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Q&A: How does Proxy Service DNS Cache work? When does revalidation occur? Time-to-live, aging, scavenging expiration behavior

Question

Silverline's Application Object Type DNS cache - Time-to-live, aging, scavenging expiration behavior, or: When would revalidation occur?

 

Environment

  • Silverline

    • Proxy / Web Application Object

      • Service Type

        • DNS CACHE

 

Answer

Silverline's DNS caching layer acts authoritative for a given DNS zone. Upon an internet-client query to the proxy for a name in that zone (presuming we've cached a previous query for the same record), we provide the client with the TTL as stated in the original response from the origin, not a live declining TTL from our cache.

This has the effect of potentially extending the sum TTL of the record the client receives. 
For example, the origin provides Silverline with a TTL of 600 for a response to a client, we cache that TTL value and begin the countdown. As new clients query, we continue to provide the static TTL for that record of 600, although our TTL is continuing to decline.

If the origin updates its record, say near our cached TTL expiration or after our cache expires, a client answer could potentially double the TTL that was originally intended and seen as stale.

In short: 
Revalidation occurs after Silverline's expiry of cached value for the duration of the set TTL on the record(s) and the client's observance of Silverline's caching expiry.

 

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