Does reqwest add the host header on its own when doing a request?

[AlexanderProd]

I'm doing a post request like this:

let client = reqwest::Client::new();
    let mut headers = HeaderMap::new();
    headers.insert("x-ms-date", HeaderValue::from_str(&date)?);
    headers.insert(
        "x-ms-content-sha256",
        HeaderValue::from_str(&content_hash_base64)?,
    );
    headers.insert(CONTENT_TYPE, HeaderValue::from_static("application/json"));
    headers.insert(
        HOST,
        HeaderValue::from_str(host)?,
    ); 
    headers.insert(
        AUTHORIZATION,
        HeaderValue::from_str(&format!(
            "HMAC-SHA256 SignedHeaders=x-ms-date;host;x-ms-content-sha256&Signature={}",
            signature
        ))?,
    );

When I include the host header like this I get an 400 error saying that may request is invalid. When I omit the Host header the request works fine.

The host header which I set is the same as in the endpoint which I call.

So I'm wondering if reqwest adds the header on its own down the line and therefore there a two headers present which would cause the 400 error.

[seanmonstar]

Yes and no. It does automatically add the host header for HTTP/1 requests, if it the header is missing. If it's there, nothing is added.

But, you might be sending HTTP/2 requests, and while the host header isn't illegal, some servers don't like it.