Danie Roux

People person, change agent. Journeyer through problem and solution space. Interested in being interested.

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Learned enough Tcl to be dangerous

August 18th, 2005 · No Comments · general

We got new toys at work. Two BIG-IP load balancers from F5.

Seriously cool devices. Some highlights include:

  • It runs Linux kernel 2.4 in conjunction with some microkernel OS that F5 calls TMOS.
  • Customized, interactive monitors for server availability can be written in Perl.
  • Traffic flow can be modified using Tcl scripts. Basically anything can be inspected and modified, on TCP level as well as HTTP. The example given is sending IE users to their own server1.

The application we deployed behind these load balancers insists on sending back a HTTP 302 redirect that forces the client to continue on plain HTTP. But we terminate SSL on the load balancers, so we really want that redirect to go back to the client as HTTPS.

So, to get this done I picked up Tcl again tonight. TclTutor was helpful to quickly grasp the basics.

I then started to write the script. To test it, I started to stub out the library functionality. Before I knew it I was reading up on namespaces in Tcl. I then realized that they are not using Tcl on the BIG-IP’s. It is in fact [incr Tcl]. So now I can do Object Oriented programming in Tcl. Well, enough of it to attempt to tickle off my foot.

1 Brilliant idea that: “No sir, it must be your browser causing the slow response time, upgrade to a decent browser.”

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