因为历史兼容性的原因,根据http1.0/1.1规范,代理必须从absolute uri中提取host,而不是从host head field中提取。

https://tools.ietf.org/html/rfc2616#section-5.1.2
The absoluteURI form is REQUIRED when the request is being made to a proxy.
...
Note that the proxy MAY forward the request on to another proxy or directly to the server specified by the absoluteURI.

section-5.2

1. If Request-URI is an absoluteURI, the host is part of the
Request-URI. Any Host header field value in the request MUST be
ignored.