On the fly content replacement using F5 Load balancers

In the modern web application world, a large proportion of sites are using SSL Offloading, be this for the added security of the web servers not having the SSL private key on them (and hence if compromised the certificate is not necessarily compromised as well) or for the performance boost associated of using hardware accelerators. This however is a double-edged sword. Its more complex for developers to test their applications against this behavior, as they need to either setup two webservers (or vhosts with proxying) on the same host to emulate this, or they need to have an actual off loading device. Both of these are not always readily available options, or easy for the development team to do.

With this in mind, I have seen many times applications that “work in development” but don’t work in production. One common issue I’ve seen is developers checking the protocol that the user has connected to the server as. When off loading, this will be HTTP, rather than HTTPS. It’s also a common practice to run SSL sites on a different port, lets say port 8080, however if the developer is using the absolute URL of the server including the port number when creating URLs this can cause issues.

The result is a url like https://www.withagrainofsalt.co.uk becomes http://web123.internal:8080/. The end user is unable to get access to this (usually) and the user experience is less than idea. The correct way to fix this would be in the application its self, however this can sometimes take weeks / months, and there may not be budget allocated to fix this defect.

With this in mind, we look to other places in the infrastructure that we would be able to fix this issue. My personal preferences are

I’ve also been informed that you can do this with

  • nginx’s HttpSubsModule (Thanks to @SamJSharpe for pointing this out)

In the case of my most recent requirement, the customer was unable to edit the application, there was no budget for Ape. The customer did however have an F5 in front of their web site.

iRule, iRock

F5 Load balancers have a TCL based scripting language that can be invoked on every request that passes through them. Due to the customer needing to do some re-writes, I got to work and crafted an iRule as below. Please be aware that you must have a STREAM profile attached to the virtual server to use this!

Breaking this down we have events triggering specific blocks of code


This is triggered when the iRule is added to a virtual server or edited

In our example we’re performing a simple set of variable, enabling debug (set this to 0 in production!)


This is the event triggered when an HTTP request from the client hits the load balancer.

We disable the stream profile at this point


This code block gets fired off when the server is replying

In our code we first check to see that the Content-Type header contains text, as this would be pointless to perform on graphics! If it is we perform the next step. If debug is enabled, we log the client IP and port to the LTM log. We set the expression that the STREAM profile will use to http://:8081/, and then enable the stream profile. If we did not see a Content-Type header contain text, we do nothing


We see this code block get executed when the STREAM profile’s expression is matched.

We simply perform a string replacement swapping http:// for https:// and :8081 for a blank string, and log this if debug is enabled.


This does have a hit to the CPU on the load balancer, so this should be used sparingly. As mentioned above you MUST have a stream profile associated with the virtual server before applying this iRule

2 thoughts on “On the fly content replacement using F5 Load balancers”

  1. One thing I haven’t been able to find yet – a dozen documents in to my research – is an explanation as to how to “apply a stream profile to a F5 virtual server?” Can you assist with this? I have not found any documentation that explains this step and working with the GUI to do this has not been intuitive. Thank you. John Weaver

    1. Hi John

      First you will need to create a stream profile (or use the pre-existing, but I prefer to create my own)

      Local Traffic > Profiles -> Other -> Stream -> Create
      Set the name to something unique for this VS and hit finish.

      On the VS its self this is set in the same location you would set your SSL Client/Server profiles or the HTTP Profile. You may however need to select advanced from the drop down if using the Gui. It should be listed as Stream Profile.

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