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chris_wood's avatar
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Jun 26, 2023

Big-IP LTM - load balancing incoming request to 2 different 'paths' on Member Pool


A strange request has come in :).  I donlt think it's possible but thought I'd reach out just encase some has an answer.

We have a VS setup that load balances to 2 different servers.

In this example we need the incoming request to load balance to a different path on each server, e.g.

incoming request > VS

VS re-writes/redirects the URL to:

Load balance to server 1 path   (https://example1/folder/folder/server1

Load balance to server 2 path  (https://example1/folder/folder/server2

Many thanks in advance!


13 Replies

  • Ok chris_wood , 

    This Irule Code should meet your need : 


    when LB_SELECTED {
    if { [IP::addr [LB::server addr] equals x.x.x.x] } { 
    log local0. " Server 1 selected , Change path to ( http://example/folder/folder/server1 ) "
    HTTP::uri "http://[HTTP::host]/folder/folder/server1"
    } elseif { [IP::addr [LB::server addr] equals x.x.x.x] } {
    log local0. " Server 2 selected , Change path to ( http://example/folder/folder/server2 ) "
    HTTP::uri "http://[HTTP::host]/folder/folder/server2"
    #Replace "x.x.x.x" Ips by your servers/poolmembers IPs. 
    #First "x.x.x.x" server1 and second "x.x.x.x" server2


    >> I have tested it and it works as expected , after capturing the traffic in both sides , I observerd the chaning in URI paths according to selected pool member inside the pool. 

    << Why I used this method >> ? 
    Because you need to use round Robin loadbalance technique and modify the request to servers at the same time. 

    try it and let me know. 
    Goodluck 🙂 

    • CA_Valli's avatar
      Icon for MVP rankMVP

      This works, I was wondering if there was a way to make it dynamic so it was scalable ..
      I've looked into [LB::server] instruction and tested it, but it only returns server IP and pool name. 

      chris_wood if you have control on the server environment and it's possible to edit the URLs, you might want to set it up this way:
      - assign any IP address you want
      - if possible, try configuring server hostname to match the last octect ex. apache30 for IP or apache12 for 
      - if possible, configure server URL to match the hostname, ex. /path/to/apache15 
      This way, iRule might support service scalability as server farm grows, 



      when LB_SELECTED {
        set selected [getfield [LB::server addr] "." 4]
        set server "apache$selected"
      #  log local0. "server $server"
        set path "/path/to/$server" ; set host "$server.local.domain"
        HTTP::host $host 
        HTTP::path $path



      This will rewrite  http(s)://<virtual server url>   to    http(s)://apacheXX.local.domain/path/to/apacheXX for any given XX , without redirects (client transparent) 

    • chris_wood's avatar
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      Thank you! I will test this afternoon and let you know!

    • chris_wood's avatar
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      I tried your script - and looks good in the F5 logs - however the server logs aren't showing showing any connections.   

      Tried on both http/https - with the http & http-transparent profile.  I'm thinking along the lines of SSL/TLS getting in the way - might need to add 

      Removing the http profile and only using 1 server in the pool worked as expected.

      • Hi chris_wood , 

        While assinging my iRule script in your virtual server, did you assign the correct pool in Virtual server resources ? 

        you sould assign a pool has 2 pool members ( and only replace first "x.x.x.x" by first pool member IP and do the same with the second pool member ip ) 

        Please Take this Packet capture to see the server and client side connections , and see the effect from F5 Bigip 

        tcpdump -nnni 0.0:nnnp -vvv -w /var/tmp/path_test.pcap host x.x.x.x
        #put Client_ip address instead of "x.x.x.x"

         I will be waiting your response 

    • chris_wood's avatar
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      Hi Mohamed,

      Thanks for your reply.

      Basically, we want to load balance (round robin for e.g) to 2 servers in the member pool - but before passing it on to the servers, we need the Virtual Server to re-write the URL path to a different specific folder on each server:

      The reading I've done so far suggests when we use the HTTP::redirect iRule, the F5 replies directly to the client and doesn't load balance (even then sending to 2 different locations is a big ask). 

      In the mean time, we are looking into completing this task using re-directs on the individual servers, as I think that makes more sense.

  • It's not optimal, but technically it should be possible.
    If you want to have it work dinamically, I'd suggest to configure the objects with a specific naming convention , for example referencing the path. 

    I haven't tested this code, but you could try something like this: 

    when LB_SELECTED {
      set path "/folder/folder/[LB::server name]"
      HTTP::path "$path"