On Tue, 2011-10-18 at 20:24 -0400, David F. Skoll wrote:
> On Tue, 18 Oct 2011 23:55:41 +0200, Karsten Bräckelmann wrote:
>
> > The DNS TTL appears to be 12 hours, and a good share of mail
> > (definitely true for ham, only partly for spam) is received from a
> > rather limited number of distinct SMTP servers, only. With a local,
> > caching DNS server the number of mail a system can handle per day
> > before exceeding the free usage limit is *much* higher.
>
> > number of mail != number of DNS lookups
>
> That's true, though caching is much less effective than you may
> suppose. In real-life measurements on real mail servers, I found a
> very low cache hit rate for common DNS{B,W}Ls, on the order of only
> 25-50% hits.
As in cache hits? That's quite substantial.
Also, is this overall, somehow a mix of both black and white-lists, as
well as different types (IP vs URI)? Given the very different TTL for
different types of lists, I suspect actual cache hit rates vary a lot.
Your users and their peers can make a huge difference, too.
And of course other related filtering, like blocking at SMTP level.
--
char *t="\10pse\0r\0dtu\0.@ghno\x4e\xc8\x79\xf4\xab\x51\x8a\x10\xf4\xf4\xc4";
main(){ char h,m=h=*t++,*x=t+2*h,c,i,l=*x,s=0; for (i=0;i<l;i++){ i%8? c<<=1:
(c=*++x); c&128 && (s+=h); if (!(h>>=1)||!t[s+h]){ putchar(t[s]);h=m;s=0; }}}
