Symantec provide a version of Squid to their Symantec.Cloud customers that they call the "Client Site Proxy". They've modified the source to add two "encrypted" headers (X-TEACUP and X-SAUCER) to each request, and only provide a Windows version of the product. These headers provide reporting information back to the centralised admin portal. I think one of them contains an encoded username, not sure about the other. They're refusing to provide a Linux version on the grounds that their modifications are "confidential" due to the "encryption" of the headers. Seeing as Squid is GNU-GPL licensed and they're providing a commercial product based upon it, aren't they required by GPL to make the source code for their modifications to squid-cache available to the consumer?