Passive information gathering is a technique where no direct contact with the target is made for gathering the information. All the information is obtained through an intermediate source which may be publicly available. The internet has many useful resources that can help us with passive information gathering. Some such techniques are discussed next.

The following diagram describes how passive information gathering works:

Here is how it works:

1. The client system first sends a request to an intermediate system
2. The intermediate system probes the target system
3. The target system sends the result back to the intermediate system
4. The intermediate system forwards it back to the client

So, there's no direct contact between the client and the target system. Hence, the client is partially anonymous to the target system.