Subject: [Uta] MTA-STS & max-age
From: "A. Schulze" <[email-masked]>
To: [email-masked]
Date: Thu, 15 Aug 2019 23:03:19 +0200
List-Id: UTA working group mailing list <uta.ietf.org>

Hello,

reading RFC 8461 again and again I find not answer on "is there is a minimum value?"

https://tools.ietf.org/html/rfc8461#section-3.2 say

      "max_age": Max lifetime of the policy (plaintext non-negative
      integer seconds, maximum value of 31557600).  Well-behaved clients
      SHOULD cache a policy for up to this value from the last policy
      fetch time. To mitigate the risks of attacks at policy refresh
      time, it is expected that this value typically be in the range of
      weeks or greater.

The RFC define a maximum but only an expectation for the minimum :-/

There are not as many implementations in use. What does implementers think on this?
How do you handle max_age > 31557600 and do you require any minimum value for max_age?

postfix-mta-sts-resolver for example require only a value >= 0 .
https://github.com/Snawoot/postfix-mta-sts-resolver/blob/c7b3d179fb10277f9bcdc77e7cd91627c879a48b/postfix_mta_sts_resolver/resolver.py#L144

Andreas