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Most firewalls operate (or should operate) from the position that nothing is permitted except that which is specifically allowed by a rule in the rule base Some early firewall products required that the administrator learn the syntax of the rule base Most firewalls available today offer an easy-to-use GUI interface Table 31-1 shows a typical rule base as one might see it from a GUI Table 31-1: A typical rule base viewed through a GUI
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The basic rule here is to put the most frequently used rule first You don t want the firewall searching through and applying a whole bunch of rules that rarely apply before it gets to the one that is needed All this searching takes precious processing power Therefore, the permission of any source, external or internal, can contact the Web server machine, but it may only use HTTP protocol (Since the port isn t specified, it is assumed to be the default port 80) Note that any source can send mail to mail server We have also installed another Web server (cust Web) especially for our customers This one is only accessible by devices attached to specific network addresses belonging to our
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customers (for example, custnet1 and custnet2) There is a list of network addresses in the firewall s rule base one for each customer There are normally other rules in the table that tend to be vendor-specific For example, the tracking column lets you specify if you want a short or long log entry every time this rule is invoked Next, the FTP users are at machines called goonie, charlie, and goofie (these are really aliases for specific IP addresses) They are the only devices allowed to FTP to FTP server In the last rule of our example, we are also restricting FTP to internal machines called lisa, albert, and shortstuff The rule allowing mail server to talk to our mail server indicates that mail server is only a mail forwarder It isn t the real mail server Only it is allowed to talk to our real mail server with our mail If somehow, mail server is hacked, it won t stop our internal mail server In addition, you don t want to put the real mail server or your internal Web server in the DMZ because then all your internal traffic has to go through the firewall increasing its burden (see firewall performance) The next to last rule allows goofie to telnet to shortstuff While this seems innocent enough, the question is: Where can the user go from here This gets into password administration, which is beyond our scope The issue is that normally a single username/password file is used system-wide so that we only have to administer access permissions in one place If we allow telnet from goofie, then the user at goofie has a login account and could telnet to other servers in the internal network The solution is to have (and maintain) separate accounts on the server lisa Note that there are no rules permitting anyone to talk to virus checker There are also no rules permitting icmp-echo (ping), so that would be hackers cannot use this tool to discover servers in the DMZ or on our network
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Although we mentioned it previously, firewall performance can be a bottleneck The more traffic we run through the firewall, the greater potential bottleneck it becomes Again, it is essential to have an understanding of the traffic flow How many packets/second must the firewall handle In our example, we would add up mail, Web, FTP, and telenet traffic Remember that mail is handled twice by the firewall and three times if the virus checker is employed After you have determined the traffic volumes, check with the hardware and software vendors of your firewall package to determine if you have sized the system correctly
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