Windows Azure Private Network behaviour change

I’ve learnt today that IP routing on Windows Azure when a private network (VPN) is configured has changed recently (not quite sure exactly when, but in the last few weeks I suspect) in a way that can be quite dramatic to many –

Previously, as soon as a site-to-site VPN was configured on a virtual network on Windows Azure, all outbound traffic from the network got routed through the VPN.

This surprised me at the time – I assumed that as the range of IP addresses exposed via the VPN is known, only traffic directed at this ranged will  get routed via the VPN and all other traffic will go directly to the internet. This assumption was proven to be wrong, as we’ve learnt at the time.

This, I was told by several people, is more secure given that VMs on Azure only have one NIC. It also provided organisation an opportunity to more tightly control traffic as all traffic got routed through the organisation’s firewall. for example – organisations who needed to present a consistent IP publically when calling remote systems could control that via their on-premises network configuration as I previously blogged about here, a post I now had to correct, see below.

The main downside of this was always that not all traffic was sensitive and routing everything through the VPN and the on-premises network added latency to the requests and load on the network. For example – a virtual machine on a private network with VPN, calling other Azure services such SQL Database, the Service Bus or the Caching Service would see all requests routed to their internal network before going back out to the internet and to, potentially, the same data centre the request originated from.

In conversation today I’ve learnt (and since confirmed, of course!) that this behaviour has changed and that Azure now behaves as I originally expected it to and now only outbound traffic directed at the IP range exposed via the VPN is routed via the VPN and all other traffic goes straight out through the internet.

This makes perfect sense, but is quite a big change and I’m surprised this wasn’t communicated clearly.

There are downsides – some customers, as I eluded earlier, enjoyed the extra level of control that routing all traffic via their network provided – be it firewall configuration or control over the IP they got routed to external services from. This is not possible at the moment, but I’m hoping that in the not too distant future Windows Azure will offer choice to customers.

Cross posted on the Solidsoft blog

About Yossi Dahan
I work as a cloud solutions architect in the Azure team at Microsoft UK. I spend my days working with customers helping be successful in the cloud with Microsoft Azure.

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