Re: [exim] Subtle delivery issue - BROKEN DNS PTR questions.

On Tue, Apr 3, 2012 at 6:52 AM, Ron White <exim.ml@???> wrote:
> I've been working with a client running Exim on a cheap shared host who
> has been having some odd delivery issues. Normally I don't get too
> involved in these, but it was interesting. It only affects some
> recipients some of the time and the only reason I can find for the
> inconstancy is what appears to be a bit of a hooky DNS set up.

Simple test: set your system resolver to use 8.8.8.8 and 8.8.4.4
instead of whatever DNS resolver it's using now.

> The host concerned has a PTR record, it's a bit of a mess, but it's
> there:
> dig -x 205.134.224.208
>
> 208.224.134.205.in-addr.arpa. 17019 IN  CNAME
> 208.128-255.224.134.205.in-addr.arpa.
> 208.128-255.224.134.205.in-addr.arpa. 65020 IN PTR
> whub28.webhostinghub.com.

SOP for doing rDNS for non 8 bit boundaries.

> However, digging this gives two A records/IP's back rotating on a round
> robin:
>
> dig +short whub28.webhostinghub.com.
> 205.134.241.17
> 205.134.224.208
> dig +short whub28.webhostinghub.com.
> 205.134.224.208
> 205.134.241.17
> dig +short whub28.webhostinghub.com.
> 205.134.241.17
> 205.134.224.208
>
> I think this may be a problem with PTR resolution because if the reverse
> lookup for a connecting IP gives the name whub28.webhostinghub.com, but
> the matching double check on that back to an IP gives two records back
> will the average mail resolver see both of these and satisfy the check,
> or will it take the top one only and spot the mismatch between the
> original connecting IP and the RrDNS?

No sites should require the forward DNS and the rDNS to be the same.
It's perfectly logical to expect rDNS to resolve to something,
anything, but not to make it match forward DNS.

I still suspect it's your DNS.

Have you verified that you have clean MTU path all the way to the
hosts which are giving you problems? Is there an overzealous firewall
that blocks all ICMP (breaking path mtu discovery)?

...Todd
--
Always code as if the guy who ends up maintaining your code will be a
violent psychopath who knows where you live. -- Martin Golding