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==================== libnids-1.24 ==================== 1. Building ----------- Libnids uses libpcap (can be retrieved from http://www.tcpdump.org/release/) and libnet (available at http://www.packetfactory.net/libnet). All credits to autors of these libs. As already mentioned in README, currently libnids will compile on Linux, any *BSD and Solaris. WIN32 port is mantained separately. In order to build libnids, issue "./configure;make" command in top directory. Library files libnids.so and libnids.a should be created in "src" directory. "make install" will install library and header files. You may wish to consult "./configure --help" output for available options. 2. Limitations -------------- In their paper, T. Ptacek and T. Newsham observed that various operating systems implement IP stack differently and can interpret differently the same packet. It means that having seen a IP packet, NIDS has to interpret it with regard to receiving operating system type. A perfect NIDS E-component should possess knowledge on all operating systems network implementation oddities. I don't know any actual NIDS implementation that takes the previous into consideration. Libnids 1.0 was meant to reliably emulate behavior of Linux 2.0.36 kernel. Thanks to libnids testing, some bugs in 2.0.36 networking code were found. One of them enabled an attacker to perform blind TCP spoofing against 2.0.x kernels, including 2.0.36 and 2.0.37 (that is NOT the vulnerability discovered by NAI; see my posting to Bugtrag from beginning of August 99). Info on spotted bugs was submitted to Linux kernel mantainers on 25th May 99 (before the release of 2.0.37), but none of them got fixed. File PATCH contains diffs against 2.0.37, which stop blind spoofing attack and one of data insertion attacks (now its equivalent is incorporated into Solar Designer's secure-linux-0.9 patch). Currently, libnids predicts 2.0.37 behavior as accurately as possible (with some unevitable exceptions - see my postings to Bugtraq from beginning of August 99). In extreme conditions, libnids can incorrectly emulate actions of other operating systems. However, libnids should cope with simple attacks (like these implemented in fragrouter 1.3) targetted at any OS type. All NIDS are vulnerable to DOS attacks. Libnids uses efficient data structures (i.e. hash tables) to minimize risk of CPU saturation. However, all NIDS (including ones based on libnids) has to define some resources (most notably, memory) limits. A determined attacker can attempt to make libnids use up all of its memory, which can result in dropping some data. Libnids will report such condition via its D-component interface. 3. Why does libnids emulate 2.0.x kernel instead of 2.2.x ? ------------------------------------------------------- First of all, libnids development started when 2.0.36 was the current stable kernel. Moreover, some people still prefer to use 2.0.x kernels, one of the reasons being the fact that there is still no Solar Designer non-executable stack patch for 2.2.x (not released oficially until July 99). Finally, 2.2.x kernels are highly configurable during run-time (for instance it's possible to change via proc interface the amount of kernel memory devoted for IP fragments queuing), which generally makes them unpredictable.