Azure DMZ Architecture

We’ve been steadily moving more and more of our servers into the azure cloud and have now started to look at moving servers from our DMZ environments there too. Traditional network security says put any internet accessible servers in a multi-tiered DMZ environment, where your web servers, application servers, and databases are in separate network segments. A security event would be localised to just that area of your network reducing the risk to the rest of your network.

Trying to replicate this way of thinking in Azure proves rather difficult currently. There is no concept of a network layer firewall (except for their Network Security Groups which I will get to) and the only security players in this field at the moment are Barracuda. With this solution (their NG firewall) you essentially create a big VPN mesh where each server is VPN’d to a firewall and you can write your enforcement policy there. A big problem with this approach is that you are reliant on a piece of software running on the server OS, any malicious user could simply disable the software and circumvent that entirely.

Using host OS firewalls such as Windows Firewall or Linux’s iptables carry the same risks as above. Anyone with access to your server can simply disable, or change the firewall rules to suit their needs.

Network Security Groups which were implemented only recently offer some form of protection, and these ACL’s can be applied to subnets or individual VM’s within a VNET. I have several gripes with these at the moment.

  • Visibility – Traffic accepted or blocked is done so silently. Traditional firewalls offer an interface which allows you to filter logs to show you whether any particular connection was allowed or blocked, and why. Without this, troubleshooting traffic is difficult, particularly with big policies.
  • No Object Groups – You cannot create an object that represents multiple hosts, or networks. For example, you cannot reference an object name ‘ActiveDirectory’ which contains all your AD servers. With most traditional firewalls you simply update that object and wherever it is referenced in your rulesets gets updated too.
  • Administration – The only way to update NSG’s at the moment is through powershell. This is administratively very laborious. Each change takes anywhere between 5 and 30 seconds to apply.

Examples shown on the Azure website show a very simplified view of this. Port 80 to the webserver, and a database call to your database. The reality is there’s so much more to it than this. Each server of ours has a requirement to connect to a lot of infrastructure services. Some of these include, Active Directory, Patch Management, AntiVirus etc. Many of these services require bidirectional rules to allow the client / server connections. In a company like ours where we are moving servers to the cloud aggressively, IP addresses are frequently changing. This change simply cannot be managed with the NSG’s with this amount of change.

I’m very much hoping Cisco, Juniper, Fortigate, Checkpoint and other firewall vendors will arrive in this space as I’m worried there is no existing solution to designing a proper DMZ architecture in Azure yet.

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