Re: DNS hiccups

Sam Varshavchik <[email protected]> Wed, 16 Apr 2025 17:45:14 -0400

Newsgroups	gmane.mail.imap.courier.general
Message-ID	<[email protected]>

Alessandro Vesely writes:

>> So, there were multiple attempts made, here.
>
>
> Only two A queries were logged.  Unfortunately, named log doesn't tell the  
> result, but someone on bind-users noticed that the name servers for  
> mail.protection.outlook.com return FORMERR for EDNS queries (5 years after  
> the DNS flag day).  Maybe the first query timed out and the second gave  
> error?
>
> I guess the resolver tries with no edns after a FORMERR.  That explains how  
> the following messages to the same MX passed.

Courier will only send a 2nd or a 3rd request if it gets no response at all.  
Any response, including an error, is processed.

Your caching resolver probably eventually got something, an error, or  
proclaimed a timeout and gave up, caching a SERVFAIL internally. So you  
probably saw the 2nd request getting passed along, but then bind finally  
received its own request (I presume that if bind forwarded a query that has  
a response pending, and then gets an additional query it'll just make it  
wait for the same response to the first passed-along request, instead of  
passing along another query) and then used it to fail Courier's request.

Subsequently all requests from Courier were handled from the cached (error)  
response.

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