Re: [exim] CNAME and recipient rewriting

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Autor: Matthias-Christian Ott
Data:  
Dla: Mike Cardwell
CC: exim-users
Temat: Re: [exim] CNAME and recipient rewriting
On Sun, Oct 10, 2010 at 09:25:38PM +0100, Mike Cardwell wrote:
> On 10/10/2010 19:41, Matthias-Christian Ott wrote:
>
> >> I recently got notified from a mailing list which is mangaged with
> >> ezmlm that E-Mails could not be delivered because my mail server
> >> rejected them. It turned out that the SMTP server or ezmlm itself
> >> rewrote the recipient of the envelope from bob@??? to
> >> bob@???, because example.com was a CNAME for b.example.com.
> >>
> >> Is such rewriting compliant with any standard (according to the
> >> rejectlog no other E-Mails got rejected for this reason)?
> >
> > As more E-Mails come in, it seems very likely that some SMTP servers
> > got confused by the CNAME and as far as I can tell from the logs
> > didn't even try to deliver the E-Mails to my server.
>
> If your MX record is pointing at a CNAME, then your DNS is broken. MX
> records are not allowed to point at CNAMEs, they're only allowed to
> point at A records.


I think this was not the problem (maybe the problem statement was
ambiguous). My DNS configuration was as follows:

example.com. IN NS 198.51.100.1
example.com. IN NS 203.0.113.1

on the nameservers I had the following:

example.com.    IN CNAME  b.example.com.
example.com.    IN MX     10  mx.example.com.  
b.example.com.  IN A      198.51.100.2
mx.example.com. IN A      198.51.100.3


According to RFC 1912 there can't be other RRs if there's already a
CNAME. It seems some SMTP servers followed the DNS RFC very closely and
didn't send E-Mails at all. But this doesn't explain why that particular
SMTP server rewrote the recipient (after I added a transitional MX
record for b.example.com ebay.com did the same).

Regards,
Matthias-Christian