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to reuse an ACK ping probe from host detection during a SYN port scan. This can greatly speed up a scan if the SYN scan finds only filtered ports. One difficulty with implementing this is that not all ping probes are appropriate for all scan types. nmap -PA -sU scanme.nmap.org would cache the ACK ping probe and send ACK pings during the UDP scan. But the pcap filter for the UDP scan doesn't catch TCP packets, so the replies would not be noticed and they would show up as dropped pings. Likewise, nmap -PR -sS 192.168.0.1 would segfault when it tried to use an uninitialized Ethernet descriptor to send an ARP ping during the SYN scan, which would use raw sockets. To fix this I added a function pingprobe_is_appropriate that determines whether a given ping probe is appropriate for the current scan type. If not, the constructor for HostScanStats just erases the ping probe. More types of ping probes could be made "appropriate." TCP timing pings work during a UDP scan if only the pcap filter is expanded to include TCP packets.
Here is some documentation for Nmap, but these files are much less comprehensive than what you'll find at the actual Nmap documentation site ( http://nmap.org ).