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The internal resolver now uses any namesever found in the host's /etc/resolv.conf as an external nameserver, and it's accessed from the host's network namespace. Before this change, when no external nameservers were found (so the host had no entries in /etc/resolv.conf) Google's DNS servers were used as fallbacks, always accessed from the container's network namespace. If a container's initial set of endpoints had IPv6 enabled, the IPv6 nameservers were included. Now we have IPv6-only networks, a similar exception would be needed for Google's IPv4 nameservers... don't include them if there are no IPv4 endpoints. However, only the initial set of endpoints was considered. As networks are connected/disconnected, IPv4 or IPv6 connectivity may be lost. Unlike nameservers read from the host's /etc/resolv.conf, there is no way to tell which fallback nameservers (v4/v6) might work from the host's namespace. So, using the host's namespace isn't a good solution. Since we want to get away from using fallback nameservers anyway, this change removes them. If a host has no /etc/resolv.conf entries, but a container does need to use DNS, it'll need to be configured with servers via '--dns'. Signed-off-by: Rob Murray <rob.murray@docker.com>