Dnmap (distributed Nmap) is a framework to distribute nmap scans among several clients. It reads an already created file with nmap commands and send those commands to each client connected to it. The framework use a client/server architecture. The server knows what to do and the clients do it. All the logic and statistics are managed in the server. Dnmap stores the Nmap output on both server and client. The only caveat of this whole set up is lack security as the framework will inherently trust the client and will execute any Nmap command sent. So, if you want to protect this setup, you might as well have it secured via ACLs, etc. Yet, the Dnmap server is capable of fighting off command injection attacks.

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http://sourceforge.net/projects/dnmap/files/