The additional answer records refer to their name, like any record in dns. This might, or might not, be a name that is also used by/in a record in the answer section.

Can the IP from the "A" record mismatch the IP address the response packet was sent from?

Yes. A mdns responder can resolve any host name to any ip address, not just its own name to its own ip address. An example of this might be a proxy with two network interfaces that connects the two networks by responding on the behalf of the hosts in the other. Or simpler, in the context of service discovery, a local host might announce a service that is running elsewhere, not under a .local name, and has an IP address outside the network.

Upvotes: 1

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