On Mar 3, 2008, at 1:51 PM, Jim Fenton wrote: > Dave Crocker wrote: >> G'day, >> >> While reviewing the posts in response to the iab draft, I am >> finding myself unclear about the reasons for Step 2, of the query >> procedure in ASP's Section 4.2.2. I'm pretty sure this is not >> merely a caffeine-deficiency-based question... >> >> What is the functional or security reason for verifying that >> the domain exists, in terms of ASP. >> >> I can imagine obvious reasons, outside of ASP, but those would not >> need to be documented in the ASP. >> >> At the least, it would help to have the document include text that >> explains the benefit of this step. > > Where this came from, as I remember, is the translation from a new > RR type to a prefixed TXT query. If ASP is published using its own > RR type, one can do a query that gets the ASP record, and find out > whether the domain exists at all. If you substitute a prefixed TXT > query, you need to do a query for the domain itself, without the > prefix, if you want to find this out.
A test should be made querying an MX record at the domain in question. If the domain does not exist, there is no reason to check the parent domain for policy records. By mandating use of MX records when publishing the policy records, this climb up the domain tree can be completely avoided. : ) > Checking for the existence of the domain is clearly a useful thing > to do, but it could be considered out of scope for ASP to check for > the existence of the domain, since a non-existent domain naturally > does not have a signing practices record (and we already know that). But avoiding a policy check at the parent would be in scope, would it not? > Another justification might be caching, but I'd need to find out > more about how negative caching works: would a negative response to > _asp._domainkey.nonexistent.example.com result in a negative cache > entry for nonexistent.example.com? If so, step 2 might occur very > quickly (and locally), potentially eliminating the step 3 query for > the parent, which would probably not be cached. See RFC 2308. Negative caching is not as certain as positive caching. This is why the MX record should be checked prior to walking up the domain tree. Negative caching should be retrained as long as the MINIMUM field of the SOA record or the TTL of the SOA itself, whichever is less. Not all resolvers are RFC 2308 compliant. Some truncate the duration of negative caching to limit the effects of transient network failures. -Doug _______________________________________________ NOTE WELL: This list operates according to http://mipassoc.org/dkim/ietf-list-rules.html
