Monitoring network services and diagnosing their problems often require active probing methods. Current probing methods, however, are becoming unreliable, because of interferences from various middleboxes, and inadequate due to their limited path metrics support. In this paper, we present the design and implementation of OneProbe, a new TCP probing method for reliable and metric-rich path monitoring. We have implemented HTTP/OneProbe (i.e., OneProbe for HTTP) which sends TCP data probes containing legitimate HTTP requests to induce HTTP responses for path measurement. Since the probing method is based on TCP's basic data transmission mechanisms, OneProbe works correctly on all major operating systems and web server software, and on 93% of the 37,874 websites randomly selected from the Internet. We also successfully deployed HTTP/OneProbe to monitor a number of network paths for over a month and obtained interesting and useful measurement results.