Iptables Tutorial 1.2.2

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On top of this, we may add Snort on each of the machines. Snort is an excellent open source network intrusion detection system (NIDS) which looks for signatures in the packets that it sees, and if it sees a signature of some kind of attack or breakin it can either e-mail the administrator and notify him about it, or even make active responses to the attack such as blocking the IP from which the attack originated. It should be noted that active responses should not be used lightly since snort has a bad behaviour of reporting lots of false positives (e.g., reporting an attack which is not really an attack).

It could also be a good idea to throw in an proxy in front of the webservers to catch some of the bad packets as well, which could also be a possibility to throw in for all of the locally generated webconnections. With a webproxy you can narrow down on traffic used by webtraffic from your employees, as well as restrict their webusage to some extent. As for a webproxy to your own webservers, you can use it to block some of the most obvious connections to get through. A good proxy that may be worth using is the Squid.

Another precaution that one can take is to install Tripwire. This is an excellent last line of defense kind of application, it is generally considered to be a Host Intrusion Detection System. What it does is to make checksums of all the files specified in a configuration file, and then it is run from cron once in a while to see that all of the specified files are the same as before, or have not changed in an illegit way. This program will in other words be able to find out if anyone has actually been able to get through and tampered with the system. A suggestion is to run this on all of the webservers.

One last thing to note is that it is always a good thing to follow standards, as we know. As you have already seen with the ICQ example, if you don't use standardized systems, things can go terribly wrong. For your own environments, this can be ignored to some extent, but if you are running a broadband service or modempool, it gets all the more important. People who connect through you must always be able to rely on your standardization, and you can't expect everyone to run the specific operating system of your choice. Some people want to run Windows, some want to run Linux or even VMS and so on. If you base your security on proprietary systems, you are in for some trouble.

A good example of this is certain broadband services that have popped up in Sweden who base lots of security on Microsoft network logon. This may sound like a great idea to begin with, but once we start considering other operating systems and so on, this is no longer such a good idea. How will someone running Linux get online? Or VAX/VMS? Or HP/UX? With Linux it can be done of course, if it wasn't for the fact that the network administrator refuses anyone to use the broadband service if they are running linux by simply blocking them in such case. However, this book is not a theological discussion of what is best, so let's leave it as an example of why it is a bad idea to use non-standards.


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