Web crawlers have been misused for several malicious purposes such as downloading server data without permission from the website administrator.Moreover,armoured crawlers are evolving against new anti-crawler mechanisms in the arm races between crawler developers and crawler defenders.In this paper,based on one observation that normal users and malicious crawlers have different short-term and long-term download behaviours,we develop a new anti-crawler mechanism called PathMarker to detect and constrain persistent distributed crawlers.By adding a marker to each Uniform Resource Locator (URL),we can trace the page that leads to the access of this URL and the user identity who accesses this URL.With this supporting information,we can not only perform more accurate heuristic detection using the path related features,but also develop a Support Vector Machine based machine leaing detection model to distinguish malicious crawlers from normal users via inspecting their different pattes of URL visiting paths and URL visiting timings.In addition to effectively detecting crawlers at the earliest stage,PathMarker can dramatically suppress the scraping efficiency of crawlers before they are detected.We deploy our approach on an online forum website,and the evaluation results show that PathMarker can quickly capture all 6 open-source and in-house crawlers,plus two exteal crawlers (i.e.,Googlebots and Yahoo Slurp).