How to log when invalid response received

You can log when LTM parses the response headers from the server in the HTTP_RESPONSE event. But I'm not sure that's going to help here as there isn't a way to log the response headers sent to the client to compare the two. To be honest though, I wouldn't expect this response to break the response sending. Maybe it's an issue with response redirect rewriting on the HTTP profile? I'd think a tcpdump on the two VLANs would be your best bet to see what's happening. You could also check the /var/log/ltm log file for any related errors.

There are a few logging iRules in the Codeshare:

http://devcentral.f5.com/Wiki/default.aspx/iRules.CodeShare

Here's one which specifically logs the clientside request and serverside response headers.

http://devcentral.f5.com/wiki/default.aspx/iRules/LogHttpHeaders.html

To run a tcpdump on both VLANs you can use:

tcpdump -i 0.0 -s 0 -w /var/tmp/sap.dmp host CLIENT_IP or host NODE_IP

Is the reference to main.htm in the root or in the same path as the request was made to (/whatever/main.htm)? You could rewrite the redirect Location header value using an iRule:

 
  when HTTP_REQUEST {  
  
      This event is only necessary if you need to log the requested path  
     set path [HTTP::path]  
  }  
  when HTTP_RESPONSE {  
  
      Check if response is a redirect  
     if {[HTTP::is_redirect]}{  
  
         Check if Location header value doesn't start with http  
        if {not ([string match -nocase "http*" [HTTP::header Location]])}{  
  
            Check if Location header value doesn't start with /  
           if {not ([string match -nocase "/*" [HTTP::header Location]])}{  
  
               Rewrite Location header value by prepending a forward slash  
              HTTP::header replace Location "/[HTTP::header Location]"  
  
               Or you could save the HTTP::path value to $path in HTTP_REQUEST and then use this  
                 to prepend the requested path to the redirect Location  
              HTTP::header replace Location [string range $path 0 [string last "/" $path]]  
           }  
        }  
     }  
  }  
   

Aaron