OS X adds "incorrect" DNS servers "on top" of correct ones OS X (latest version of High Sierra) is doing something strange when I try to connect to my campus wireless network.

On a regular basis, it is adding an incorrect DNS server (10.0.0.255) "on top" of the list of DNS servers that actually is appropriate for my campus network. I have to go in and manually delete the incorrect server, which appears by itself; when I do so, the full correct list shows up and everything returns to normal.

How do I prevent OS X from doing this automatically?
Firstly, I'm going to assume that your wireless IP address is provided by DHCP (Domain Host Configuration Protocol), in addition to DNS settings from the same DHCP server. 

It’s not clear how  10.0.0.255 is not a valid and correct DNS as opposed to the DNS handling many requests and responding slowly.

If the DNS of 10.0.0.255 really does not work, you may want to check with the campus wireless providers. For all you know; you could be connecting to someone pretending to be the campus wireless; and also failed to setup their fake AP correctly.

You could also just manually enter the one correct DNS in your settings which will remove the second response and wait until the network gets fixed.