Securing Your Network With OpenBSD

By Kurt Seifried, [email protected]


This isn't a totally conventional article. I'm not going to be recommending you replace every UNIX server you've got with OpenBSD.  Although the OpenBSD team did release 2.7 just last week, making it very hard not to cover its new features, this article will be a bit of both. 

Most networks share a few common traits:

So how can OpenBSD help with these problems? Well, first off: if you plan to track, probe and monitor your network from a central machine, it had better be secure. If someone breaks into these boxes, chances are you won't notice their activity on the network since it looks like normal internal scans/etc. The data collected by these machines will tend to be sensitive (i.e. a list of all your mail servers and what versions of Sendmail they are running). OpenBSD is the perfect candidate for this type of work; it's got solid security and an extremely capable network stack (important if you processing and generating a lot of network traffic. One major new upgrade with OpenBSD is support for gigabit network cards, so you can attach it to anything from the slowest LAN to the fastest LAN. Once you have installed your OpenBSD boxes, you need various software packages to aid you in your quest for network security. OpenBSD makes this trivially easy to do with the ports collection. Instead of shipping pre-compiled packages and having some huge central repository that attempts to contain them all, the BSD family uses the ports system. The ports system is essentially a gigantic collection of software packages, with comments, a description, MD5 sums of the tarballs, and makefiles (detailed instructions for the compiler on how to build the package). You simply find the package you want, run "make" to compile it, and then "make install" to create an install package and install it. If you want to update something, simply get a newer ports file and redo the process. This process is also very difficult to subvert. Since the ports package ships with MD5 (Message Digest version 5), a relatively secure cryptographic hash function, the chances of an attacker being able to create a source code package with the same MD5 sum are very slim. So which packages are useful for securing your network? I'll break them down into several main areas:

I'm not too sure of the terminology since I've seen varied usage, so I'll explain each area. "Port/network scanning programs" in my mind are programs that either probe IP addresses or TCP/UDP/ICMP ports to see if services are available or not. This also includes programs like Firewalk which attempts to map out ports a firewall is allowing and blocking. "Intrusion/penetration scanning programs" are programs that test for known vulnerabilities, For example, looking at the version of Sendmail a server has and telling you if it is vulnerable or not, and optionally may actually try to exploit the problem to see if it works; this can of course result in problems. The last is "General network management", tools like MRTG, Multi Router Traffic Graph, which helps you spot unusual network behavior and can also help you plan out future needs. The latest copy of the snapshot file is: /pub/OpenBSD/snapshots/ports.tar.gz and I would recommend it over the copy in /pub/OpenBSD/2.7/ simply because a lot of the software gets frequent updates; with software, especially security related software, it's important to be up to date. Simply download the ports file and unpack it in the /usr/ directory (i.e. cd /usr ; tar -zxf ~/ports.tar.gz).

My favorite port scanning program is by far nmap. This thing has dozens of options and can conduct a wide variety (probably the widest) of scans from simple SYN scans to bouncing very strange packets off of open ftp servers. You will find nmap in the net/nmap/ directory; it should compile without any problems. The man page for nmap is exhaustive, options include controlling the speed of scans (useful for slipping past IDS systems, or stress testing firewalls), generating random IP's to test (useful for creating statistics and "discovering" servers). Scanning from a relatively slow machine, like sub P100, running OpenBSD you can easily scan a class B address for most interesting ports in a day or two. Your main limits will be bandwidth and handling opening ports. This means most organizations can easily afford to scan their entire network daily. This can be very beneficial to security; by comparing daily reports of the open and closed ports, you can track additions of new servers and ensure they are secured properly. Simply comparing daily files with diff or uniq can generate a relatively comprehensive report of what services are available on your network. Another scanner is strobe; while somewhat more limited then nmap for options, it is blazingly fast and simple to use, you can find it in security/strobe/. Now that you have a list of open ports (and generally speaking they will map to the expected services, but not always), you can now do some more in- depth probing.

Probably the best free network security scanner is Nessus. Using a client / server architecture, it is very platform independent. With the server running on UNIX and NT, the client portion runs on UNIX, Windows and in Java. There are over 400 tests, written in a custom plug-in language which makes the writing of new tests extremely easy, and additional information such as the solution can be included in the test script. The reports it generates are also very nice, with solutions (if offered in the test script) and output formatted in HTML, text, LaTeX, and ".nsr" format (which can be read in by Nessus clients and used to view the data). The tests also include denial of service attacks, so be careful when running Nessus against production machines. However, it is probably a wise idea to hammer production machines during off time, since attackers will probably hit them during prime time. For pure CGI scanning (useful if you are a web hosting provider or similar), the tool of choice would be whisker, in security/whisker/. Whisker is a pure CGI scanner with a lot of nifty options and features; it's ideal for finding out what you left enabled on the new web server. Whisker comes with a database of scans to attempt and you can easily add items, so if you purchase some new web based product you can easily scan it for problems.

With all this cool scanning going on you probably want to try to avoid detection by IDS systems and firewalls. By using fragrouter in security/fragrouter/ you can severely mangle packets and slip them past many systems. Additionally, some systems will crash under the stress of fragmented packets. It's better to find out on your own than let an attacker exploit it. Many older IDS systems did not properly, if at all, check fragmented packets; this is an ideal way of finding IDS systems that need to be updated in your organization.

This last network scanning tool didn't really fit into network scanning so I stuck it out on its own. Oftentimes when an attacker gains access to a machine on a network, the first tool they will install is a network sniffer. This allows them to suck up passwords and other interesting data easily. Antisniff is ideal for detecting these types of network sniffers (of course it isn't 100% accurate, but it's a whole lot better than nothing). Antisniff will not detect hardware based sniffers or induction taps or other "sophisticated" methods of sniffing, but as a rule of thumb, these require physical access to the network, which prevents most attackers from using them.

In conclusion, an OpenBSD box or two loaded up with free software can help you secure your network and make collecting information significantly easier. If possible, try to have an OpenBSD box on each network segment between firewalls so you can completely monitor the network, also have an offsite box or two that will allow you to conduct scans with access equivalent to what most attackers would have. Old cast-off Pentium machines are perfect for the task, and since the software is free, usually the main obstacle is getting permission to conduct potentially hazardous scans on your network(s). Probably the easiest way to convince management is by pointing out that attackers will do it anyway, and if you are feeling particularly brave to do it and present them with the findings (and the fact that you probably went undetected). Next week I'm going to look at various methods for placing backdoors in machines, and methods for preventing/detecting/disabling them.


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Last updated 15/2/2002

Copyright Kurt Seifried 2002