At 11:32 -0400 7/29/04, Kevin Darcy wrote:
I would argue that if an application is using the results of a previous DNS lookup which it has stored in memory, then that is "caching" by any meaningful definition of the term. And if an app is caching, we should provide a way for the app to do so in a coherent manner. This implies making the TTL visible. Whether this is in getaddrinfo() or some other API, I'm not qualified to comment.
Yes, it is caching by the application and it is up to the application to decide how long it will internally cache the data. (Will SSH close a connection after 8 hours because it feels the original lookup data is no longer fresh enough?)
The DNS TTL though is not a good measure of freshness - by that I mean freshness at the authoritative source. E.g., on one run an SSH client might get an address via a cache that had to get a fresh authoritative answer. In this case, the DNS TTL might be 10800. On a later run, the same client may get the same address - but following an HTTP client lookup by an hour. In this case, the TTL is now 7200.
What does that mean to the SSH client, other than the data was sitting in an intermediary for an hour? It says nothing about the freshness of the data at the origin. (Note that the client can't determine that it was sitting in the intermediary - there's no indication of how far a TTL has ticked down in a reply. It might be that the source has lowered the TTL. Also, even if the application can be made to decide that it wants fresher data - it can't force the cache to get it. The app would have to do it's own query iteration.)
Like others, I've had problems with apps that "lock on" to a particular IP address. Particularly in dynamically load-balanced scenarios.
I've seen that. I attribute it to poor application design, not a limitation of the interface to DNS (or other lookup service).
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