Which HTTP header is most commonly used to indicate a request passed through a proxy server?

• A. Via
• B. Content-Type
• C. Authorization
• D. Referer

Answer: (B)

The key idea is how HTTP signals proxies in the request path. The Via header is specifically designed to indicate that a request (or response) has passed through one or more proxy servers. It records the protocol version and the proxy identifiers along the path, so you can trace the hop-by-hop traversal of the traffic. That’s why you’d see entries like “Via: 1.0 proxy.example.com, 1.1 cache.example.org,” which directly shows proxy involvement.

Content-Type, in contrast, tells the recipient what kind of data the payload contains (for example, text/html or application/json). It has no role in signaling that a proxy was involved in delivering the request, so it’s not the indicator you’d use to determine proxy traversal.

There are other headers like X-Forwarded-For that can carry the original client’s IP address when a proxy forwards the request, but the header that best indicates that a request passed through a proxy is the Via header.