How to – Troubleshoot Azure Firewall

Networking in Azure is one of my favourite topics. As my work has me focus primarily on Azure Virtual Datacenter builds, networking is key. When Microsoft introduced Azure Firewall (AFW), I was excited to see a platform based option as a hopeful alternative to the traditional NVAs. Feature wise in preview, AFW lacked some key functionality. Once it went GA a lot of the asks from the community were rectified however there are still some outstanding issues like cost, but all of that is for a blog post for another day!

AFW is used in a lot of environments. It’s simple to deploy, resilient and relatively straight forward to configure. However, once active in the environment, I noticed that finding out what is going wrong can be tricky. Hopefully this post helps with that and can save you some valuable time!

I don’t know about you, but the first thing I always check when trying to solve a problem is the most simple solution. For AFW that check is to make sure it’s not stopped. Yes that’s right you can “stop” AFW. It’s quick and easy to do via shell:

# Stop an existing firewall

$azfw = Get-AzFirewall -Name "FW Name" -ResourceGroupName "RG Name"
$azfw.Deallocate()
Set-AzFirewall -AzureFirewall $azfw

But how do you check if it has been stopped? Very simply, via the Azure Portal. On the overview blade for AFW it shows provisioning state. If this is anything but “Succeeded” you most likely have an issue.

So, how do you enable it again should you find your AFW deallocated? Again, quite simply via shell, however, it must be allocated to the original resource group and subscription. Also, while it deallocates almost instantly, it takes roughly the same amount of time to allocate AFW as it does to create one from scratch.

# Start a firewall

$azfw = Get-AzFirewall -Name "FW Name" -ResourceGroupName "RG Name"
$vnet = Get-AzVirtualNetwork -ResourceGroupName "RG Name" -Name "VNet Name"
$publicip = Get-AzPublicIpAddress -Name "Public IP Name" -ResourceGroupName " RG Name"
$azfw.Allocate($vnet,$publicip)
Set-AzFirewall -AzureFirewall $azfw

So, your AFW is active and receiving traffic via whatever method (NAT, Custom Route Tables etc.) and you have created rules to allow traffic as required. Don’t forget all traffic is blocked by default until you create rules.

By default the only detail you can get from AFW are metrics. These can show a small range of traffic with no granular detail, such as rules hit count.


To get detailed logs, like other Azure services, you need to enable them. I recommend doing this as part of your creation process. In terms of what to do, you have to add a diagnostic setting. There are two logs available, and I recommend choosing both.

  • AzureFirewallApplicationRule
  • AzureFirewallNetworkRule

In terms of where to send the logs, I like the integration offered by Azure Monitor Logs and there is a filtered shortcut right within the AFW blade too.

Once enabled, you should start seeing logs flowing into Azure Monitor Logs within five to ten minutes. One aspect that can be viewed as a slight negative is that logs are sent in JSON. As a result most of the interesting data you want is part of an object array:

{
  "category": "AzureFirewallNetworkRule",
  "time": "2018-06-14T23:44:11.0590400Z",
  "resourceId": "/SUBSCRIPTIONS/{subscriptionId}/RESOURCEGROUPS/{resourceGroupName}/PROVIDERS/MICROSOFT.NETWORK/AZUREFIREWALLS/{resourceName}",
  "operationName": "AzureFirewallNetworkRuleLog",
  "properties": {
      "msg": "TCP request from 111.35.136.173:12518 to 13.78.143.217:2323. Action: Deny"
  }
}

So, when running your queries, you need to parse that data. For those who have strong experience in Kusto, this will be no problem. For those who don’t, Microsoft thankfully provide guidance on how to parse both logs including explanatory comments

For ApplicationRule log

AzureDiagnostics
| where Category == "AzureFirewallApplicationRule"
//using :int makes it easier to pars but later we'll convert to string as we're not interested to do mathematical functions on these fields
//this first parse statement is valid for all entries as they all start with this format
| parse msg_s with Protocol " request from " SourceIP ":" SourcePortInt:int " " TempDetails
//case 1: for records that end with: "was denied. Reason: SNI TLS extension was missing."
| parse TempDetails with "was " Action1 ". Reason: " Rule1
//case 2: for records that end with
//"to ocsp.digicert.com:80. Action: Allow. Rule Collection: RC1. Rule: Rule1"
//"to v10.vortex-win.data.microsoft.com:443. Action: Deny. No rule matched. Proceeding with default action"
| parse TempDetails with "to " FQDN ":" TargetPortInt:int ". Action: " Action2 "." *
//case 2a: for records that end with:
//"to ocsp.digicert.com:80. Action: Allow. Rule Collection: RC1. Rule: Rule1"
| parse TempDetails with * ". Rule Collection: " RuleCollection2a ". Rule:" Rule2a
//case 2b: for records that end with:
//for records that end with: "to v10.vortex-win.data.microsoft.com:443. Action: Deny. No rule matched. Proceeding with default action"
| parse TempDetails with * "Deny." RuleCollection2b ". Proceeding with" Rule2b
| extend 
SourcePort = tostring(SourcePortInt)
|extend
TargetPort = tostring(TargetPortInt)
| extend
//make sure we only have Allowed / Deny in the Action Field
Action1 = case(Action1 == "Deny","Deny","Unknown Action")
| extend
    Action = case(Action2 == "",Action1,Action2),
    Rule = case(Rule2a == "",case(Rule1 == "",case(Rule2b == "","N/A", Rule2b),Rule1),Rule2a), 
    RuleCollection = case(RuleCollection2b == "",case(RuleCollection2a == "","No rule matched",RuleCollection2a),RuleCollection2b),
    FQDN = case(FQDN == "", "N/A", FQDN),
    TargetPort = case(TargetPort == "", "N/A", TargetPort)
| project TimeGenerated, msg_s, Protocol, SourceIP, SourcePort, FQDN, TargetPort, Action ,RuleCollection, Rule

