What is – Azure Route Server

Back in March 2021 at Ignite, Microsoft announced a new network feature in Preview, Azure Route Server. As the name suggests, this service is designed to act as a routing service within your Azure footprint. Specifically, it should simplify the management of dynamic routing between your network virtual appliance (NVA) and your Virtual Network. It does so by allowing you to exchange routes via BGP between the NVA and the Azure Software Defined Network (SDN) in your Virtual Network. As it’s a fully managed service, this means you no longer need to implement and maintain multiple route tables.

This service has just gone GA, and as a result, I thought I would share some thoughts relative to its implementation and functionality.

A note, if you’re on a CSP subscription, you will se an error in the portal if you try to deploy. However, you can work around this by using PowerShell or any of the other deployment options.

First up, a little bit more on how it works. Microsoft have an excellent graphic to show you the functionality at a high-level:

Diagram showing Azure Route Server configured in a virtual network.

However, the key to understanding this service is having familiarity with implementing an NVA in an environment without it. So, say we build a Palo Alto, or any vendor NVA in our VNET. It is a flat VNET which has several subnets. And we want all our traffic to route via the NVA for filtering. Without Route Server, we have to implement single, or multiple, Route Tables on the subnets and manually manage route modifications on the NVA. Meaning a lot of admin overhead and room for potential user error.

Route Server removes this requirement, which believe me, on complex NVA hub-spoke designs is very welcome. The service leverages BGP to expose the system routing table of the VNET to allow the NVA write directly. Similarly, the NVA can learn routes from the VNET as they update, for example, the addition of a peering.

Creation of the service itself is very simple, a dedicated subnet of /27 or larger named RouteServerSubnet and a Public IP. Essentially a similar creation process to Azure Bastion. Guides for deployment are well documented, and include IaC options (although no Bicep example as yet).

Once Route Server is built, you then peer, and complete configuration as required with your NVA(s). Route Server advertises an ASN and IPs from the dedicated subnet for this, as below

I’ve only mentioned NVAs to this point, but Route Server also works with Virtual Network Gateways, supporting both VPNs and ExpressRoute. Integration here is even tighter, you don’t need to configure the peering, you just enable the switch within Route Server. This switch is called Route Exchange, and when enabled, your NVA and ExpressRoute wil learn each others routes.

One thing I have found so far is that this seems more like a service that should be enabled on the VNET itself, like DNS or Service Endpoints, rather than a resource you have to build and configure. Having it “built into” every VNET would seem like the better option to me, and would definitely be cleaner.

Also, there doesn’t appear to be any integration offered with Azure Firewall as yet, however, I want to dig a bit more on that and will update this post accordingly.

Perhaps someday we will get a VNET Version 2.0 which includes the likes of Route Server, Bastion etc all within the one blade and configuration item!

Until then however, if you want to try out Route Server, the absolute best option is to follow the Docs tutorial which covers everything except Virtual Network Gateway, although you could add that on yourself manually if required as I did (ping me if you need help). The full tutorial is here and is a really clean implementation and removal which gives you the ability to test each feature.

One thing that did catch me out here was advertising the default route, however, while I realised my mistake and corrected it with a UDR, I was glad to see this configuration issue, and others are already documented.

Before deploying to production, consider the FAQ to ensure you understand limits and supported scenarios. Peer and route limits are specifically important here for larger designs!

As always, if you have any questions, get in touch!

How To – Troubleshoot Azure Virtual Network Peerings

Recently, Microsoft announced a new preview feature relative to Virtual Network Peerings. This preview allows you to resize the address space of a peered virtual network. Up until the point of this preview, any resize operation on a Virtual Network with a peering activated would fail. The only previous workaround was to delete the peering, complete the required address space modifications, then recreate the peering. While this can be completed quite quickly, it does cause downtime. This preview feature is definitely a welcome addition.

Address space change error example due to peering

So while there is a new feature available, but it’s in preview, let’s have a look at the current state of Virtual Network Peering. Starting off with what exactly they are. A peering allows you to seamlessly connect two Virtual Networks. There are a couple of the usual caveats like non overlapping adress spaces but configuration is as simple as can be. One item of note, peerings are not transient. That means that in the graphic below, even though the Hub Virtual Network is peered to both A and B, traffic cannot traverse from A directly to B or vice versa.

virtual network peering transit
Hub/spoke network example with non-transient peering

Using peerings allows for the creation of well architected, and secure, network footprints in Azure. If you’re new to network on Azure, and/or new to peerings, I really recommend reading this routing page on Docs, it gives clear examples and explanations of common scenarios. And for more specifically on peering, check out this page on Docs. Both will help set the context for some of the areas we are going to cover in this post, and if you’re not familiar with them, may be a challenge.

Configuration

For all explanations we will use the following architecture:

Hub/spoke virtual networks with peerings

The first step to create a peering is also the first place you should confirm when troubleshooting. Often, it can simply be a missed setting in this configuration that causes your issue. A peering contains three elements of configuration, and they are repeated on both sides, leaving you with six settings in total that can impact your peering.When creating a peering, we have the following choices. These choices impact how your peering will function and should be your first check, every time, when you have an issue with a peering.

