CNAME and TTL

[Matus UHLAR - fantomas]

On 09.12.13 16:37, sumsum 2000 wrote:
What i meant was that,
> when a DNS query is made to emp.bbci.co.uk, the record for it is present
>in the cache with TTL of 436. and since it is present, there should not be
>any query to the DNS server,the same goes for emp-live.bbc.net.uk and
>emp.bbci.co.uk.edgesuite.net
>
>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.
>
> For all these three CNAME, there should not be  any lookup that needs to
>be  sent to the DNS server, as they are present in the cache and their TTL
>entries have not expired and are present in the cache. Only when it comes
>to the last chain a336.g.akamai.net, which expires after TTL of 6 and when
>is not present in the cache,  the DNS query for this should be sent for
>lookup.
>
>But I see that though the entries for CNAME's are present in the cache,
>the look up happens again for the CNAME,

how do you know? do they appear again with higher TTL? Did you sniff the
network to see they are looked up again?

it looks correct here:

uhlar at fantomas% dig +noauth +noadd +nostats +nocmd +nocomments emp.bbci.co.uk.
;emp.bbci.co.uk.                        IN      A
emp.bbci.co.uk.         796     IN      CNAME   emp-live.bbc.net.uk.
emp-live.bbc.net.uk.    196     IN      CNAME   emp.bbci.co.uk.edgesuite.net.
emp.bbci.co.uk.edgesuite.net. 2632 IN   CNAME   a336.g.akamai.net.
a336.g.akamai.net.      10      IN      A       80.239.149.26
a336.g.akamai.net.      10      IN      A       80.239.247.14

uhlar at fantomas% dig +noauth +noadd +nostats +nocmd +nocomments emp.bbci.co.uk.
;emp.bbci.co.uk.                        IN      A
emp.bbci.co.uk.         740     IN      CNAME   emp-live.bbc.net.uk.
emp-live.bbc.net.uk.    140     IN      CNAME   emp.bbci.co.uk.edgesuite.net.
emp.bbci.co.uk.edgesuite.net. 2576 IN   CNAME   a336.g.akamai.net.
a336.g.akamai.net.      20      IN      A       80.239.149.26
a336.g.akamai.net.      20      IN      A       80.239.247.14

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