Reverse IPv6 caches well. You just can't pre-populate servers with PTR
records for all 2^64 ptr records in a normal IPv6 subnet. You need to
use tools that add records for nodes that actually exist. Those tools
are a decade old now.
Over in e-mail land, we've been pondering the behavior of spammers, who
will likely hop to a different IPv6 address for every spam. If you do rDNS
lookups, your cache will fill up with useless entries, maybe PTR, maybe
NXDOMAIN, it hardly matters. DNSBLs and DNSWLs, if done the same way as
they are in IPv4, have the same problem. These issues are well known in
the mail ops community, where it's now the standard advice not to try rDNS
lookups on incoming IPv6 mail.
John Levine, johnl(_at_)iecc(_dot_)com, Primary Perpetrator of "The Internet for
Please consider the environment before reading this e-mail. http://jl.ly
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