feat(parser): reject missing/duplicate/invalid Host (RFC 9112 §3.2)

parse_head never inspected Host, so a missing (HTTP/1.1), duplicate, or
syntactically invalid Host all passed through to the router (h1spec #8/#9/
#10 -> 404 instead of 400). Add per-header validity (RFC 3986 host[:port]
charset via valid_host) plus a post-loop presence/uniqueness check: 1.1
MUST carry exactly one valid Host; 1.0 may omit it but a duplicate/invalid
one is still 400. Unit matrix mirrors the three h1spec cases with reg-name/
port/IPv6-literal positive controls.

src/parser.rs

@@ -100,6 +100,8 @@ pub fn parse_head(buf: &[u8], max_headers: usize) -> Result<ParsedHead, ParseErr
     let mut connection_hdr = None;
     let mut chunked        = false;
     let mut expect_100     = false;
+    let mut host_count     = 0usize;
+    let mut host_ok        = true;
 
     for h in req.headers.iter() {
         let name_lower = h.name.to_ascii_lowercase();
@@ -126,11 +128,29 @@ pub fn parse_head(buf: &[u8], max_headers: usize) -> Result<ParsedHead, ParseErr
             "expect" if value.eq_ignore_ascii_case("100-continue") => {
                 expect_100 = true;
             }
+            "host" => {
+                // Presence/uniqueness enforced post-loop; validity here.
+                host_count += 1;
+                if !valid_host(value) {
+                    host_ok = false;
+                }
+            }
             _ => {}
         }
         headers.append(&name_lower, value.to_string());
     }
 
+    // Host (RFC 9112 §3.2): an HTTP/1.1 request MUST carry exactly one valid
+    // Host; a missing, duplicate, or malformed Host is a 400. HTTP/1.0 may
+    // omit Host, but a duplicate or invalid one is still rejected on any
+    // version (ambiguous / malformed authority).
+    if host_count > 1 || !host_ok {
+        return Err(ParseError::Malformed);
+    }
+    if version == HttpVersion::Http11 && host_count == 0 {
+        return Err(ParseError::Malformed);
+    }
+
     if chunked {
         // Transfer-Encoding is an HTTP/1.1 mechanism; a 1.0 request
         // carrying it is malformed. And a request carrying BOTH a
@@ -163,6 +183,32 @@ pub fn parse_head(buf: &[u8], max_headers: usize) -> Result<ParsedHead, ParseErr
     })
 }
 
+/// Conservative RFC 3986 check for a `Host` field-value: non-empty and every
+/// byte drawn from the `host[:port]` productions (reg-name / IP-literal
+/// brackets / port colon). This is charset-level, not full structural
+/// validation (no bracket matching, no pct-encoding well-formedness) — enough
+/// to reject the smuggling-relevant garbage (whitespace, controls, `@`, `/`,
+/// `?`, `#`) while accepting every legitimate host. Tighter structural checks
+/// (bracketed IPv6, single port colon) are a possible follow-up.
+fn valid_host(value: &str) -> bool {
+    !value.is_empty()
+        && value.bytes().all(|b| {
+            b.is_ascii_alphanumeric()
+                || matches!(
+                    b,
+                    // unreserved punctuation
+                    b'-' | b'.' | b'_' | b'~'
+                    // sub-delims
+                    | b'!' | b'$' | b'&' | b'\'' | b'(' | b')'
+                    | b'*' | b'+' | b',' | b';' | b'='
+                    // pct-encoded lead
+                    | b'%'
+                    // IP-literal brackets + port separator
+                    | b'[' | b']' | b':'
+                )
+        })
+}
+
 // ---------------------------------------------------------------------------
 // Conn assembly
 // ---------------------------------------------------------------------------
@@ -403,6 +449,55 @@ mod tests {
         }
     }
 
+    // --- Host (RFC 9112 §3.2) -------------------------------------------
+
+    #[test]
+    fn parse_missing_host_http11_is_malformed() {
+        let req = b"GET / HTTP/1.1\r\n\r\n";
+        match parse_head(req, 64) {
+            Err(ParseError::Malformed) => {}
+            _ => panic!("expected Malformed for missing Host on 1.1"),
+        }
+    }
+
+    #[test]
+    fn parse_missing_host_http10_is_allowed() {
+        // Host is optional in HTTP/1.0.
+        let req = b"GET / HTTP/1.0\r\n\r\n";
+        assert!(parse_head(req, 64).is_ok(), "1.0 may omit Host");
+    }
+
+    #[test]
+    fn parse_duplicate_host_is_malformed() {
+        let req = b"GET / HTTP/1.1\r\nHost: a\r\nHost: b\r\n\r\n";
+        match parse_head(req, 64) {
+            Err(ParseError::Malformed) => {}
+            _ => panic!("expected Malformed for duplicate Host"),
+        }
+    }
+
+    #[test]
+    fn parse_invalid_host_value_is_malformed() {
+        // Embedded whitespace — invalid in an RFC 3986 authority.
+        let req = b"GET / HTTP/1.1\r\nHost: bad host\r\n\r\n";
+        match parse_head(req, 64) {
+            Err(ParseError::Malformed) => {}
+            _ => panic!("expected Malformed for invalid Host"),
+        }
+    }
+
+    #[test]
+    fn parse_valid_hosts_accepted() {
+        // Positive controls: reg-name, reg-name:port, and IPv6-literal:port.
+        for req in [
+            b"GET / HTTP/1.1\r\nHost: example.com\r\n\r\n".as_slice(),
+            b"GET / HTTP/1.1\r\nHost: example.com:8080\r\n\r\n".as_slice(),
+            b"GET / HTTP/1.1\r\nHost: [::1]:443\r\n\r\n".as_slice(),
+        ] {
+            assert!(parse_head(req, 64).is_ok(), "should accept a valid Host");
+        }
+    }
+
     #[test]
     fn serialise_basic_200() {
         let conn = Conn::new().put_status(200).put_body("hi");