Utilize RegEx variable to minimize Rules within Azure Front Door Rule Sets

Question

Is there a way to use RegEx in Azure FrontDoor Standard Subscription Rule Set, to extract a value lets say from a Request Header Accept-Language and use the extracted value in thr URL Rewrite Action to do some localization managing. For example: Accept-Language is "ja-JP" i write a regex to extract any supported languages lets say (en|es|fr|ja). Since "ja" is included in the header, I want to use that value in my URL Rewrite path, for example /gallery/letters/{lang}/writtenArticle.svg where {lang} is the value found in the provided regex. This can do so much good since currently every time I want to do some localization paths from lets say the CDN i have to write a seperate rule for each language. If I support 6 languages and I have let's say 20 images written in those 6 languages, i would need 6 x 20 = 120 rules. Instead of utilizing the variable extraction for the regex and using that, making only 20 rules that contain each supported language within. I hope what i wrote makes sense.

Answer 1

Hi @Nikola Oravec

Welcome to the Microsoft Q&A Platform! Thank you for asking your question here.

Please go through the following points:

• Unfortunately, Azure Front Door Standard/Premium Rules Engine doesn't support extracting values from request headers using RegEx and using those extracted values in URL rewrite actions.
• The Rules Engine primarily focuses on matching conditions and predefined actions.
• While it supports regular expressions in match conditions, it doesn't provide the capability to capture and manipulate extracted values.
• As you mentioned, you currently need to create separate rules for each language.
• While this can be tedious, it's the most straightforward approach within the limitations of Azure Front Door. For your reference: https://learn.microsoft.com/en-us/azure/frontdoor/standard-premium/how-to-configure-rule-set#configure-rule-set-in-azure-portal
• For more complex scenarios, consider using Azure Functions. You can create a function that receives the request, extracts the language code from the Accept-Language header using RegEx, modifies the request URL accordingly and forwards the request to the origin server. For your reference: https://learn.microsoft.com/en-us/azure/azure-app-configuration/quickstart-azure-functions-csharp?tabs=isolated-process
• You can then configure Azure Front Door to route requests to this Azure Function. For your reference: https://learn.microsoft.com/en-us/samples/azure/azure-quickstart-templates/front-door-standard-premium-function-public/#:~:text=Front%20Door%20profile%2C%20endpoint%2C%20origin%20group%2C%20origin%2C%20and%20route%20to%20direct%20traffic%20to%20the%20Azure%20Functions%20application
• If you have more flexibility in your architecture, you can use a custom domain with a reverse proxy to handle the localization logic.
• The reverse proxy can extract the language code and rewrite the URL before forwarding the request to Azure Front Door.

Kindly let us know if the above helps or you need further assistance on this issue.

Thanks,

Sai.