Detect port scanning without psad - write own IDS

[wakatana]

Hi gurus,

1st:
Is possible to detect port scanning just by using utilities included in linux (netstat, iptables...), Yes there is utility called psad but I would write some scripts for my own and learn something new

2nd:
Could you point me to good tutorial for writing own Intrusion Detection System?
I am not a C++ geeg, so first thing that comes to my mind is that i will use simple BASH SCRIPT that will interact with iptables and monitor user activity, network activity etc. (but I thing that would not be good solution due to performance and capabilities, and also I am not familliar with memory in linux at all).
I was reading aboud SNORT HIDS, NIDS...
AFAIK some information can be obtainet from /proc but I have no idea which values should be monitored.
Could you please point me to some good resources It would be good if there would be some example code.

Thanks a lot

[jippie]

tcpdump

it has extensive manual page docs: check at least the man pages for tcpdump and pcap-filter.

From a security point of view, I really doubt it is very wise to program and trust on your own IDS, but I'll leave the boring preaches to another poster.