sendmail's implementation of DANE determines whether DNSSEC validation was
successful based on the presence of the AD bit in the response to the DANE
record lookup.
An equivalent dig lookup would be:
% dig TLSA _25._tcp.smtp.gshapiro.net.
...
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 160
;; flags: qr rd ra ad; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1
...
; ANSWER SECTION:
_25._tcp.smtp.gshapiro.net. 5 IN TLSA 3 1 1
8B2B0BF34A1D650A91399A28D5E6BBF377FB5319E9850078538164F5 557CD5BA
As you can see above the flags returned include "ad".
However, if sendmail is run on a server that lists the authoritative nameserver
for a domain as a resolver (/etc/resolv.conf), the AD bit is not returned for
lookups of those authoritative domains. For example, when running the above
dig command pointing at ns.gshapiro.net (running BIND 9.16.24), the AD bit is
not returned:
> dig TLSA _25._tcp.smtp.gshapiro.net. @ns.gshapiro.net
...
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 45940
;; flags: qr aa rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1
...
;; ANSWER SECTION:
_25._tcp.smtp.gshapiro.net. 120 IN TLSA 3 1 1
8B2B0BF34A1D650A91399A28D5E6BBF377FB5319E9850078538164F5 557CD5BA
Two questions:
1. Is there a reason when BIND is running as both a recursive server and an
authoritative server for a domain, it doesn't set the AD bit when answering
resolver queries for one of its authoritative domains?
2. Should sendmail not be trusting the AD bit in replies from the admin
configured (i.e., trusted by admin) resolvers? I.e., should sendmail be doing
something different for DANE DNSSEC validation? Note that DANE doesn't allow
for treating the authoritative server differently so I don't believe we can use
the AA bit as a substitute for the AD bit.
Thanks!
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