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From Jens Schleusener <>
Subject [users@httpd] Apache reverse proxy in front of a Windows NLB cluster
Date Mon, 04 Sep 2006 14:59:44 GMT

we try to implement an Apache Linux server as a reverse proxy for 
different kinds of backend web servers to achieve a homogeneous web space. 
One of the backend servers is an Microsoft IIS server with an NLB (Network 
Load Balancing) Cluster of two systems. On that IIS server runs a CMS 
system that requires Load Balancing Affinity. That all works well but the 
problem is that the NLB service sees always the same IP address (that of 
the Apache reverse proxy) so only one of the two real servers got the 

So I tried to assign the hostname of the reverse proxy two IP-addresses 
and assign to the only single physical network-interface that two 
IP-adresses. In a first test I set up two IP-based virtual servers hoping 
that round robin DNS resolution would lead to a nearly equally workload of 
the so realized two reverse proxy servers. That seems to work well in the 
first simple tests but unfortunately the outgoing requests to the backend 
servers (via a "RewriteRule" using the "P" flag) always contain the 
IP-address of the "main" (the first assigned) IP-adress and not the two 
required IP-adresses to make the NLB probably "happier".

So another very raw idea of using (instead of the non-perfect DNS round 
robin approach) two further internal reverse proxy servers with different 
IP addresses and splitting the traffic conditionally via a "SetEnvIf 
Remote_Addr ..." directive seems superfluous.

Any ideas to handle this situation (second network-interface, iptables, 
additional server hardware or ...) ?

I hope my question isn't too off-topic.



Dr. Jens Schleusener            T-Systems Solutions for Research GmbH
Tel: +49 551 709-2493           Bunsenstr.10
Fax: +49 551 709-2169           D-37073 Goettingen

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