Yesterday I experienced something interesting while setting up a Wi-Fi access point. The access point was connected to the Internet via a local modem (gateway), which had no DNS server address configured. The clients (Windows notebooks, Android phones, iPhones) successfully connected to the access point, and got their network configuration via DHCP, including the DNS server, which was the IP address of the local internet gateway (the modem). But no DNS servers were configured on the modem, so you couldn't surf the web as usual (only by typing in the IP address of a website so no DNS was necessary).
Only if I manually configured the DNS on a Windows machine in the wireless adapter settings, "normal" surfing was working again. Also Android clients couldn't access the Internet (I didn't try to configure DNS manually on a client).
But the iPhone surprised me (running the latest iOS): It could load any web page without troubles, albeit the DNS server (the local gateway) served via DHCP was not working, as there no DNS server was configured.
So my question is: does the iPhone (or more precisely: iOS) have an additional, internal, backup DNS server address configured, which it uses if it gets no working DNS server address via DHCP?