Wednesday, July 08, 2015

SIP DHCP Option 120 DeMystified

On the surface, Option 120 seems simple - it's the Pool FQDN, encrypted.

Everyone knows to run the DHCPUtil.exe -sipserver and it spits out a long hex value for you.  Option 43 is a little fancier, and gets more press.  Option 120 seems straightforward in comparison.  What if I asked you to create the hex code without DHCPutil?  Sure sounds easy, you take that FQDN, run off to your local hex to asc convertor and punch in your FQDN, you get a non-human readable string, and you set your DHCP options.  Done and done -  What do you mean it's not working?

So, reading through RFC3361 (as I am known to do in front of the fireplace, pouring over dot-matrix printed copies, burning the parts I have already committed to memory) we learn that Option 120 has a VERY particular format.

That format can be seen below - where "##" is the hex code for the length of the ASCII section following it.


That's confusing, but for a machine with limited CPU/RAM, that's a pretty sweet input.

So, let's un-Lync this for a minute and pretend our SIP server was Keep in mind, the 00-FF range is 0-255 characters in each section of the FQDN.  I don't know a TLD above 10 characters, but I guess they really wrote this code openly!

Section Characters Hex Code
www 3 03
bing 404

I'll avoid the haiku here but the encoding for this would look like:

00 03 HEXCODE(www) 04 HEXCODE(bing) 03 HEXCODE(com) 00

Or this (bolded the Hex count codes)
Armed with information, I began working in Excel to make it so I could build these codes without having RDP to a Lync or S4B server available to me, or from a place where I might not have DHCPUtil.exe access.

Taking an input in Excel using the FQDN, you need to do some fair complicated text manipulation to break down the sections, perform the ASC to Hex conversions and concatenate the text.  I am pleased to report that in my search for this code already existing, I was able to find this Design Document Generator written by Alessio Giombini.

I have taken the tab and modified it some to release it separately to you here.
Download from the TechNet Gallery!

Here's a quick Office Mix I made on this.


Paul Bloem said...

Hi Chris,
I am looking for a "Best Practice" statement with regard to DHCP setup in an environment where an SBA is in use. So, while demystifying option 120 can you perhaps shed light on where it SHOULD be pointing (SBA\Primary Pool\HLB)??


Chris Lehr said...


If you have a standard edition, it should point at that FQDN. If enterprise, the internal web FQDN - which would likely be an HLB.