NO-1XX-HTTP10

What it sends

An HTTP/1.0 POST with Expect: 100-continue and a body, designed to test whether the server incorrectly sends a 100 Continue interim response to an HTTP/1.0 client.

POST / HTTP/1.0\r\n
Host: localhost:8080\r\n
Expect: 100-continue\r\n
Content-Length: 5\r\n
\r\n
hello

What the RFC says

“Since HTTP/1.0 did not define any 1xx status codes, a server MUST NOT send a 1xx response to an HTTP/1.0 client.” – RFC 9110 Section 15.2

Why it matters

HTTP/1.0 clients do not understand interim responses. If a server sends 100 Continue to an HTTP/1.0 client, the client may interpret the 100 status line as a malformed final response, discard it as garbage, or enter an undefined state. This is especially dangerous in proxy chains where an HTTP/1.0 hop cannot forward 1xx responses correctly.

Sources

• RFC 9110 §15.2 – Informational 1xx