34Protecting Yourselfwith a FirewallAgood firewall (Cheapest web hosting) is important to
34Protecting Yourselfwith a FirewallAgood firewall is important to any network because it s your firstline of defense against intruders. Obviously, if your system has nocontact with the outside world (either via the Web or by modem), you don t need a firewall. If your system does have contact with the Web, you should consider screening what comes through your networkinterfaces. Network information travels in packets groups of data that are encasedin layers of envelopes. Each layer of the packet contains a different kindof information about the data within the packet. The outermost layercontains information about the hardware that the packet is coming from. The next layer within contains the IP address for the source and the destination. Inside that envelope is the TCP information the port num- bers and sequence numbers of both the source and destination machines. At the heart of the packet is the packet payload the real data thatyou re interested in. A good firewall can filter the incoming or outgoing packets based on the information contained in any of the envelope layers. Because theenvelopes are nested within one another, if you toss the outer enve- lope, the inner envelopes go with it, and the data never reaches yourapplication. In this technique, we show you ways to set up a firewall that s tightenough to protect your system, but loose enough to allow useful data inand out. You ll save time with a safer system that gives everyone theaccess they need to get their jobs done. Finding Your FirewallStarting with kernel version 2.4, most Linux distributions include asophisticated packet filtering package called netfilter. You can use netfil- ter to build firewalls, perform network address translation (NAT), per- form port translation, and alter network packets as they traverse yourTechniqueSave Time By Installing an intruder- proof firewall Preserving networkresources and limitingexposure to nasty virusesby limiting Internetaccess Keeping a tight firewallbut allowing SSH access42_
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