Using same authoritative NSes multiple times in delegation

[Kevin Darcy]

Andrey G. Sergeev (AKA Andris) wrote:
> Greetings,
>
>
> does the following setup violate any DNS RFCs or is it in the conflict 
> with any best practices?
>
> ----------------------------------------------------------------------
> [andris at strigidae ~]$ dig +nocmd +nocom +noque +nosta domain1.tld1. ns
> domain1.tld1. 86400 IN NS ns1.domain1.tld1.
> domain1.tld1. 86400 IN NS ns2.domain1.tld1.
> domain1.tld1. 86400 IN NS ns1.domain2.tld2.
> domain1.tld1. 86400 IN NS ns2.domain2.tld2.
> domain1.tld1. 86400 IN NS ns1.domain3.tld3.
> domain1.tld1. 86400 IN NS ns2.domain3.tld3.
> ns1.domain1.tld1. 86400 IN A IP.Add.ress.1
> ns2.domain1.tld1. 86400 IN A IP.Add.ress.2
> ^^^^^^^^^^^^^
> ns1.domain2.tld2. 86400 IN A IP.Add.ress.3
> ^^^^^^^^^^^^^
> ns2.domain2.tld2. 86400 IN A IP.Add.ress.4
> ns1.domain3.tld3. 86400 IN A IP.Add.ress.2
> ^^^^^^^^^^^^^
> ns2.domain3.tld3. 86400 IN A IP.Add.ress.3
> ^^^^^^^^^^^^^
> ----------------------------------------------------------------------
>
> As we can see above, the ns2.domain1.tld1 / ns1.domain3.tld3 are 
> actually the same physical host with the IP.Add.ress.2 and the 
> ns1.domain2.tld2 / ns2.domain3.tld3 are actually the same machine with 
> the IP.Add.ress.3.
The DNS standards only say that every zone must have at least 2 
nameservers. That doesn't appear to be violated here. The fact that some 
of the nameservers have multiple names, doesn't reduce the 
availability/robustness of the delegations (which is apparently the 
whole point of the rule), the only minor negative effect is that there 
is some confusion over where the PTR records should point. But even that 
is pretty much irrelevant, since doing a reverse lookup of an 
authoritative nameserver is not required by any standard, nor something 
that is done in the normal course of operation.
>
> What are the benefits of this setup?
4 nameservers are cheaper than 6 (??)

- Kevin