[rbldnsd] initial IPv6 support

[Michael Tokarev]

Hello.

I completed initial IPv6 support in rbldnsd, in a form of
ip6tset dataset, in similarity with ip4tset (trivial set).

The difficulty with IPv6 is that the address size is large
(so it requires both more memory and complex arithmetic),
and potential amount of addresses is huge too.

Initially I considered using only first half of IPv6 address
for listing, ie, listing by /64 networks (which is a end-user
allocation size).  This is easy, since it will require just
a sorted array of 64bit integers, and all modern computers
are 64bits and supports 64bit integers natively (so comparisons
will be fast).

But it quickly turned to be insufficient.  Imagine a single
end-user /64 allocation with a mail server in there and a lot
of infected machines - we should have a way to whitelist at
least that single mailserver (/128) but keep other zombies
in the same /64 away.

Using /128 (or any size between /65 and /128 actually) for
listings isn't really productive either, since it's completely
controllable by the "bad guys", and it's possible to fill up
all resources on the DNSBL side by using some round-robin
mechanism for all your 2^64 addresses.  This can be detected
and addressed on the collector side somehow by aggregating
"many" listings into larger blocks, so at least in theory
this can be worked around somehow.

So for now, I implemented a first preview/RFC variant, which
has quite some limitations, and which I never ever actually
tested with any real software (like an MTA) - I can perform
queries using a command-line dns client and see adequate
replies, and that's about it for now... ;)

Currently, it has 2 lists: one "main" list, for networks of
size /64 _only_, and another "whitelist", for individual
/128 addresses only.

So it works for a scenario where you want to list whole
end-user /64 but have an ability to whitelist a single
mailserver or two inside.

This is just a test implementation, -- in a somewhat long
time I didn't do any C programming, and this served me as
a excersise to remember how things are done to start with,
so it had to be something smallish.  But at the same time
it should already be useful and I want to hear opinions
from others about what _should_ be done.  So it is quite
possible that the semantics will change or even whole
this dataset be removed later, when real-use scenarious
will be considered.

Current (preview) version can be found at
http://www.corpit.ru/mjt/rbldnsd/rbldnsd-0.996-ipv6-0.tar.gz
and the actual code is in the "ipv6" branch of the git tree,
see http://git.corpit.ru/?p=rbldnsd.git;a=shortlog;h=ipv6

Any comments about how it should be done are apprecated.

Thanks!

/mjt