Bertrand Jacquin
17f3e1fb3b
Since 748a548e1b ("resolved: set cache-flush bit on mDNS responses"),
every RR of an mDNS zone, except DNS-SD service enumeration PTRs,
carries DNS_ANSWER_CACHE_FLUSH, and dns_packet_append_key() turns it
into the cache-flush bit regardless of the protocol of the packet.
When a lookup via the DNS stub is answered from our own mDNS zone, e.g.
the reverse lookup of one of our link-local addresses, the RR is sent
with class 0x8001. In unicast DNS bit 15 is part of the class value, so
clients see CLASS32769 instead of IN:
$ dig +nocmd +nocookie -x fe80::d1f:2970:a730:5ab8 @127.0.0.53
;; Warning: Message parser reports malformed message packet.
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 30061
;; flags: qr aa rd ra ad; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 65494
;; QUESTION SECTION:
;8.b.a.5.0.3.7.a.0.7.9.2.f.1.d.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.e.f.ip6.arpa. IN PTR
;; ANSWER SECTION:
8.b.a.5.0.3.7.a.0.7.9.2.f.1.d.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.e.f.ip6.arpa. 120 CLASS32769 PTR lady-voodoo.local.
;; Query time: 1 msec
;; SERVER: 127.0.0.53#53(127.0.0.53) (UDP)
;; WHEN: Wed Sep 02 21:40:55 IST 2026
;; MSG SIZE rcvd: 132
The cache-flush bit is defined for mDNS only, hence only set it on mDNS
packets.
See: https://github.com/systemd/systemd/pull/43627
(cherry picked from commit 21159c052bbf28983115d842dbc2597869177d42)
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