Books on hacking, cracking, exploiting, and breaking software seem to get all of the attention in the security world. However, we need more works like Mike Schiffman's 'Building Open Source Network Security Tools' (BOSNST). I regret having waited so long to read BOSNST, but I'm glad I did. Schiffman's book is for people who want to build, not break, software, and the way he describes how to create tools is enlightening.The major theme I captured from BOSNST was the importance of creating useful code libraries. Six of the book's 12 chapters focus on libraries which provide functions for application programmers. While not all have gained the same amount of fame or use, the author's approach remains sound. Libraries are the building blocks around which numerous tools can and should be built. This theme helped me understand the evolution of RFP's Whisker CGI scanner, released in Oct 1999 and deprecated in May 2003. Whisker lives on as a library, Libwhisker, in the Nikto Web server scanner. Similarly, Schiffman's chapter on Libsf mentions the utility of creating a library offering the functionality of the popular Nmap scanning tool. (Unfortunately, I haven't seen progress on this. Nmap author Fyodor last mentioned 'Libnmap' in his 2003 Nmap features survey, and it's not apparent in the tool's latest version.)I found the six library chapters to be helpful. Some of the code has stagnated since 2002 (Libnids, Libsf), while some has continued to evolve (Libpcap, Libdnet, OpenSSL). Schiffman provides good explanations of buffer overflow and format string attacks in ch 10, and I thought his state machine-based port scan detector (Descry) in ch 11 was innovative.One of the strongest sections of BOSNST is ch 12, where the author provides a 25-page code walkthrough of his Firewalk tool. This chapter is the model for anyone seeking to explain tool internals. Schiffman offers flowcharts, context charts, and explanations of code snippets. He doesn't simply dump page after page of C code in front of the reader. (Most chapters of BOSNST do conclude with the full source code for sample tools, however.)I have no real complaints with BOSNST. I found minor errors in two diagrams (p 220, 223 should show the SYN/ACK or RST reply coming from the target, not to the target). Schiffman's writing style is clear and engaging, which makes a difference when explaining functions in code. Those who want to learn how to assemble their security expertise in the form code libraries should read BOSNST. Those who wish to use the libraries found in the book, or those with similar functionality, should also read BOSNST. I look forward to Schiffman's next book, where hopefully he will finally update his biography to say 'AFIWC' (for 'Air Force Information Warfare Center') instead of 'AFWIC' (aka the UN's 'AFrican Women In Crisis' program).