Let’s break those down a little. First, “Traffic to remote virtual network”, this enables communication between the virtual networks and allows resources connected to either virtual network to communicate with each other with the same bandwidth and latency as if they were connected to the same virtual network. So, you may ask why is there a block option if I am enabling a peering!? Good question! It’s mostly there to facilitate temporary blocks, saving you from deleting and recreating the peering. How this works is slightly complicated, as it’s based on manipulating the service tag “Virtual Network” rather than explicitly blocking traffic.

If you want to block all traffic – delete the peering.

The VirtualNetwork service tag for network security groups encompasses the virtual network and peered virtual network when this setting is Enabled. Read more about network security group service tags here. When this setting is disabled, traffic doesn’t flow between the peered virtual networks by default; however, traffic may still flow if explicitly allowed through a network security group rule that includes the appropriate IP addresses or application security groups.

Next, “Traffic forwared from remote virtual network”, when enabled, this allows traffic that didn’t originate from the virtual network. This is best explained using a routing example; take our three virtual networks, VNET A is peered to VNET B and to VNET C, however, B and C are not peered. Don’t forget, peerings are not transient, so without additional configuration, B and C cannot communicate. WE can create a service chain by using VNET A as the next hop for B to reach C and vice versa, we do this by using route tables and a network virtual appliance, such as a router or firewall etc. However, for that traffic to be allowed use the peering, we need the forwarded traffic flag enabled on BOTH sides of the peering.

Finally, “Virtual network gateway or Route Server”, this can be enabled depending on whether the services exist or not in your peered virtual networks. Using our architecture, creating a peering between A and B. On the A side, we could choose “Use this virtual network’s gateway…” and on B “Use the remote virtual network’s gateway…”. This would allow B to use the connections terminated on the gateway in A. Without this enabled, for example on our peering between A and C, traffic cannot use the connections on the gateway.

Important note here, you cannot peer and use gateways if both virtual networks have gateways. You can peer and use the other two options, but not the gateway.

One often over looked feature is the ability to use the VNG in A to route traffic from B to C. It’s a supported configuration, requiring route tables etc. and isn’t very well documented but it works! Ping me if you need help with it.

Constraints

Now, if you have confirmed that all of these settings are correct, but you are still facing issues, there are some common constraints that may be the cause of your trouble.

  • Problem Resources – There are a set of resources that do not work across global peering – Listed here.
  • Classic Virtual Network – while you can peer an ARM and a classic, you cannot peer two classics. Upgrade those vnets to ARM!
  • Peering status – Every peering has a status when viewed within the virtual network. If it shows Initiated, traffic wont flow. Check both sides of the peering and update configuration until it shows Connected
  • DNS – If you’re using the default DNS with your virtual network, you cannot resolve names in a peered virtual network. You will need custom DNS, or Azure DNS Private.
  • P2S – If you have P2S configured and then add a peering, you must donwload the client config again to pick up the peered virtual network routes.

Scenarios

While 99% of the issues I have seen are resolved with some combination of the above, there are a few specific scenarios that require a slightly different focus, Microsoft have documented these all on one page.

Finally

Don’t forget, peerings are all about the system route table of the virtual networks, that’s how they function. Understanding and validating your route tables is key to successful troubleshooting. Having said that, if you are ever stuck, please get in touch, I will do my best to help!

How To – Enable Web Application Firewall Geomatch Custom Rules

At the end of July, Microsoft announced the general availability of geomatching via Custom Rules in Web Application Firewall. This is a feature I am quite fond of, and is excellent at reducing your attack surface. Thankfully, it’s also quite simple to implement!

First, what is Web Application Firewall (WAF)? Well it’s a service that provides protection for your web apps from common exploits and vulnerabilities. It can be deployed with Azure Application Gateway, Azure Front Door, and Azure Content Delivery Network (CDNN is preview for now). WAF allows for central management, meaning you can react to threats faster, instead of securing each individual web application. There are a couple of specific differences depending on the service you attach a WAF policy to, and you can read about them here.

WAF overview
Nice graphical explainer of WAF from Microsoft

Next, what are Custom Rules? As part of each WAF policy, you have to configure a set of standard rules, such as Prevention or Detection mode, and managed rules (OWASP). However, you can also create your own rules, and in WAF these are simply called Custom Rules (CR). A CR is made up of one or more conditions followed by an action. All CRs for a WAF policy are match rules. You can have multiple CRs per policy and they are processed in order of priority in a range of 1-100, with 1 being highest priority, or processed earliest.

Now that we have the foundation, let’s look at the geomatch option. This is available within your CR as a match Condition.

Create your CR with an appropriate name and priority, then choose ‘Geo location’ from the Match type drop down as above. Next, you’ll want to ensure you choose RemoteAddr as the match variable, and decide what logic you want to apply. By logic I mean the pattern that will fire the rule. In this example, I want all traffic except Ireland blocked. So I will choose the Operation ‘Is not’, then location Ireland, then Deny. If I wanted all traffic allowed and Ireland blocked, I would simply choose the Operation ‘Is’. I recommend figuring out your pattern then working your way through the final section of the CR.

