qtype=any messages are cached especially in bind9 resolver?

[zhanglikun]

Hello,

When I dig at my bind9 resolver with qtype=ANY,  I find the message never
expire and being refetched until the last rrset expire, if there are several
rrsets in answer section, when one rrset in answer section expires,  the
query client will get one cached message just without the expired rrset.
It's different with unbound and google dns 8.8.8.8, which will refetch the
entire message from authoritative server when any rrset in answer section
expire.

You can do the same test with qname=cnnic.cn, cnnic.cn mx record's ttl is
only 10s.

But, Bind9 resolver will refetch the qtype!=ANY message which have cname and
other type records in answer section, when any rrset expires. 

So my question is why bind9 do like that?



谢谢

Zhang Likun
Software Department, CNNIC 
Phone: 8610-58813250
Email: zhanglikun at cnnic.cn
MSN: kerryzhang1227 at hotmail.com