Re: IPv6 recap

Sam Varshavchik <[email protected]> Fri, 05 Apr 2024 17:28:41 -0400
Newsgroups	gmane.mail.imap.courier.general
Message-ID	<[email protected]>
Alessandro Vesely writes:

> On Fri 05/Apr/2024 14:08:27 +0200 Sam Varshavchik wrote:
>> Alessandro Vesely writes:
>>
>>> this happened again, but this time caused a bounce:
>>>
>>>  <<< 554 resimta-c2p-559421.sys.comcast.net resimta- 
>>> c2p-559421.sys.comcast.net 2a02:29e1:500:6c00::4 Comcast requires that all  
>>> mail servers must have a PTR record with a valid Reverse DNS entry.  
>>> Currently your mail server does not fill that requirement. For more  
>>> information, refer to: https://postmaster.comcast.net/smtp-error- 
>>> codes.php#554
>>>
>>> Is it possible to inhibit use of IPv6 a little bit harder?
>>
>> Can you block all IPv6 connections to port 25 on your firewall?
>
>
> Yes, I could.  However, that way I'd lose the possibility to reach IPv6-only  
> MXes (if there are any).
>
> If that depends on DNS queries timing, it should be enough to enlarge the  
> timeout a little bit.  Is that configurable?

Nope, but it won't be difficult to make it configurable to some degree.  
Handling of DNS query timeouts already has some logic to it, there's going  
to be a retry if there's no response, and all nameservers from resolv.conf  
will be tried, then if there's no response there will be additional rounds,  
with progressively higher timeouts. This can be tweaked as long as the  
timeout is the same for all DNS queries. It'll be harder to make the timeout  
different depending on what's being queried.

I don't know if this will help though. The baked-in logic makes three  
attempts using an exponential backoff, timing out each attempt after 5, 25,  
then 125 seconds. Per resolver. The last, 125 second timeout, is pretty  
generous.

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