error after adding new irule

First, the error should only be manifesting when the client source doesn't match your whitelist data group, where you subsequently send a 503. In the order that you have your iRules applied to the VIP (assuming new one on top), you're potentially sending a 503 response (an egress mechanism) BEFORE attempting to add a header to the ingress stream. You should be able to solve this two ways:

1. Re-order the iRules in your VIP configuration, or

2. Apply priority tags to the HTTP_REQUEST events to force a specific order, like this:

   when HTTP_REQUEST priority 50 { 
       if { [HTTP::header exists X-Forwarded-For] } { 
           HTTP::header replace X-Forwarded-For [substr [IP::client_addr] 0 "%"] 
       } else { 
           HTTP::header insert X-Forwarded-For [substr [IP::client_addr] 0 "%"] 
       } 
   }   
   
   when HTTP_REQUEST priority 100 { 
       if { ( [class match [IP::client_addr] equals whitelist] ) } { 
           pool Pool_1 
       } else { 
           HTTP::respond 503 content [b64decode [class element -name 0 DG_MAINTENANCE_HTML]] "Content-Type" "text/html" 
       } 
   }
   

thanks for reply Kevin I have reordered the irules but we are still recieving errors . Mon Jan 6 09:48:44 GMT 2014 err local/tmm3 tmm3[6222] 01220001 TCL error: iR_XFF_RewriteOrInsert - Operation not supported (line 4) invoked from within "HTTP::header insert X-Forwarded-For [substr [IP::client_addr] 0 "%"]"

would you mind changing the existing irule to something like this?

root@(ve11a)(cfg-sync In Sync)(Active)(/Common)(tmos) list ltm rule xff
ltm rule xff {
    when HTTP_REQUEST_SEND {
  clientside {
    HTTP::header remove X-Forwarded-For
    HTTP::header insert X-Forwarded-For [substr [IP::client_addr] 0 "%"]
  }
}
}

you could try "event disable all" behind the HTTP::respond

HTTP::respond 503 content [b64decode [class element -name 0 DG_MAINTENANCE_HTML]] "Content-Type" "text/html" 
event disable all

its resolved with chsnging the sequence of the irules I was not doing it on all the VS earlier . Apology for time and thanks for help