Re: Really not prefer IPv6 MX

On Tue 03/Dec/2024 14:30:26 +0100 Sam Varshavchik wrote:
> Alessandro Vesely writes:
> 
>> Hi Sam,
>>
>> as I have IPv6 addresses, I'd like to be able to (try to) send messages to 
>> domains that have no IPv4 addresses.  However, I have no chance to set up 
>> reverse PTR, as my provider itself didn't claim its reverse zone.  Hence, if 
>> it's possible to use IPv4, I'd rather re-queue than use IPv6.
>>
>> I have ESMTP_PREFER_IPV6_MX=0 in courierd.  It doesn't seem to have any 
>> effect, as I introduced it after having fared without it for a while and 
>> didn't see much difference (similar preference is probably driven by 
>> gai.conf).  I get sporadic rejects like so:
> 
> How recent is the code you're running? There were some issues related to that 
> which were fixed in 1.3.7, but having said that:
> 
> ESMTP_PREFER_IPV6_MX=0 does not mean no IPv6 ever, it means that IPv4 is tried 
> first before trying IPv6. So if there was a connection failure to the IPv4 
> address then the IPv6 address gets attempted, so in the outlined scenario you 
> could still get an occasional failure, because of that.


I'm still at 1.3.5.  I browsed the code and that confirms me that setting 
ESMTP_PREFER_IPV6_MX=0 is the same as not setting it.  To get the behavior I'd 
like one would have to code something like, say,

    if (rc == MX_OK && *prefer_ipv6 == 'N') rc2 = MX_NONE;

That is, if IPv6 is really not preferred and IPv4 exist, pretend IPv6 does not. 
  But I won't propose a patch of that kind if I'd be the only one who ever uses 
it.  What you think?


> If you have a limited number of problematic domains you can try to put them 
> into esmtproutes, as a bandaid. The best, long term solution would be to set 
> aside an IPv4-only smarthost, and use esmtproutes to direct the problem domain 
> to the smarthost.


Too much work.  For the time being, this only happens with unimportant messages 
sent to comcast, so I just ignore it.


Best
Ale
-- 







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