Bug#1051477: dnssd: IPv6: fails to resolve services

[Tj]

Package: systemd-resolved
Version: 252.14-1~deb12u1
Severity: normal
Tags: ipv6 upstream

Reported upstream and tracked as:

https://github.com/systemd/systemd/issues/29120

Two hosts on the same link of an IPv6-only network. Host A is unable to resolve a service record advertised by host B using `resolvectl` where each has multiple IPv6 addresses (link-local, ULA, and global).

When host A does `resolvectl -i LOCAL service _apt-proxy._tcp local.` it sends from the link-local address. Host B ignores it.
When host A does `avahi-browse -rt _apt-proxy._tcp` it sends from a global address. Host B responds but sends the response from its link-local address to the multicast group.

This affects tools like `auto-apt-proxy` that uses `apt-helper
srv-lookup ...` and that talks to the system resolver configured in
`/etc/resolv.conf` which is systemd-resolve's stub entry when
systemd-resolved is active.

I was hoping to figure it out and have been working in the source-code
but so far not identified anything. It looks to be due to how the
multicast DNS (`src/resolve/resolved-mdns*`) discards packets for
various reasons on reception.

-- System Information:
Debian Release: 12.1
  APT prefers stable-updates
  APT policy: (500, 'stable-updates'), (500, 'stable-security-debug'), (500, 'stable-security'), (500, 'stable-debug'), (500, 'proposed-updates-debug'), (500, 'stable'), (100, 'proposed-updates')
Architecture: amd64 (x86_64)
Foreign Architectures: i386

Kernel: Linux 6.5.1+debian+tj (SMP w/16 CPU threads; PREEMPT)
Kernel taint flags: TAINT_WARN, TAINT_OOT_MODULE, TAINT_UNSIGNED_MODULE
Locale: LANG=en_GB.UTF-8, LC_CTYPE=en_GB.UTF-8 (charmap=UTF-8), LANGUAGE=en_GB:en
Shell: /bin/sh linked to /usr/bin/dash
Init: systemd (via /run/systemd/system)
LSM: AppArmor: enabled

Versions of packages systemd-resolved depends on:
ii  dbus [default-dbus-system-bus]  1.14.8-2~deb12u1
ii  dbus-broker [dbus-system-bus]   33-1
ii  libc6                           2.36-9+deb12u1
ii  libssl3                         3.0.9-1
ii  libsystemd-shared               252.14-1~deb12u1
ii  systemd                         252.14-1~deb12u1

Versions of packages systemd-resolved recommends:
ii  libnss-myhostname  252.14-1~deb12u1
ii  libnss-resolve     252.14-1~deb12u1

Versions of packages systemd-resolved suggests:
ii  policykit-1  122-3
ii  polkitd      122-3

-- no debconf information