domain name system - What is the difference between the NS records and the glue records?

I have two glue records at Godaddy, for example:

NS1.MYDNSSERVER.COM
NS2.MYDNSSERVER.COM

And I have three NS records in my domain:

NS1.MYDNSSERVER.COM

NS2.MYDNSSERVER.COM
NS3.MYDNSSERVER.COM

1. If I have glue records why do I need the NS records?


2. The extra record (NS3.MYDNSSERVER.COM) is a real DNS server but from some reason I can't update this record as a glue record. Can this cause any problems?

Answer

Glue records are "hint" IP addresses that the parent servers (i.e not GoDaddy) provide in addition to the answer when a resolver asks what the NS records are for your domain. They are not the authoritative answer. The records you have labeled above as glue records are not glue records, however GoDaddy probably use those records to tell the parent nameserver what the glue should be. The canonical DNS answer describes this much better than I am able to.

Some resolvers will use the glue records without double-checking and some will query the NS records from the authoritative name servers (the ones provided in the glue records) to make sure they are actually correct.

I know from experience that PowerDNS is in the former group and bind9 is in the latter. I have no experience with any other resolvers.

Not having NS records at your nameservers will cause problems with bind9. The kind of problems where anyone whose ISP is using bind9 will not be able to visit your website.

Having an extra NS record at your nameservers that isn't at the parent nameservers will just mean that nameserver gets less traffic than the other two. It's the other way around in this situation and bind9 will use all the nameservers and PowerDNS will not.