How are proxies searched?

Question

proxy

I am writing a small parser for proxy sites.
And the following question arose:
How do these sites collect working proxy addresses? Do they scan all IPs and all ports?

Answer 1

That's right, they scan all addresses and ports (sometimes only some ports).
I wrote my proxy, requests from such auto-search engines came in darkness. Usually this is something like GET mysite.ru/proxytest.txt HTTP/1.0
, that is, they try to get their file through me and look at the result. But then spam is usually sent through my proxy :( CONNECT requests to the SMTP port and sending letters. I had to attach an authorization mechanism