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d29767431c
The internal resolver needs to know whether to make requets
to external DNS servers from the container's network namespace
or the host's.
The original rule was that requests were always made from the
container's namespace, unless the nameserver was on a localhost
address on the host. IPv6 nameservers were left in the container's
/etc/resolv.conf.
Commit 4e8d9a4 modified that so that IPv6 nameservers were also
used as external nameservers. The internal resolver accessed
them from the host namespace if the container's initial set of
endpoints were IPv4-only, or the nameserver address contained
a zone-id, (or the nameserver was on the IPv6 loopback address).
That would break if initial IPv6 endpoints were disconnected from
the container, leaving it with no IPv6 address.
Once IPv6-only networks are allowed, another exception would need
to be made for IPv4 nameservers (they'd need to be accessed from
the host's namespace).
Instead of doing that ... this change simplifies things, if a
nameserver address is read from the host's /etc/resolv.conf, it'll
work in the host's namespace. So, the rule is now simply that
nameservers read from the host's resolv.conf are accessed from the
host's namespace. DNS servers added as overrides ('--dns') are
accessed from the container's namespace (as before).
Signed-off-by: Rob Murray <rob.murray@docker.com>