[Dnsmasq-discuss] dnsmasq drops CNAME cache entries when CNAMEs collide

[Simon Kelley]

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Answered here:


http://lists.thekelleys.org.uk/pipermail/dnsmasq-discuss/2017q1/011068.h
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Cheers,

Simon.

On 02/02/17 11:23, Aaron Jacobs wrote:
> Hi all,
> 
> I've found what seems to me an odd behavior in dnsmasq related to
> CNAME caching, and am wondering if it's a bug, or just a subtlety
> of the DNS standard that I don't understand.
> 
> I'm using dnsmasq 2.76, invoking in a way that should enable
> caching:
> 
> % ./src/dnsmasq -v Dnsmasq version 2.76
> 
> % sudo ./src/dnsmasq --no-resolv --server= --keep-in-foreground 
> --no-hosts --bind-interfaces --listen-address=127.0.2.1 
> --cache-size=400 -dq
> 
> Normally it caches fine. The first query for a domain takes awhile,
> and the second takes no time at all:
> 
> % dig @127.0.2.1 photos.yahoo.com ; sudo killall -USR1 dnsmasq 
> [...] ;; ANSWER SECTION: photos.yahoo.com.       299     IN
> CNAME   rc.yahoo.com. rc.yahoo.com.           299     IN      CNAME
> src.g03.yahoodns.net. src.g03.yahoodns.net.   155     IN      A
> 98.137.236.150
> 
> ;; Query time: 144 msec ;; SERVER: 127.0.2.1#53(127.0.2.1)
> 
> % dig @127.0.2.1 photos.yahoo.com ; sudo killall -USR1 dnsmasq 
> [...] ;; ANSWER SECTION: photos.yahoo.com.       289     IN
> CNAME   rc.yahoo.com. rc.yahoo.com.           289     IN      CNAME
> src.g03.yahoodns.net. src.g03.yahoodns.net.   145     IN      A
> 98.137.236.150
> 
> ;; Query time: 0 msec ;; SERVER: 127.0.2.1#53(127.0.2.1)
> 
> Note that photos.yahoo.com has a CNAME of rc.yahoo.com, which in
> turn has a CNAME of src.g03.yahoodns.net. When I query another
> domain with that CNAME, it appears to kill the cache entries for
> those CNAMES; the next query for photos.yahoo.com is slow again,
> clearly not hitting cache:
> 
> % dig @127.0.2.1 www.flicker.com ; sudo killall -USR1 dnsmasq 
> [...] ;; ANSWER SECTION: www.flicker.com.        1799    IN
> CNAME   rc.yahoo.com. rc.yahoo.com.           299     IN      CNAME
> src.g03.yahoodns.net. src.g03.yahoodns.net.   264     IN      A
> 124.108.105.150
> 
> ;; Query time: 143 msec ;; SERVER: 127.0.2.1#53(127.0.2.1)
> 
> % dig @127.0.2.1 photos.yahoo.com ; sudo killall -USR1 dnsmasq 
> [...] ;; ANSWER SECTION: photos.yahoo.com.       299     IN
> CNAME   rc.yahoo.com. rc.yahoo.com.           299     IN      CNAME
> src.g03.yahoodns.net. src.g03.yahoodns.net.   299     IN      A
> 124.108.105.150
> 
> ;; Query time: 147 msec ;; SERVER: 127.0.2.1#53(127.0.2.1)
> 
> Here are the cache dumps after each query above:
> 
> ======= dnsmasq: Host                                     Address 
> Flags      Expires dnsmasq: photos.yahoo.com
> rc.yahoo.com CF         Thu Feb  2 22:13:43 2017 dnsmasq:
> src.g03.yahoodns.net           98.137.236.150 4F         Thu Feb  2
> 22:11:19 2017 dnsmasq: rc.yahoo.com
> src.g03.yahoodns.net CF         Thu Feb  2 22:13:43 2017 ======= 
> dnsmasq: Host                                     Address Flags
> Expires dnsmasq: photos.yahoo.com               rc.yahoo.com CF
> Thu Feb  2 22:13:43 2017 dnsmasq: src.g03.yahoodns.net
> 98.137.236.150 4F         Thu Feb  2 22:11:19 2017 dnsmasq:
> rc.yahoo.com                   src.g03.yahoodns.net CF         Thu
> Feb  2 22:13:43 2017 ======= dnsmasq: Host
> Address Flags      Expires dnsmasq: photos.yahoo.com CF         Thu
> Feb  2 22:13:43 2017 dnsmasq: src.g03.yahoodns.net
> 124.108.105.150 4F         Thu Feb  2 22:13:22 2017 dnsmasq:
> rc.yahoo.com                   src.g03.yahoodns.net CF         Thu
> Feb  2 22:13:57 2017 dnsmasq: www.flicker.com
> rc.yahoo.com CF         Thu Feb  2 22:38:57 2017 ======= dnsmasq:
> Host                                     Address Flags
> Expires dnsmasq: photos.yahoo.com               rc.yahoo.com CF
> Thu Feb  2 22:14:00 2017 dnsmasq: src.g03.yahoodns.net
> 124.108.105.150 4F         Thu Feb  2 22:14:00 2017 dnsmasq:
> rc.yahoo.com                   src.g03.yahoodns.net CF         Thu
> Feb  2 22:14:00 2017 dnsmasq: www.flicker.com CF         Thu Feb  2
> 22:38:57 2017
> 
> It appears that when dnsmasq discovers the CNAME for
> www.flicker.come, it knocks the CNAME for photos.yahoo.com out of
> the cache (and vice versa). Is this intended behavior?
> 
> Thanks, Aaron
> 
> _______________________________________________ Dnsmasq-discuss
> mailing list Dnsmasq-discuss at lists.thekelleys.org.uk 
> http://lists.thekelleys.org.uk/mailman/listinfo/dnsmasq-discuss
> 
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