F5 LTM VE – Configuring iRules

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iRules are perhaps one of the coolest features of the F5 devices.  They allow you to write rules that can manipulate the load load balancing decision.  The rules can evaluate variables that the F5 can detect.  In this entry, we are going to examine the user-agent string which is part of the HTTP header.  Based on the use-agent field, we can determine what type of browser the client is connecting with.  Looking at this information can also help us determine what kind of device the client is connection from.

First off, I wanted to add another pool to my LTM.  Rather than spin up new VMs for this purposes I told apache on each WebHost instance to listen on port 81 and direct requests incoming on that port to a different folder.  So I added this to my httpd.conf on each WebHost…

Listen 81
<VirtualHost *:81>
        ServerName localhost
        DocumentRoot /var/www/html/81

Then I put a modified copy of my index.html file in the 81 folder on each WebHost.  Then I created a new pool that looked like…


Now that that’s done we can write our iRule.  iRules are pretty easy to understand just by looking at them but on the other hand, rather picky about syntax, spacing, etc.  So lets take a look at the iRule we are going to use in this test.  I wanted to create a iRule that would send requests coming from a iPhone to the new HTTP_MOBILE_POOL (the port 81 group we just created) and send the rest to the default HTTP_POOL.  So I came up with…


So as I said earlier, this is pretty easy to read.  If I detect “iPhone” in the user-agent filed, I’ll send the request to the mobile pool.  If I don’t, send it to the regular HTTP pool.  

Now that we wrote the rule we have to apply it to the virtual server.  Open your virtual server and click on the ‘Resources’ tab.  Since this iRule is setting the pool for us and we have a ‘else’ statement, I’m going to set the default pool to none.  Then set the iRule accordingly…


That’s it, give it a test! From out desktop we see the normal page…

And from the iPhone we see…

Not very inventive of me but you get the point.

So as you can see iRules are very powerful.  I hope to write some more soon and post them up as templates and examples but for now I just wanted to let you see how powerful they can be.

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