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Date: Tue, 14 Apr 2020 11:57:17 +0200
From: Florian Weimer <>
To: Rich Felker <>
Cc: Christian <>,
Subject: Re: Resolver routines, Postfix DNSSEC troubles - how to check for incompatibilities?

* Rich Felker:

> On Mon, Apr 13, 2020 at 05:52:34PM +0200, Florian Weimer wrote:
>> * Christian:
>> > So Viktor did some digging:
>> >
>> > "The comment on line 25:
>> >
>> >
>> >
>> > is not encouraging.  It suggests that _res is unused. If so, Postfix
>> > DNS does not work correctly with this C library.  And not just for DANE, since Postfix is also unable to to control RES_DEFNAMES and RES_DNSRCH.
>> Are these changes to the RES_DEFNAMES and RES_DNSRCH flags really
>> necessary? Why doesn't Postfix use res_query (or perhaps res_send) as
>> appropriate?
> But to actually answer these questions, modifying the flags is
> presumably because traditional req_query builds an rfc1035 query or
> edns query based on these flags derived from from resolv.conf, and
> Postfix either assumes or wants to support the case where resolv.conf
> is not already configured for edns, perhaps because it was generated
> by a dhcp client.

In my comment above, I specifically meant RES_DEFNAMES and RES_DNSRCH.

RES_USE_EDNS0 seems different; I would expect applications to use
their own DNS libraries if they need to access DNSSEC data and
non-address record types (where there is no benefit gained form
integrating with /etc/hosts or other data sources).

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