CDNs don’t actually block traffic at the edge; they redirect it to a different edge server that is configured to block it.
Let’s look at how you’d actually implement this. Imagine you have a website hosted in AWS S3, and you’re using CloudFront as your CDN. You want to block all traffic originating from Russia.
First, you need to understand that CloudFront’s geo-blocking rules are not based on IP address lists you maintain. Instead, CloudFront uses a third-party geolocation database. This database maps IP address ranges to countries. When a request comes in, CloudFront checks the IP address against this database.
Here’s how you’d configure it in the AWS console:
- Navigate to your CloudFront Distribution: Go to the CloudFront service in your AWS console and select the distribution you want to configure.
- Edit the Distribution Settings: Click the "Edit" button.
- Locate "Geo Restriction": Scroll down to the "Geo Restriction" section.
- Choose Restriction Type: You’ll have two options:
- Allow list: Only allow traffic from specified countries.
- Deny list: Block traffic from specified countries.
- For our example, we’ll choose Deny list.
- Add Countries to Deny List: Click "Add item" and search for "Russia". Select it from the dropdown. You can add multiple countries if needed.
- Save Changes: Scroll to the bottom and click "Save changes".
CloudFront will then start deploying these changes. This process usually takes a few minutes, but can sometimes take up to 15 minutes. During this time, the old rules are still in effect.
Once deployed, when a user from Russia tries to access your site, their request will hit a CloudFront edge server. That edge server will look up the user’s IP address in the geolocation database. If it finds that the IP belongs to Russia, it will return an HTTP 403 Forbidden error to the user. The request never even makes it to your origin server (your S3 bucket in this case).
The surprising part for many is that CloudFront doesn’t maintain its own country IP lists. It relies entirely on a third-party provider. This means the accuracy and recency of the blocking depend on the updates to that external database. If a new block of IPs is assigned to a country, or if the database lags in updating, there might be a temporary window where traffic from that country isn’t blocked as expected.
Let’s say you’re using a different CDN, like Akamai. The configuration would be similar in principle but differ in the UI. You’d typically go into your "Property Manager" (Akamai’s configuration tool), find the specific rule set for your website, and add a "Geo-IP" condition. You’d set this condition to match "Country Code" is "RU" and then set the action for that condition to "Deny". The underlying mechanism is the same: the edge server checks the IP against a database and returns a 403 if it matches.
The key takeaway is that geo-blocking at the CDN level is a coarse-grained tool. It’s effective for broad country-level blocking but not for nuanced regional restrictions or blocking specific IP ranges within a country. The accuracy is dependent on the CDN’s geolocation data provider.
If you need to block traffic from specific IP addresses or subnets, you’d typically implement that at your origin server’s firewall or using a Web Application Firewall (WAF) service, like AWS WAF or Cloudflare’s WAF.
After successfully blocking traffic from Russia, the next common configuration challenge is managing cached content for different regions, which often leads to understanding CDN caching behaviors and cache invalidation strategies.