Host Header

The Host header is the only header where RFC 9112 explicitly mandates a 400 response for violations. This makes the Host header tests the strictest in the entire suite — close or timeout is NOT acceptable.

The Rule

"A server MUST respond with a 400 (Bad Request) status code to any HTTP/1.1 request message that lacks a Host header field and to any request message that contains more than one Host header field line or a Host header field with an invalid field value." — RFC 9112 Section 3.2

This single sentence covers three violations:

Missing Host header
More than one Host header line (duplicate)
Host header with an invalid field value

Tests

MISSING-HOST

No Host header present. MUST respond with 400.

DUPLICATE-HOST

Two Host headers with different values. MUST respond with 400.

DUPLICATE-HOST-SAME

Two Host headers with identical values. MUST respond with 400.

HOST-WITH-USERINFO

Host header with userinfo (user@host). Invalid field value.

HOST-WITH-PATH

Host header with path component. Invalid field value.