Current exclusions list from --excludefile takes linear time to match
against. Using a trie structure, we can do matching in O(log n) time,
with a hard maximum of 32 comparisons for IPv4 and 128 comparisons for
IPv6. Each node of the trie represents an address prefix that all
subsequent nodes share; matching stops when one is matched exactly or
when the candidate address does not match any prefix of the addresses in
the trie.
For now, only numeric addresses without netmask are supported. We plan
to extend this to addresses with netmasks, including resolved names.
Storing IPv4 ranges and wildcards in this structure would be
prohibitively complex, so the existing linear match method will be used
for those. It is unlikely that any users are using large exclusion lists
of these types of specifications, so performance impact is small.
Potential future features could use the trie structure to implement
custom routing or scope-limiting.
This was a todo list item based on this report:
https://seclists.org/nmap-dev/2012/q4/420
324965d1d2