copy
fix get valid ifaces in nameservers_bsd
nameservers on MacOS can be found in resolv.conf reliably
nameservers on MacOS can be found in resolv.conf reliably
exclude local IPs from MacOS resolve conf check
use scutil for MacOS, simplify reinit logic to prevent duplicate calls
add more dns server fetching options
never skip OS resolver in IsDown check
split dsb and darwin nameserver methods, add delay for setting DNS on interface on network change.
increase delay to 5s but only on MacOS
Without verbose log, we use internal log writer with log level set to
debug. However, this will affect other writers, like console log, since
they are default to notice level.
By adopting FilteredLevelWriter, we can make internal log writer run in
debug level, but all others will run in default level instead.
