CNAME and TTL

[Matus UHLAR - fantomas]

On 06.12.13 15:52, sumsum 2000 wrote:
>I am having only a forward only option in bind configuration.
>When i dig on some host which has CNAME, the cache contains  a longer TTL
>for the CNAME than the TTL for the  final resolution of the IP.

Yes, every record has its own TTL, including every record in CNAME chain.

>However, in the example below, the CNAME  is  queried again when the TTL
>for a336.g.akamai.net. is up.

Pardon?

>I was expecting that the TTL for CNAME will be used from the cache and the
>lookup will not happen again until the TTL has not expired.

When a name is queried, query is processed always the same way - every name
of a chain is validated again and missing/expired names are resolved again.

Note that records can removed from memory even without expiring, e.g. when
memory is full.

>Is there a way to avoid additional lookups when the value is already
>present in the cache for CNAME entries.

not without violating DNS standard.

>;; ANSWER SECTION:
>emp.bbci.co.uk. 436 IN CNAME emp-live.bbc.net.uk.
>emp-live.bbc.net.uk. 253 IN CNAME emp.bbci.co.uk.edgesuite.net.
>emp.bbci.co.uk.edgesuite.net. 3368 IN CNAME a336.g.akamai.net.
>a336.g.akamai.net. 6 IN A 58.27.124.225
>a336.g.akamai.net. 6 IN A 58.27.124.200

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