MacOS – No DNS resolution if custom configured DNS server is not reachable – how to fallback to default DNS

To get the settings:

networksetup -getdnsservers Airport

To change them:

networksetup -setdnsservers Airport xx.xx.xx.xx

These settings are non-persistent, so you could clear your manual settings, let the public wi-fi DNS do its thing, then use the -setdnsservers to set it to what you want until next time.

Yep. You can do this, /etc/resolver is your friend.

$ man 5 resolver:

domain
Domain name associated with this resolver configuration. This option is normally not required by the Mac OS X DNS search system when the resolver configuration is read from a file in the /etc/resolver directory. In that case the file name is used as the domain name.

Simply put you can place nameserver 127.0.0.1 inside a file you create called /etc/resolver/dev

You'll need to reload/restart. It'll then direct any requests to *.dev to your local nameserver.

Your network connection DNS remains on automatic, so it will cope with any changes in your environment.

Edit: I've just found https://serverfault.com/a/164215/163311 and http://www.echoditto.com/blog/never-touch-your-local-etchosts-file-os-x-again which have a few more details and include dnsmasq setup details.