What are the ways to manage a DDoS attack, preferably using open source? Don't say Cloudflare because they're an extortionist firm.
Even if you go all out and buy a bunch of huge IP transit links, you are not gonna be able to stop the IXP 800 miles away from getting congested and blocking your customers from accessing your site anyways. You need access to a backbone to route traffic differently to avoid those kinds of issues, which is why DDoS scrubbing services will partner with a T1 ISP to do most of the work.