Apache - websocket握手期间出错，意外响应代码：502

Question

我有以下Apache的设置，但它失败并出现以下错误：

与'wss：//mydoain.com/socket.io/1/websocket/zpxKUhj9zN3MsTdXIkrL'的WebSocket连接失败：WebSocket握手期间出错：意外响应代码：502

#Server version: Apache/2.2.15 (Unix)
#Server built:   Jul 23 2014 14:17:29

NameVirtualHost *:443
<VirtualHost *:443>
  Header set Access-Control-Allow-Origin: "https://from.mydomain.com"
  SSLEngine on
  SSLProxyEngine On
  ProxyPreserveHost On

  SSLProtocol all -SSLv2
  SSLCipherSuite ALL:!ADH:!EXPORT:!SSLv2:RC4+RSA:+HIGH:+MEDIUM:+LOW
  SSLCertificateFile /etc/key.crt
  SSLCertificateKeyFile /etc/key.key
  SSLCertificateChainFile /etc/key.ca

  ServerName to.mydomain.com
  ServerAlias to.mydomain.com
  ProxyPass / http://localhost:8888/
  ProxyPassReverse / http://localhost:8888/
  #ProxyPassReverseCookiePath /MYSITE/ /
        CacheDisable *
</VirtualHost>

注意：http://localhost:8888的服务器是基于websocket的信令服务器