Developer's guide: Manage traffic with CEL expressions and connection variables

ngrok's Traffic Policy Module uses Common Expression Language (CEL) variables to help developers create policies that shape or manipulate traffic based on information embedded within every connection, request, and response that passes through the endpoint. Connection variables contain information about the raw connection between a client and the ngrok endpoint, including geographic origin, TLS usage, and connection timestamp. These variables can be used to target specific traffic and dynamically apply policies, such as filtering connections by IP address or geo-based location, and interpolating content into upstream headers or logs. The module also allows developers to create custom responses during planned maintenance using a combination of CEL expressions and connection variables.