iRule to a variable for chosen pool member

The LB_SELECTED event happens after the ACCESS_ACL_ALLOWED event, but then the HTTP_REQUEST_SEND event happens after all of that, so please give this a try:

when HTTP_REQUEST_SEND {
    if { [class match [LB::server addr] equals my_host_lb_dg] } {
        clientside {
            HTTP::header replace Host [class match -value [LB::server addr] equals my_host_lb_dg]
        }
    }
}

where "my_host_lb_dg" is an arbitrary string-based data group that maps the pool member IPs to corresponding host names. Example:

10.70.0.1 := foo.example.com
10.70.0.2 := bar.example.com
10.70.0.3 := test.example.com

i have more code in the when ACCESS_ACL_ALLOWED.. i am wondering i can move it all to the HTTP_REQUEST_SEND event..

what we have now

when ACCESS_ACL_ALLOWED {
   set user [ACCESS::session data get "session.logon.last.username"]
   set uri [string range [HTTP::uri] 3 end]
   HTTP::header replace "APP_USER" $user
   HTTP::header replace Host "server1.domain.com:8080"
   HTTP::uri $uri
}
when HTTP_RESPONSE {
if { [HTTP::header "Location"] contains "server1" } {
HTTP::header replace Location [string map {"server1.domain.com:8080" "app.example.com/app"} [HTTP::header value Location]] }

}

What i think we need.

when HTTP_REQUEST_SEND {
    if { [class match [LB::server addr] equals prod_servers] } {
        serverside {
            set user [ACCESS::session data get "session.logon.last.username"]
            set uri [string range [HTTP::uri] 3 end]
            HTTP::header replace "APP_USER" $user
            HTTP::header replace Host "[class match -value [LB::server addr] equals prod_servers]:8900"
            HTTP::uri $uri
        }
    }
}
    when HTTP_RESPONSE {
    if { [HTTP::header "Location"] starts_with "prodserver" } {
    HTTP::header replace Location [string map {"[class match -value [LB::server addr] equals prod_servers]:8080" "prod_app.example.com/app"} [HTTP::header value Location]] }
}

What do you think??

You should be guaranteed that the access session exists when you get to the HTTP_REQUEST_SEND event, so that should work.

Kevin, when using the HTTP_REQUEST_SEND, i get this error in the logs -

TCL error: /Common/irule-app-test - Illegal argument. Can't execute in the current context. (line 2) invoked from within "HTTP::uri"invalid serverside expression (line 3) invoked from within "serverside { set user [ACCESS::session data get "session.logon.last.username"] set uri [string range [HTTP::uri] 3 end] ..."

You're passing headers to the server in the client side context, so you need the clientside expression, not serverside.

i actually had to split the events to do the different things i needed.. i ended up with this-

when ACCESS_ACL_ALLOWED { 
    set user [ACCESS::session data get "session.logon.last.username"] 
    set uri [string range [HTTP::uri] 3 end] 
    HTTP::header replace "APP_USER" $user
        HTTP::uri $uri
    }
when HTTP_REQUEST_SEND {
    if { [class match [LB::server addr] equals server-dev] } {
        clientside {
            HTTP::header replace Host "[class match -value [LB::server addr] equals server-dev]:9000"
            }
        }
    }   
when HTTP_RESPONSE {
    if { [HTTP::header "Location"] starts_with "devserver" } {
    HTTP::header replace Location [string map {"[class match -value [LB::server addr] equals server-dev]:9000" "company.domain.com/app"} [HTTP::header value Location]] }
}

Thanks for your valuable input on this..