http_proxy: print proxy errors in response body

[Choraden]

Responses will now contain proxy name and error:
First line is a generic message, second the error.

curl -x localhost:3128 not.exist   
forwarder failed to connect to remote host
dial tcp: lookup not.exist on 192.168.1.1:53: no such host

Header:

X-Forwarder-Error: forwarder dial tcp: lookup www on 10.255.3.12:53: no such host

[mmatczuk]

Can it be like

forwarder failed to connect to remote host
dial tcp: lookup not.exist on 192.168.1.1:53: no such host

[mmatczuk]

Also, check the response headers, the proxy name must be both in headers and body.

[mmatczuk]

How about we fixed the messages so that they do not start with upper case?

[Choraden]

ok

[mmatczuk]

let's add some text here, "encountered an unexpected internal error: "

[mmatczuk]

So do we need strings.ToLower here?

[mmatczuk]

This does not play well with config.Name

[mmatczuk]

Nah, just skip it if MITM is enabled.

[Choraden]

The issue is that our current httpexpect implementation also does the same trick, so all https requests made by the client do not return an original response in case of a connect error.

func (c *Client) do(req *http.Request) (*http.Response, error) {
	if c.trace != nil {
		req = req.WithContext(httptrace.WithClientTrace(req.Context(), c.trace))
	}
	resp, err := c.rt.RoundTrip(req)

	// There is a difference between sending HTTP and HTTPS requests.
	// For HTTPS client issues a CONNECT request to the proxy and then sends the original request.
	// In case the proxy responds with status code 4XX or 5XX to the CONNECT request, the client interprets it as URL error.
	//
	// This is to cover this case.
	if req.URL.Scheme == "https" && err != nil {
		for i := 400; i < 600; i++ {
			if err.Error() == http.StatusText(i) {
				return &http.Response{
					StatusCode: i,
					Status:     http.StatusText(i),
					ProtoMajor: 1,
					ProtoMinor: 1,
					Header:     http.Header{},
					Body:       http.NoBody,
					Request:    req,
				}, nil
			}
		}
	}

	return resp, err
}

Solution 1. is to keep this test and later address it in both proxy http client and our http expect client.

Solution 2. is to limit the scope and make it a dedicated MITM flag test.

[Choraden]

I actually wanted to print a warning there, but I'm afraid, it won't be visible in a non-debug mode. So there we have this "half skip" - it always checks the status code.

[mmatczuk]

Add text checking only for http requests.

[Choraden]

ok