A successful TCP connection and a working HTTPS handshake don't necessarily mean a proxy will work with a particular destination. A 403 could involve IP reputation, ASN-level filtering, geolocation inconsistencies, TLS or HTTP fingerprint differences, session state, or destination-specific access policies. The challenge is separating proxy-level failures from application-layer restrictions. I'm curious how experienced engineers approach this diagnostically. Do you start by comparing multiple IPs within the same subnet, testing across different ASNs, examining response headers, or checking whether the failure follows the session rather than the IP? I'd be interested in hearing about methods that help isolate the actual cause without jumping to conclusions.   submitted by   /u/One_Television_6504 [link]   [comments]