Re: (DNS) consensus wording
Maurizio Codogno (mau@beatles.cselt.stet.it)
Wed, 3 Apr 1996 14:44:31 +0200
% From dwm@shell.portal.com Mon Apr 1 18:04:05 1996
% It is my understanding that MUST and SHOULD are defined terms and
% strongly encouraged is not as far as RFCs are concerned. Thus, I
% offer the following editorial alternative to Koen's suggestion (which
% I endorse):
%=20
% If a client caches the result of a DNS lookups, it should observe =
the
% TTL (Time To Live) reported by the DNS server. If the TTL value is
% not available, the client must not cache the result of a DNS lookup
% for longer than XX minutes. In either case, the client must =
immediately=20
% discard a name lookup result if a network error occurs when using =
the=20
% result to initiate a connection.
%=20
% Rationale for other changes:
% 1. I believe this paragraph is about DNS name lookups and should be
% specific
% 2. We don't care what the motivation is for the caching
% 3. I'm not sure that 10 minutes is the right number ... my IPSs tell =
me
% that 24 hours must be allowed for DNS change propigation. Given
% rational expectation for rate of change of the value, I would =
prefer
% a larger number ... or if we have a DNS expert, perhaps there is
% a DNS defined default TTL for cases where not is specified.
% I can live with the 10 minutes but it was a detail which I felt =
should
% surface for expert comment.
RFC 1034 does not say anything about intended time of propagation -=20
it rather specifies that "if a change can be anticipated, the TTL can be
reduced prior to the change", and it can even be set up to 0, meaning =
that
it must not be cached. Of course this should be done only in=20
particular cases (prior to a major change), but I think it is=20
exploited by multi-IP servers to share workload (IMO a bad move - a TTL
of 10 minutes would have been ok)
My own understanding of TTL is that *every* request has a TTL - if not=20
explicitly, it is set up from the MINIMUM field in the Zone Authority =
data.
If DNS lookup is considered a threat for the Net, I would suggest to set
the maximum caching at 30 minutes - I am not sure if is a Good Thing,
however. I'd rely to DNS data.
.mau.