We present a novel technique for estimating the round trip time network latency between two off-path end hosts. That is, given two arbitrary machines, A and B, on the Internet, our technique measures the round trip time from A to B. We take advantage of information side-channels that are present in the TCP/IP network stack of modern Linux kernels to infer information about off-path routes. Compared to previous tools, ours does not require additional resources, machines, or require additional protocols beyond TCP. The only requirements are that one end host have an open port and be running a modern Linux kernel and that the other end host responds to unsolicited SYN-ACK packets with a RST packet. We evaluate our technique 'in the wild' and compare our off-path estimates to on-path measurements. Our experiments show that our technique provides accurate, real-time estimates of off-path network latency. In over 80% of measurements our technique provides off-path round trip time measurements within 20% of the actual round trip time. We also discuss possible causes of errors that impact the accuracy of our measurements.