Network environment:

Https Client<=============>Proxy Server<==============>Https Server
                                                  192.168.17.11<-----extranet------>192.168.17.22
10.100.21.10<----intranet----->10.100.21.11

ps: Http Client without default gateway, but it can ping to 10.100.21.11

Description:

OS: Ubuntu 12.04 on 3 hosts
Https Client: Implement with java(openjdk-6).Have one network-interface.
Proxy Server: Apache2.2.Have two network-interfaces.
Https Server: Tomcat6.Have one network-interface.

I use two method to implement httpsurlconnection through proxy:
(For facilitate I do not write down about ssl handle function for checking serverTrusted and hostnameVerifier issue.If need I will update.)

1.Proxy class

InetSocketAddress proxyInet = new InetSocketAddress("10.100.21.11",80);
Proxy proxy = new Proxy(Proxy.Type.HTTP, proxyInet);
URL httpsUrl = new URL("https://192.168.17.22:8443/test");
HttpsURLConnection httpsCon = (HttpsURLConnection) httpsUrl.openConnection(proxy);

httpsCon.setDoOutput(true);
httpsCon.setDoInput(true);
httpsCon.setRequestMethod("POST");
OutputStream out = httpsCon.getOutputStream();
OutputStreamWriter owriter = new OutputStreamWriter(out);

owriter.write("<request>test</request>");
owriter.flush();
owriter.close();
...

This method workable and I observed packets flow also met my expectation.
HttpClient ---> ProxyServer ---> HttpServer

But when I use set Pro