Re: [LEAPSECS] Proposed use of DNS to Disseminate Leap Second Information

From: Clive D.W. Feather <>
Date: Fri, 25 Apr 2003 08:57:41 +0100

Ed Davies said:
> A while ago I mentioned in this list an idea for using the
> Internet Domain Name System to disseminate leap second
> information.


Brief comments:

(1) Yes, it's an abuse of DNS. The fact that others do it doesn't justify

(2) If you're going to use DNS, the information ought to be in a TXT record
rather than playing games with A records. The way that blacklists work is
because someone glommed on to existing code in sendmail, rather than
thinking propoerly about the problem.

(3) It doesn't provide an easy way to find out when the last and next
leap seconds are, requiring you to do binary search or linear search (and I
bet many people are lazy enough to do the latter). There's two obvious
approaches to this:
  (A) Have a second sub-hierarchy that returns that information.
  (B) Return more than one A record for each month in your main hierarchy.
  For example, have 127.2.N.P mean "the previous leap second was at the
  end of the month P months ago, and the next one will be at the end of the
  month in N months time" (N >= 0, P > 0). Use P = 254 for "this is before
  the first leap second" and N = 254 for "no more scheduled so far". Use
  255 for "this field is invalid".

(4) To be future-proof you need to deal with IPv6. IPv6 only has one
loopback address. However, it also has two ways of embedding IPv4
addresses into IPv6 addresses. I suggest that you use:

    ::FFFF:7Faa:bbcc aka

(that is, the equivalent IPv4 address with 80 zeros and 16 ones prefixed).

Clive D.W. Feather  | Work:  <>   | Tel:    +44 20 8495 6138
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Received on Fri Apr 25 2003 - 00:57:55 PDT

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