I am trying to get a full understanding of the meaning of DNS resolution process, through +trace option (in order to make the resolver simulate the NS default server role). I have taken the following output from a didactic example, then I added some lines that I found in a different context (I am sure that this is a possible scenario). These lines that I added are the last 2 of the output (regarding the NS of XXX.mit.edu and WWW.YYY.ZZZ).
dig www.mit.edu +trace ; <<>> DiG 9.2.4 <<>> www.mit.edu +trace ;; global options: printcmd . 2788 IN NS b.root-servers.net. . 2788 IN NS c.root-servers.net. . 2788 IN NS d.root-servers.net. . 2788 IN NS e.root-servers.net. . 2788 IN NS f.root-servers.net. . 2788 IN NS g.root-servers.net. . 2788 IN NS h.root-servers.net. . 2788 IN NS i.root-servers.net. . 2788 IN NS j.root-servers.net. . 2788 IN NS k.root-servers.net. . 2788 IN NS l.root-servers.net. . 2788 IN NS m.root-servers.net. . 2788 IN NS a.root-servers.net. ;; Received 276 bytes from 188.8.131.52#53(184.108.40.206) in 1 ms edu. 172800 IN NS A3.NSTLD.COM. edu. 172800 IN NS C3.NSTLD.COM. edu. 172800 IN NS D3.NSTLD.COM. edu. 172800 IN NS E3.NSTLD.COM. edu. 172800 IN NS G3.NSTLD.COM. edu. 172800 IN NS H3.NSTLD.COM. edu. 172800 IN NS L3.NSTLD.COM. edu. 172800 IN NS M3.NSTLD.COM. ;; Received 302 bytes from 220.127.116.11#53(b.root-servers.net) in 194 ms mit.edu. 172800 IN NS STRAWB.mit.edu. mit.edu. 172800 IN NS BITSY.mit.edu. mit.edu. 172800 IN NS W20NS.mit.edu. ;; Received 138 bytes from 18.104.22.168#53(A3.NSTLD.COM) in 182 ms www.mit.edu. 60 IN A 22.214.171.124 mit.edu. 21600 IN NS BITSY.mit.edu. mit.edu. 21600 IN NS STRAWB.mit.edu. mit.edu. 21600 IN NS W20NS.mit.edu. mit.edu. 21600 IN NS XXX.mit.edu mit.edu. 21600 IN NS WWW.YYY.ZZZ ;; Received 154 bytes from 126.96.36.199#53(STRAWB.mit.edu) in 150 ms
I understood everything that happen before those lines: "." information are kept by root name servers. The list of root name servers is given by 188.8.131.52, which is my default name server. "edu" information are given by the name servers in the "second block" (XX.MSTLD.COM). And this is information is given by a root name server. "mit.edu" information are given by STRAWB, BITSY and W20NS (which are authority NS for mit.edu). This is also fine. So, in the "last block": what do the "XXX.mit.edu" and "WWW.YYY.ZZZ" mean? They are also autorithy NS for mit.edu, but why weren't they listed previously, along with the other 3 autorithy NS? It is like if only STRAWB.mit.edu knows that there are more NS containing information relating to mit.edu zone.. does it mean that they are auxiliary name servers, maybe for redundancy?
I hope my question was clear.