Finding A Zombie Host
The first step in executing an idle scan is to find an appropriate zombie. It needs to assign IP ID packets incrementally on a global (rather than per-host it communicates with) basis. It should be idle (hence the scan name), as extraneous traffic will bump up its IP ID sequence, confusing the scan logic. The lower the latency between the attacker and the zombie, and between the zombie and the target, the faster the scan will proceed.
Note that when a port is open, IPIDs incerement by 2. Following is the sequence:
1. Attacker to target -> SYN, target to zombie ->SYN/ACK, Zombie to target -> RST (IPID increment by 1)
2. Now attacker tries to probe zombie for result. Attacker to Zombie ->SYN/ACK, Zombie to Attacker -> RST (IPID incerement by 1)
So, in this process IPID incerements by 2 finally. (Example by: Nutan Vishwakarma)
When an idle scan is attempted, tools (for example nmap) tests the proposed zombie and reports any problems with it. If one doesn't work, try another. Enough Internet hosts are vulnerable that zombie candidates aren't hard to find. A common approach is to simply execute a ping sweep of some network. Choosing a network near your source address, or near the target, produces better results. You can try an idle scan using each available host from the ping sweep results until you find one that works. As usual, it is best to ask permission before using someone's machines for unexpected purposes such as idle scanning.
Funny—simple network devices often make great zombies because they are commonly both underused (idle) and built with simple network stacks which are vulnerable to IP ID traffic detection.
While identifying a suitable zombie takes some initial work, you can keep re-using the good ones. Alternatively, there have been some research on utilizing unintended public web services as zombie hosts to perform similar idle scans. Leveraging the way some of these services perform outbound connections upon user submissions can serve as some kind of poor's man idle scanning.
Read more about this topic: Idle Scan
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