When measuring activity with the Piwik Tracking API, the following HTTP headers may be used:
Host– the hostname of the HTTP request, the hostname of the Piwik server.
Referer– used to track Referrer information such as website, search engine, social network.
User-Agent– used to detect the information about browsers, operating system, devices by device-detector library.
Cookie– when Opted-out from tracking, or when Third party cookies are used.
Origin– used when cross-origin resource sharing is enabled.
Accept-Language– used to detect the users’ language
X-Forwarded-For– when Piwik is setup behind a proxy
DNT– used to detect whether a visitor’s device is opted-out from tracking (Piwik privacy)