https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=281391

--- Comment #4 from John W. O'Brien <[email protected]> ---
I am experiencing this too. On 14.3-RELEASE-p8 amd64, reproduced via several
different net drivers (em, re, ixl). When sending to an IPv6 group, the first
frame emitted on behalf of the socket is destined to the expected multicast
MAC, and then every subsequent frame is destined to the Solicited Node
multicast MAC of the default gateway.

The terminal excerpts below show the system default gateway (least significant
24-bits of fe:b4:0c:c5) followed captured frames (source address gently
anonymized) while a test program sends four datagrams to ff0e::1234:5678,
terminates, then starts again and sends four datagrams to ff0e::fedc:ba09.

$ netstat -rnfinet6 | grep default
default                           fe80::ce36:cfff:feb4:cc5%re0  UG             
re0
$ tcpdump -i re0 -e -n ip6 net ff0e::/16
tcpdump: verbose output suppressed, use -v[v]... for full protocol decode
listening on re0, link-type EN10MB (Ethernet), snapshot length 262144 bytes
14:50:15.809235 00:01:2e:70:f4:17 > 33:33:12:34:56:78, ethertype IPv6 (0x86dd),
length 76: 2001:db8::8679:4d04:a971:1cb3.30231 > ff0e::1234:5678.1234: UDP,
length 14
14:50:16.845027 00:01:2e:70:f4:17 > 33:33:fe:b4:0c:c5, ethertype IPv6 (0x86dd),
length 76: 2001:db8::8679:4d04:a971:1cb3.30231 > ff0e::1234:5678.1234: UDP,
length 14
14:50:17.846461 00:01:2e:70:f4:17 > 33:33:fe:b4:0c:c5, ethertype IPv6 (0x86dd),
length 76: 2001:db8::8679:4d04:a971:1cb3.30231 > ff0e::1234:5678.1234: UDP,
length 14
14:50:18.854446 00:01:2e:70:f4:17 > 33:33:fe:b4:0c:c5, ethertype IPv6 (0x86dd),
length 76: 2001:db8::8679:4d04:a971:1cb3.30231 > ff0e::1234:5678.1234: UDP,
length 14
14:50:31.343242 00:01:2e:70:f4:17 > 33:33:fe:dc:ba:09, ethertype IPv6 (0x86dd),
length 76: 2001:db8::8679:4d04:a971:1cb3.33200 > ff0e::fedc:ba09.1234: UDP,
length 14
14:50:32.358560 00:01:2e:70:f4:17 > 33:33:fe:b4:0c:c5, ethertype IPv6 (0x86dd),
length 76: 2001:db8::8679:4d04:a971:1cb3.33200 > ff0e::fedc:ba09.1234: UDP,
length 14
14:50:33.369699 00:01:2e:70:f4:17 > 33:33:fe:b4:0c:c5, ethertype IPv6 (0x86dd),
length 76: 2001:db8::8679:4d04:a971:1cb3.33200 > ff0e::fedc:ba09.1234: UDP,
length 14
14:50:34.424186 00:01:2e:70:f4:17 > 33:33:fe:b4:0c:c5, ethertype IPv6 (0x86dd),
length 76: 2001:db8::8679:4d04:a971:1cb3.33200 > ff0e::fedc:ba09.1234: UDP,
length 14
^C

My test program sets IPV6_MULTICAST_HOPS to 10, and does not set
IPV6_MULTICAST_IF. Per ip6(4):

> ...
> For hosts with multiple interfaces, each multicast transmission is sent
> from the primary network interface.
> ...
> A value of zero specifies the default interface.

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