IPv6 automatic reverse DNS

Steve Atkins steve at blighty.com
Fri Oct 28 23:28:44 UTC 2016


> On Oct 28, 2016, at 4:02 PM, Baldur Norddahl <baldur.norddahl at gmail.com> wrote:
> 
> Hello
> 
> Many service providers have IPv4 reverse DNS for all their IP addresses. If nothing is more relevant, this will often just be the IPv4 address hashed somehow and tagged to the ISP domain name. For some arcane reason it is important to have the forward DNS match the reverse DNS or some mail servers might reject your mails.
> 
> However with IPv6 it is not practical to build such a complete reverse DNS zone. You could do a star entry but that would fail the reverse/forward match test.
> 
> It should be simple to build a DNS server that will automatically generate a hostname value for every reverse lookup received, and also be able to parse that hostname value to return the correct IPv6 address on forward lookups.
> 
> Does any DNS server have that feature?

It's easy enough to implement with plugins on some servers.

> Should we have it?

Meh.

> Why not?

Because having an automatically generated reverse DNS is a sign that the IP address is not really intended to be offering public services, rather it's a malware-infested end user machine.

> 
> I know of some arguments for:
> 
> 1a) mail servers like it

... because it's a sign that the mail is coming from a real mailserver configured by a competent admin, rather than being a random compromised machine. That's not the case if you're just synthesizing reverse DNS for arbitrary IP addresses on your network.

> 
> 1b) anti spam filters believe in the magic of checking forward/reverse match.

For the same reason as above. Spam filters are also often smart enough to recognize, and treat as dubious, synthesized reverse DNS.

If you have synthesized reverse DNS on your smarthost you're likely to have a bad time, perhaps initially, perhaps the first time someone notices bad mail coming from it and doesn't recognize it as a legitimate smarthost.

> 
> 2) traceroute will be nicer

Most of those hosts a traceroute goes through should hopefully have stable IP addresses and meaningful, not synthesized, reverse DNS, I'd think. Consumer endpoints are the only ones where you might expect that not to be the case and synthesized reverse DNS might be an improvement there.

> 
> 3) http://ipv6-test.com/ will give me 20/20 instead of 19/20 (yes that was what got me going on this post)
> 
> 4) Output from "who" command on Unix will look nicer (maybe).
> 
> Regards,
> 
> Baldur

Cheers,
  Steve




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