http cross origin request, server behavior

Question

What ensures origin header is set(by client side or hacker! )
What servers usually do if Origin header is not set up?
And how do they survive CORS requests?
2. Also tell me what the servers(usually) will do to escape the attack if a lot of requests are sent to the server(by hacker) ?

Answer 1

Jazz i think proxies that exist between client and server set up some headers. is it true?