On 13/07/16 17:01, Claus Aßmann wrote:
> Mike Scott wrote:
>
>> data.scotts: Host [39.54.122.34] claimed to be 39.54.122.34
>
>> Can someone say exactly why this merits a warning please? A host
>> claiming to be itself doesn't seem so dreadful a thing! :-}
>
> AFAICT:
>
> [39.54.122.34] != 39.54.122.34
>
>
Well, true.
But the host isn't pretending to be who it isn't.
After a quick look at rfc5321, I'm wondering if the problem is down to
the client using a bad ehlo line. If I read correctly, if you're not
using a domain name, you use an address literal, which has square
brackets. So is the example I gave complaining that my server has
checked the client IP, and is saying it expects the client to say
ehlo [39.54.122.34]
whereas it /actually/ says
ehlo 39.54.122.34
Right reasoning???
Which leads me to a long-standing mis-configuration of my own mail
server - it's behind a natted router on a dynamic IP - which conspires
to mean it's nigh impossible to send a fully correct ehlo line. The
sendmail server doesn't know the public IP, so I can't use an address
literal. Any domain name has to be an A (AAAA) record for the public IP
(why???!), which again doesn't really work unless I use the dynamic IP
provider name which doesn't seem a good thing to make public (and bad in
principle). The saving grace is the server "MUST NOT" refuse mail
because of a mismatch here. Is there a good solution though?