For NetworkRule log

AzureDiagnostics
| where Category == "AzureFirewallNetworkRule"
//using :int makes it easier to pars but later we'll convert to string as we're not interested to do mathematical functions on these fields
//case 1: for records that look like this:
//TCP request from 10.0.2.4:51990 to 13.69.65.17:443. Action: Deny//Allow
//UDP request from 10.0.3.4:123 to 51.141.32.51:123. Action: Deny/Allow
//TCP request from 193.238.46.72:50522 to 40.119.154.83:3389 was DNAT'ed to 10.0.2.4:3389
| parse msg_s with Protocol " request from " SourceIP ":" SourcePortInt:int " to " TargetIP ":" TargetPortInt:int *
//case 1a: for regular network rules
//TCP request from 10.0.2.4:51990 to 13.69.65.17:443. Action: Deny//Allow
//UDP request from 10.0.3.4:123 to 51.141.32.51:123. Action: Deny/Allow
| parse msg_s with * ". Action: " Action1a
//case 1b: for NAT rules
//TCP request from 193.238.46.72:50522 to 40.119.154.83:3389 was DNAT'ed to 10.0.2.4:3389
| parse msg_s with * " was " Action1b " to " NatDestination
//case 2: for ICMP records
//ICMP request from 10.0.2.4 to 10.0.3.4. Action: Allow
| parse msg_s with Protocol2 " request from " SourceIP2 " to " TargetIP2 ". Action: " Action2
| extend
SourcePort = tostring(SourcePortInt),
TargetPort = tostring(TargetPortInt)
| extend 
    Action = case(Action1a == "", case(Action1b == "",Action2,Action1b), Action1a),
    Protocol = case(Protocol == "", Protocol2, Protocol),
    SourceIP = case(SourceIP == "", SourceIP2, SourceIP),
    TargetIP = case(TargetIP == "", TargetIP2, TargetIP),
    //ICMP records don't have port information
    SourcePort = case(SourcePort == "", "N/A", SourcePort),
    TargetPort = case(TargetPort == "", "N/A", TargetPort),
    //Regular network rules don't have a DNAT destination
    NatDestination = case(NatDestination == "", "N/A", NatDestination)
| project TimeGenerated, msg_s, Protocol, SourceIP,SourcePort,TargetIP,TargetPort,Action, NatDestination

Using either query gives you clear readable that you can filter. One tip however, is to add a sort command to the end of the queries, normally I use by TimeGenerated to show me the latest data. So to condense and add that for the NetworkRule query above, it would look like:

AzureDiagnostics
| where Category == "AzureFirewallNetworkRule"
| parse msg_s with Protocol " request from " SourceIP ":" SourcePortInt:int " to " TargetIP ":" TargetPortInt:int *
| parse msg_s with * ". Action: " Action1a
| parse msg_s with * " was " Action1b " to " NatDestination
| parse msg_s with Protocol2 " request from " SourceIP2 " to " TargetIP2 ". Action: " Action2
| extend SourcePort = tostring(SourcePortInt),TargetPort = tostring(TargetPortInt)
| extend Action = case(Action1a == "", case(Action1b == "",Action2,Action1b), Action1a),Protocol = case(Protocol == "", Protocol2, Protocol),SourceIP = case(SourceIP == "", SourceIP2, SourceIP),TargetIP = case(TargetIP == "", TargetIP2, TargetIP),SourcePort = case(SourcePort == "", "N/A", SourcePort),TargetPort = case(TargetPort == "", "N/A", TargetPort),NatDestination = case(NatDestination == "", "N/A", NatDestination)
| project TimeGenerated, msg_s, Protocol, SourceIP,SourcePort,TargetIP,TargetPort,Action, NatDestination
| sort by TimeGenerated desc

Using parsed data, you can immediately see all the traffic hitting AFW and, for example, filter on options such as Action to see only denied traffic.

Microsoft also provide pre-cooked visualisation should you prefer it, you can download from here – https://raw.githubusercontent.com/Azure/azure-docs-json-samples/master/azure-firewall/AzureFirewall.omsview – then import into Azure Monitor. The detail is great for quick glance work, I really like the ApplicationRule breakout

Application rule log data

That about sums it up. Hopefully you are now informed and equipped to troubleshoot traffic issues in your Azure Firewall instance. As always, if there are any questions, please get in touch!

If you need more info on how to enable logs – https://docs.microsoft.com/en-us/azure/firewall/tutorial-diagnostics

Log and metrics concepts – https://docs.microsoft.com/en-us/azure/firewall/logs-and-metrics

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