So, based on my example with Ireland, my CR now looks like this:

Now most commonly, I would see a geomatch policy applied to an Application Gateway, but it can be applied to Front Door also. On of the nice features of a WAF policy when using it with Application Gateway is that you get association choices. This means you can be as granular as having different policies per listener, or even per path rule. Giving you huge flexibility when combined with CRs.

Finally, I have obviously shown you how to create this CR in the portal, but there are other methods, like Powershell. These can be very handy if you need to recreate a geomatch CR, especially if you have multiple conditions with many countries (there is a max of 10 per condition). Note, if using these methods, you need to use country codes, like IE, and these are all listed here.

And that’s it! You have your CR created and associated. WAFs take effect pretty quickly so you should be able to test within a couple of minutes. If you want to perform a simple verify, you can switch the action from Deny traffic to Allow and retest.

Finally, a nice tip when you’re happy with your configuration is to setup an alert against your WAF should there be a change made, or worse, deleted (apply resource locks people). This runs as a signal alert from Azure Monitor against an action group as per standard

That’s it for this post, as always, if you have any questions, please get in touch!

Global Azure Virtual 2020

As a result of the current global health crisis, the Global Azure events have moved entirely online. Spread over three days (23rd-25th April) it promises to be an amazing mix of content from contributors from all corners of the world.

In the UK & Ireland, we have a local event. Organised by a great team, all of the details are here – https://azureglobalbootcamp2020.azurewebsites.net/

There is a mix of live, and pre-recorded sessions with a wide range of topics.

I’ve contributed a session myself, pre-recorded, on Azure Foundations. If anyone you know is looking to make a start on Azure, have them check it out! #GlobalAzureVirtual

How to – Use Azure Firewall IP Groups

If you’re familiar with Azure Firewall you would know that the introduction of an IP Group resource is most welcome. IP Groups are still in preview at the moment, so as usual be cautious on production environments as there is no SLA. However, it’s always nice to try out a service to see if it can work for you, or make your life easier.

IP Groups themselves are a relatively simple resource. They can contain a single IP address, multiple IP addresses, or one or more IP address ranges. They can then be used for DNAT, Network, or Application rules in Azure Firewall.

They currently have some interesting limitations that are a little bit confusing at first. From Docs:

For 50 IP Groups or less, you can have a maximum of 5000 individual IP addresses each per firewall instance. For 51 to 100 IP Groups, you can have 500 individual IP address each per firewall instance.

What this means is that while your rules should already be scoped accurately, you may need to use a couple of extra IP groups if you’re working with large address ranges. A simple example is a /16 will simply not work in an IP Group, /20 is basically your limit per IP Group.

I actually tried this on my own sub and it appears to actually work for now. Expect that to change as preview progresses.

If you’ve worked with Azure Firewall, I’m sure you’ve already thought of several places these rules can really help. For me, it was within Network Rule Collections.

However, as the service is in preview, there are a few aspects to be ironed out. Unfortunately, one of those is the ability to add an IP Group as a destination within a network rule when using the Portal. See below

UPDATE: As expected, this is now resolved! However, read on to see how to do this at scale.

At this point, I am going to flag extreme caution if your Azure Firewall is in production and you are trying this. It is very easy to overwrite all of your collections, take your time and export them before making any changes!

I’m a Windows guy, so I’m going to explain how to do this with Powershell, but it also works for CLI. Similarly, I’m showing a Network Rule, same process works for Application Rules.

First up, you need to all of the details for your Azure Firewall as we will work with it’s config as a variable and finally update it.

#Get the AFW I want to edit
$afw = Get-AzFirewall -Name wda-afw-test -ResourceGroupName rg-wda-afw

#Save current Network Rule Collection to a variable for reference
$oldcol = $afw.NetworkRuleCollections

#Get the IP Group I want to use
$ipg = Get-AzIpGroup -Name wda-group1 -ResourceGroupName rg-wda-afw

#Create my new network rule
$newrule = New-AzFirewallNetworkRule -Name "rule2" -Protocol TCP -SourceAddress * -DestinationIpGroup $ipg.Id -DestinationPort 445

Now this is where it can get a bit tricky. Collections are stored as nested arrays. My AFW has two collections, I want to add my new rule to the second one which means I need to reference index 1. See the collections below, the one we’ll be editing is “collection2” which currently only has “rule2”

#view all collections
$afw.NetworkRuleCollections

#view the specific collection rules using place in array
$afw.NetworkRuleCollections[1].Rules | ft

#add my new rule to my collection
$afw.NetworkRuleCollections[1].AddRule($newrule)

#if you like, check it has updated as desired
$afw.NetworkRuleCollections[1].Rules | ft

#If as expected, update AFW
Set-AzFirewall -AzureFirewall $afw

The last command can take a minute or two to complete. Once it has, you can see the rule is now added to my collection2. The Portal will display it correctly, but you cannot edit correctly with the glitch.

And that’s it! You’ve successfully added an IP Group as a destination to your Azure Firewall. Again, please be careful, the above is only a guide and I cannot be responsible for your Azure Firewall 🙂