"Downstream" Reverse-DNS
How do downstream ISPs (those that acquire their IP address blocks from an upstream ISP, as opposed to directly from IANA), manage reverse-DNS (RDNS)? Unlike forward DNS lookups, RDNS lookups go to the upstream ISP's DNS servers (who "owns" the IP blocks) where the downstream domain name information may be absent. [How do THEY know that 'www.xyz.com' became the primary domain for 192.168.142.13 five minutes ago? ...unless you pester the hell out of them for every minor update?]
I have asked my upstream ISP to allow us (or, at least, our automated systems which translate customer domain changes to DNS zone files on the fly) to write RDNS info to their DNS servers but they have instead suggested using CNAME records on their DNS servers to forward RDNS queries for our assigned IP blocks to one domain (one that will never go away) on our own DNS servers. There, we would provide the requisite PTR records for each and every assigned IP. This does give us the accessibility we need to make unlimited RDNS changes but seems a little kludgy. Is this the way all downstream ISPs do it?
Any advice is appreciated.
Dave
