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Thread: "Downstream" Reverse-DNS
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09-07-2003, 08:47 PM #1WHT Addict
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- Feb 2001
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"Downstream" Reverse-DNS
Hi all,
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
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09-07-2003, 09:08 PM #2Web Hosting Guru
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- Jan 2003
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- San Diego, California
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- 336
The upstream ISP delegates reverse to the folks downstream.
If they are using bind and your IP's were like 172.16/16 and they assigned you 172.16.3/24 the zone file would look like:
16.172.in-addr.arpa:
3 IN NS your server here
IN NS your secondary here
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09-08-2003, 12:51 PM #3Web Hosting Master
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- Jun 2003
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- 673
If you have a /24 or larger, then ARIN can delegate the /24 blocks directly to your DNS servers instead of to your ISP's servers. Otherwise, your ISP will need to delegate the space to you on their DNS servers, either by supplying NS records for each of your IP address (like Niosys wrote) or by using the cheesy technique described in RFC 2317.
(edit: English is my native language, really!)
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09-08-2003, 02:00 PM #4WHT Addict
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- Feb 2001
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- Lake Tahoe
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Thank you very much! Both of your recommendations are ideal, being that they utilize our DNS servers as is and without requiring any modification to our automation scripts. The solution proposed by our upstream provider would require extensive modification.
Yes, we do operate a /24. I'll take this up with our provider and ARIN. If no go, I will happily propose Niosys' solution.
Again, THANK YOU, BOTH!!!