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619f1ddd05
Since commit 925b484 ("No fallback nameservers for internal
resolver"), if the host's resolv.conf has no nameservers and
no servers are supplied via config, the internal resolver will
not use Google's DNS - so the container will not be able to
resolve external DNS requests.
That can happen when container's are "restart-always" and the
docker daemon starts before the host's DNS is configured.
So, to highlight the issue (which may not be an error, but
probably is), include a warning in the container's resolv.conf
file.
Also, log a warning - logs currently say "No non-localhost DNS
nameservers are left in resolv.conf. Using default external
servers". But, that's misleading because it's from an initial
resolv.conf setup, before the internal resolver configured without
those fallbacks - we'll drop the fallbacks completely once the
default bridge has an internal resolver).
Signed-off-by: Rob Murray <rob.murray@docker.com>