Description:

Libnids is a library that provides a functionality of one of NIDS
(Network Intrusion Detection System) components, namely E-component.
It means that libnids code watches all local network traffic, cooks
received datagrams a bit (quite a bit ;)), and provides convinient
information on them to analyzing modules of NIDS. Libnids performs:
+ assembly of TCP segments into TCP streams;
+ IP defragmentation;
+ TCP port scan detection.
More technical info can be found in MISC file.

So, if you intend to develop a custom NIDS, you don't have to build
low-level network code. If you decide to use libnids, you have got
E-component ready - you can focus on implementing other parts of NIDS.