title: The proxy server received an invalid response from an upstream server
body: <p>We have an application deployed on tomcat.
  To access application we are using apache which sends requests to tomcat.
  At random browser gives following response.</p>
  
  <pre><code>Proxy Error
  The proxy server received an invalid response from an upstream server.
  The proxy server could not handle the request GET /sampleapp/&lt;http://samplehost.com:8080/sampleapp/&gt;.
  Reason: Error reading from remote server
  ________________________________
  Apache/2.2.21 (Unix) Server at samplehost.com Port 8080
  </code></pre>
  
  <p>This error is not coming always and so far we are getting this in a client machine with IE only. On the same client machine with Firefox it works well.</p>
  
  <p>Following is the error trace from Apache logs for a failed request</p>
  
  <pre><code>[Thu Mar 22 02:51:08 2012] [error] [client 10.36.64.137] (20014)Internal error: proxy: error reading status line from remote server localhost:8081, referer: http://samplehost.com:8080/sampleapp/opp/showSearchHome.htm
  [Thu Mar 22 02:51:08 2012] [error] [client 10.36.64.137] proxy: Error reading from remote server returned by /sampleapp/opp/searchResult.htm, referer: http://samplehost.com:8080/sampleapp/opp/showSearchHome.htm
  </code></pre>
  
  <p>Nothing is getting printed on tomcat logs when a request is failing.</p>
  
  <p>Following is the information related installed version of apache</p>
  
  <pre><code>-bash-3.00$ ./httpd -v
  Server version: Apache/2.2.21 (Unix)
  Server built:   Sep 21 2011 20:00:58
  </code></pre>
  
  <p>When trying to find out modules loaded I get following error</p>
  
  <pre><code>-bash-3.00$ /usr/local/apache2/bin/apachectl -t -D DUMP_MODULES
  httpd: Syntax error on line 64 of /usr/local/apache2/conf/httpd.conf: Cannot load /usr/local/apache2/modules/mod_authnz_ldap.so into server: ld.so.1: httpd: fatal: libldap-2.4.so.2: open failed: No such file or directory
  </code></pre>