Decoding HTTPS CONNECT in Proxy Environments
Understanding the difference between HTTPS CONNECT and GET methods is essential in any environment where proxies are used, such as with Zscaler. This piece sheds light on the CONNECT method and explains why not seeing GET requests in logs doesn’t necessarily mean there’s no connection.
Zscaler Agent’s Function
In environments where Zscaler or similar proxies are deployed, the Zscaler agent facilitates secure internet access. It starts with a CONNECT request to the proxy when a user tries to visit an HTTPS site. This request is about setting up a secure channel, not fetching the actual web content.
The Essence of CONNECT Method
The CONNECT method is often misunderstood. Its primary function is to establish a tunnel for encrypted data to pass through, not to request content like the GET method does. When a CONNECT request is successful, indicated by a 200 OK response, it means the secure tunnel is established, ready for data transfer.
Analysts’ Common Misconception
A common point of confusion for analysts monitoring network traffic is the absence of GET requests following a CONNECT. This absence doesn’t imply a lack of connection. Instead, it reflects the nature of encrypted traffic within the established tunnel, which hides the specifics of the data transfer from view.
Proxy Configurations and Visibility
The visibility into the data transferred through the CONNECT method depends on the proxy’s configuration. In a standard setup, the proxy forwards encrypted data without insight into its contents. However, with SSL inspection enabled, proxies like Zscaler can decrypt, inspect, and re-encrypt traffic, providing a glimpse into the encrypted exchanges.
It’s crucial to use SSL inspection carefully, considering privacy and compliance. Zscaler offers guidance on configuring SSL inspection to balance security with privacy (Zscaler SSL Inspection).
Conclusion
In environments using proxies, a CONNECT request followed by a 200 OK response signifies a successful establishment of a secure browsing session, not the absence of a connection. Analysts should understand the secure, encrypted channels’ workings to effectively interpret network traffic logs.
Ressources
https://datatracker.ietf.org/doc/html/draft-luotonen-web-proxy-tunneling-01#section-3.2