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Linux Firewalls by Michael Rash

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Abusing the Transport Layer

Because the transport layer is, in a sense, the last gateway before communicating up the stack with a networked application, it's a juicy target for an attacker. Much of the suspicious activity that involves transport layer information falls into the category of reconnaissance efforts instead of outright attacks.

Port Scans

A port scan is a technique used to interrogate a host in order to see what TCP or UDP services are accessible from a particular IP address. Scanning a system can be an important step along the way toward a successful compromise, because it gives information to an attacker about services that may be accessed and attacked.

That said, a port scan can also be an important step to just seeing what services ...

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