Since 4.3.0 the guard resolves a name to every address it has, and a
single blocked address refused the whole name. LAN DNS servers commonly
publish a host's fe80:: address next to its IPv4, so a self-hosted IdP on
such a name failed every login with "Requests to link-local /
cloud-metadata addresses are not allowed". Immich, Synology and the other
strict-guard integrations on the same kind of name failed the same way.
Both guards now leave a blocked address out of the list the socket is
pinned to and connect over the rest, and a name that resolves to nothing
else is still refused. A private address among the remaining ones still
needs ALLOW_INTERNAL_NETWORK under the strict guard, and
ALLOW_LINK_LOCAL_IPS works as before.
Leaving addresses out is only safe if the blocked set is complete, so the
strict guard now blocks everything the admin lane does. The AWS IMDSv6
(fd00:ec2::/32) and Alibaba (100.100.100.200, 100.100.100.100) metadata
addresses sit in ULA and CGNAT space, which ALLOW_INTERNAL_NETWORK opened
to any URL a user can type. Both guards also read an IPv4-mapped or
IPv4-compatible address as the IPv4 it carries: Node prints an AAAA
record of ::a9fe:a9fe as ::169.254.169.254, which passed both guards, and
::ffff:100.100.100.200 passed the admin lane. Neither guard connects to a
link-local or metadata address now, and the strict guard never connects
to loopback either.
Maurice
0887f251ce