os.IsNotExist doesn't unwrap errors. errors.Is does.
The ioutil.ReadFile ones happened to be fine but I changed them so
we're consistent with the rule: if the error comes from os, you can
use os.IsNotExist, but from any other package, use errors.Is.
(errors.Is always would also work, but not worth updating all the code)
The motivation here was that we were logging about failure to migrate
legacy relay node prefs file on startup, even though the code tried
to avoid that.
See golang/go#41122
Amazingly, there doesn't seem to be a documented way of updating network
configuration programmatically in a way that Windows takes notice of.
The naturopathic remedy for this is to invoke ipconfig /registerdns, which
does a variety of harmless things and also invokes the private API that
tells windows to notice new adapter settings. This makes our DNS config
changes stick within a few seconds of us setting them.
If we're invoking a shell command anyway, why futz with the registry at
all? Because netsh has no command for changing the DNS suffix list, and
its commands for setting resolvers requires parsing its output and
keeping track of which server is in what index. Amazingly, twiddling
the registry directly is the less painful option.
Fixes#853.
Signed-off-by: David Anderson <danderson@tailscale.com>
