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If you’re working with an Application Load Balancer (ALB) on AWS and want to set up host header matching with regex patterns, you’ll find that doing this directly through the AWS console is possible. When creating a listener rule in the console, you can choose between two options for matching host headers: “Value matching” and “Regex matching.” The value matching option lets you use wildcards such as ‘*’ and ‘?’, while regex matching allows for more complex pattern recognition using regular expressions.
However, if you try to define a regex pattern in CloudFormation—AWS’s infrastructure-as-code tool—things get tricky. Changes made through the console might show that values are removed or altered, and CloudFormation may not fully recognize or support regex match patterns for host headers at this time. This can lead to configuration drift, where the actual setup differs from what’s defined in the template.
Here’s what you can specify for host header conditions in the console:
– For value matching, you can set up to 128 characters, case-insensitive, using characters like a-z, A-Z, 0-9, and special characters such as -_.; and wildcards (* and ?).
– For regex matching, you can also set up to 128 characters, which gives you flexible pattern matching.
Since CloudFormation doesn’t currently support regex matching for host headers, you might want to keep an eye on AWS updates or check their roadmap. They often add new features over time.
If you need to use regex patterns for host headers, consider using transforms for listener rules. Transforms let you alter requests before they reach your targets and support regex-based modifications for host headers. This way, you can achieve more sophisticated routing rules without waiting for CloudFormation to add direct support.
For detailed instructions and updates, refer to the AWS documentation on condition types, adding rules, and transforms for listener rules.
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