Attacks on a firewall should be approached with

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Attacks on a firewall should be approached with extreme caution. An attack that modifies a firewall or causes it to crash can disrupt an entire network for hours and even days. We do not open up a network to security problems it did not otherwise have before we began testing. Consequently, if a firewall is vulnerable to an attack in which we find that we could change a configuration file or binary, we stop the test at this point and explain to the client what we have discovered and why we will not proceed any further with this particular part of the test. Once we have launched all attacks against the firewall, we turn our attention next to host machines within the client’s network. We first attempt to  telnet , then use login to obtain a shell on these machines. Using different IP addresses from different domains to launch these attacks provides a better test of the robustness of the IP address screening rules. We also test for trusted host access. The likelihood of success using telnet/rlogin and rlogin is, however, typically small. Next we determine whether we can use available services to gain access to these systems. Some of the most useful services to attack are the Network File System (NFS), the Network Information Service (NIS), and the mail daemon (sendmail). Hacking tools shared with us by organizations that have experienced intrusions are extremely useful in this regard. In addition, many hacker tools are available on Internet ftp sites. Such tools frequently come with source code that can be examined to determine whether the tool contains malicious code such as Trojan Horse programs before the tool ever used for firewall testing. In some respects, therefore, hacker tools are more useful than commercial attack tools, which almost without exception do not come with source code. Not all types of attacks, however, are suitable for use in Layer 3 and the subsequent layer because they are likely to cause damage or disruption. IP spoofing and session hijacking attacks provide two excellent examples. IP spoofing attacks require that a legitimate client host’s ports be wedged; session hijacking causes a user connection to a destination host to be dropped (Tomsen, 1995). We recommend, therefore, that instead of actually launching these types of attacks, the testing team should instead evaluate 4 We have made this decision because making specific information concerning how to attack systems freely available is likely to aid the perpetrator community in breaking into systems, an outcome we feel is our duty to guard against.
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the potential for these attacks to be successful by following procedures such as inspecting routing tables to ensure that incoming packets that indicate they originate from within a network are rejected at the gate.
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