Malformed reply from SOCKS server whereas I use a HTTP proxy (with Apache HTTP library)

Question

I know that there are a lot of questions regarding the error Malformed reply from SOCKS server which mostly point to a wrong configuration for the proxy.

However, in my case, I'm using the system HTTP(!) proxy for a POST request with the apache httpclient library 4.3.5 (httpcore 4.3.2) like this:

SystemDefaultRoutePlanner routePlanner = new SystemDefaultRoutePlanner(
        ProxySelector.getDefault());
CloseableHttpClient httpclient = HttpClients.custom()
        .setRoutePlanner(routePlanner)
        .build();

This is what has also been advised in https://mcmap.net/q/678315/-how-do-i-replace-deprecated-httpclient-getparams-with-requestconfig

In most situations this httpclient and the following http POST request work very well. At one customer however, it fails with the following log error:

Mrz 05, 2015 10:11:04 AM org.apache.http.impl.execchain.RetryExec execute
Information: I/O exception (java.net.SocketException) caught when processing request to {}->http://proxy.local:80->http://my-webservice.tld:80: Malformed reply from SOCKS server
Mrz 05, 2015 10:11:04 AM org.apache.http.impl.execchain.RetryExec execute
Information: Retrying request to {}->http://proxy.local:80->http://my-webservice.tld:80

The local system proxy settings (on Windows 7) aren't configured as a SOCKS proxy, but as an HTTP proxy! I confirmed this by logging different RoutePlanner and Proxy parameters:

TunnelType: PLAIN
TargetHost: http://my-webservice.tld:80
ProxyHost: http://proxy.local:80
ProxyPort: 80
ProxyType: HTTP

However, my POST request isn't beeing sent correctly. In the proxy logfiles it appears as follows:

2015-03-05 10:11:04 119973 192.168.124.111 0 TCP_ERR_MISS 0 4 unknown - - / - - - - - - PROXIED - - xxx.xxx.xxx.xxx SG-HTTP-Service

This is my uploadFile() method which uses the mentioned SystemDefaultRoutePlanner and creates and executes the HttpPost:

private String uploadFile(File fileToUpload) throws Exception {

    SystemDefaultRoutePlanner routePlanner = new SystemDefaultRoutePlanner(ProxySelector.getDefault());
    CloseableHttpClient httpclient = HttpClients.custom()
            .setRoutePlanner(routePlanner)
            .build();

    try {
        HttpPost httppost = new HttpPost(webserviceURL);

        MultipartEntityBuilder requestEntity = MultipartEntityBuilder.create();

        requestEntity.addPart("project", new StringBody(paramProjekt, ContentType.TEXT_PLAIN));
        requestEntity.addPart("param1", new StringBody(param1, ContentType.TEXT_PLAIN));
        requestEntity.addPart("param2", new StringBody(param2, ContentType.TEXT_PLAIN));
        requestEntity.addPart("debug", new StringBody(paramDebug, ContentType.TEXT_PLAIN));
        requestEntity.addPart("xmlFile", new FileBody(fileToUpload));

        ProgressHttpEntityWrapper.ProgressCallback progressCallback = new ProgressHttpEntityWrapper.ProgressCallback() {

            @Override
            public void progress(float progress) {
                int min = 5;
                int max = 40;
                int diff = max - min;
                gui.updateProgress(min + (int)(diff * progress / 100));

                if (progress == 100) {
                    gui.setStep(GUI.STEP.PROCESSING);
                }
            }
        };

        httppost.setEntity(new ProgressHttpEntityWrapper(requestEntity.build(), progressCallback));

        logger.info("executing request " + httppost.getRequestLine());
        CloseableHttpResponse response = httpclient.execute(httppost);

        try {
            logger.info("Response Status: " + response.getStatusLine());
            HttpEntity resEntity = response.getEntity();

            if (resEntity != null) {
                String responseString = EntityUtils.toString(resEntity);
                return responseString;
            }

            EntityUtils.consume(resEntity);
        } catch (Exception e) {
            e.printStackTrace();
            return null;
        } finally {
            response.close();
        }

    } finally {
        httpclient.close();
    }

    return null;
}

The last thing in my logfile is the executing request log info but the Response Status: log information never shows up...

Mrz 05, 2015 10:09:04 AM de.company.product.WebserviceClient.WebserviceClient uploadFile
Information: executing request POST http://my-webservice.tld HTTP/1.1
Mrz 05, 2015 10:11:04 AM org.apache.http.impl.execchain.RetryExec execute
Information: I/O exception (java.net.SocketException) caught when processing request to {}->http://proxy.local:80->http://my-webservice.tld:80: Malformed reply from SOCKS server
Mrz 05, 2015 10:11:04 AM org.apache.http.impl.execchain.RetryExec execute
Information: Retrying request to {}->http://proxy.local:80->http://my-webservice.tld:80

Does anyone has a clue why

• it is showing the Malformed reply from SOCKS server error when there is a HTTP proxy defined in the system and I use the system proxy settings?
• how to configure either the httpclient or the httppost in the right way to recognize system HTTP proxy settings and work well with and without HTTP proxy?

Adenine