1

mDNS (multicast DNS) queries are sent to 224.0.0.251:5353. While this is a multicast address, don't all hosts on the network segment have to receive this traffic in order to have the correct DNS entries? If that is the case, then why not just sent the queries to 255.255.255.255 or another broadcast address? If not, what devices in the network segment are not part of the group that receives traffic sent to 224.0.0.251:5353?

Thanks.

Improve this question

1 Answer 1

Reset to default
2

Only hosts that care about mDNS will instruct their NICs to listen to the Ethernet multicast MAC address that 224.0.0.251 maps to, which is 01:00:5e:00:00:fb.

So, for example, by default, Windows machines won't be bothered by these multicasts, since their NIC hardware will drop these frames without passing them up to the OS. But if you install Bonjour for Windows, it'll tell the NIC to start passing those multicasts up to the OS so they can reach the mDNSResponder process.

Improve this answer
2
  • So how is that really that different from just doing a regular broadcast? Would mDNS queries go up to the OS instead of being dropped at the NIC?
    – I_Am_The_Batman782
    Mar 20, 2014 at 1:36
  • @I_Am_The_B Broadcasts are always passed up to the OS. Multicasts are only passed up on machines that want that kind of multicast, as specified by which multicast MAC addresses the OS has instructed the NIC to receive.
    – Spiff
    Mar 20, 2014 at 2:53

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .