TIP: *tcpdump* is a common packet analyzer that runs under the command line. It allows the user to display TCP/IP and other packets being transmitted or received over a network to which the computer is attached. Distributed under the BSD license, tcpdump is free software.
Netfilter offers various functions and operations for *packet filtering*, *network address* translation, and *port translation*, which provide the functionality required for *directing packets* through a network and *prohibiting packets* from reaching sensitive locations within a network.
*iptables* is a command line utility for configuring Linux kernel firewall implemented within the Netfilter project. The term ''iptables'' is also commonly used to refer to this kernel-level firewall. It can be configured directly with iptables, or by using one of the many
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==== http://docs.swarmlab.io/SwarmLab-HowTos/labs/sec/ex-3a_iptables-flow-chart.adoc.html[Traversing of tables and chains^]
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When a packet first enters the firewall, it hits the hardware and then gets passed on to the proper device driver in the kernel.
Then the packet starts to *go through a series of steps in the kernel*, before it is either *sent to the correct application* (locally), or *forwarded to another host* - or whatever happens to i
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=== http://docs.swarmlab.io/SwarmLab-HowTos/labs/sec/ex-4_iptables.adoc.html[Iptables with shorewall! - Three-Interface Firewall^]
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*Shorewall* is an open source firewall tool for Linux that builds *upon the Netfilter (iptables/ipchains)* system built into the Linux kernel, making it easier to manage more *complex configuration schemes* by providing a higher level of abstraction for describing rules using text files.
A *virtual private network (VPN)* extends a private network across a public network, and enables users to send and receive data across shared or public networks as if their computing devices were *directly connected to the private network*.
*SSH Tunneling,* is the ability to use ssh to *create a bi-directional encrypted network connection* between machines over which data can be exchanged, typically TCP/IP.
