We present an Internet measurement technique for finding machines that are hidden behind firewalls. That is, if a firewall prevents outside IP addresses from sending packets to an internal protected machine that is only accessible on the local network, our technique can still find the machine. We employ a novel TCP/IP side channel technique to achieve this. The technique uses side channels in 'zombie' machines to learn information about the network from the perspective of a zombie. Unlike previous TCP/IP side channel techniques, our technique does not require a high packet rate and does not cause denial-of-service. We also make no assumptions about globally incrementing IPIDs, as do idle scans. This paper addresses two key questions about our technique: how many machines are there on the Internet that are hidden behind firewalls, and how common is ingress filtering that prevents our scan by not allowing spoofed IP packets into the network. We answer both of these questions, respectively, by finding 1,296 hidden machines and measuring that only 23.9% of our candidate zombie machines are on networks that perform ingress filtering.