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If a CNAME has a TTL of 1 hour that points to an A record with a TTL of 1 minute, will someone who looks up this CNAME have the entire result cached for 1 hour, or will it will keep having to lookup the A record every 1 minute?

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3 Answers

up vote 1 down vote accepted

You can see that the CNAME and subsequent record have different TTLs by using dig..

 dig docs.nwesd.org

; <<>> DiG 9.5.1-P3 <<>> docs.nwesd.org
;; global options:  printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 28244
;; flags: qr rd ra; QUERY: 1, ANSWER: 3, AUTHORITY: 4, ADDITIONAL: 4

;; QUESTION SECTION:
;docs.nwesd.org.                        IN      A

;; ANSWER SECTION:
docs.nwesd.org.         7200    IN      CNAME   ghs.google.com.
ghs.google.com.         16662   IN      CNAME   ghs.l.google.com.
ghs.l.google.com.       195     IN      A       74.125.95.121

;; AUTHORITY SECTION:
google.com.             32196   IN      NS      ns1.google.com.
google.com.             32196   IN      NS      ns4.google.com.
google.com.             32196   IN      NS      ns3.google.com.
google.com.             32196   IN      NS      ns2.google.com.

;; ADDITIONAL SECTION:
ns1.google.com.         32193   IN      A       216.239.32.10
ns2.google.com.         32193   IN      A       216.239.34.10
ns3.google.com.         70187   IN      A       216.239.36.10
ns4.google.com.         242861  IN      A       216.239.38.10

;; Query time: 102 msec
;; SERVER: 127.0.0.1#53(127.0.0.1)
;; WHEN: Wed Mar 17 18:40:05 2010
;; MSG SIZE  rcvd: 232

To show that you get both the CNAME and the what it is pointed at is returned in a single request to a recursive server.

18:40:05.418435 IP 192.168.32.10.24712 > 216.146.36.113.53: UDP, length 43
        0x0000:  4500 0047 4e58 0000 4011 4e98 c0a8 200a  E..GNX..@.N.....
        0x0010:  d892 2471 6088 0035 0033 aae5 f66c 0100  ..$q`..5.3...l..
        0x0020:  0001 0000 0000 0001 0464 6f63 7305 6e77  .........docs.nw
        0x0030:  6573 6403 6f72 6700 0001 0001 0000 2910  esd.org.......).
        0x0040:  0000 0080 0000 00                        .......
18:40:05.519081 IP 216.146.36.113.53 > 192.168.32.10.24712: UDP, length 243
        0x0000:  4500 010f b93a 0000 3511 eded d892 2471  E....:..5.....$q
        0x0010:  c0a8 200a 0035 6088 00fb 6ceb f66c 8180  .....5`...l..l..
        0x0020:  0001 0003 0004 0005 0464 6f63 7305 6e77  .........docs.nw
        0x0030:  6573 6403 6f72 6700 0001 0001 c00c 0005  esd.org.........
        0x0040:  0001 0000 1c20 0010 0367 6873 0667 6f6f  .........ghs.goo
        0x0050:  676c 6503 636f 6d00 c02c 0005 0001 0000  gle.com..,......
        0x0060:  4116 0008 0367 6873 016c c030 c048 0001  A....ghs.l.0.H..
        0x0070:  0001 0000 00c3 0004 4a7d 5f79 c030 0002  ........J}_y.0..
        0x0080:  0001 0001 11ac 0006 036e 7333 c030 c030  .........ns3.0.0
        0x0090:  0002 0001 0001 11ac 0006 036e 7332 c030  ...........ns2.0
        0x00a0:  c030 0002 0001 0001 11ac 0006 036e 7334  .0...........ns4
        0x00b0:  c030 c030 0002 0001 0001 11ac 0006 036e  .0.0...........n
        0x00c0:  7331 c030 c0a2 0001 0001 0000 7dc1 0004  s1.0........}...
        0x00d0:  d8ef 200a c07e 0001 0001 0000 7dc1 0004  .....~......}...
        0x00e0:  d8ef 220a c06c 0001 0001 0002 0204 0004  .."..l..........
        0x00f0:  d8ef 240a c090 0001 0001 0003 b4ad 0004  ..$.............
        0x0100:  d8ef 260a 0000 2910 0000 0080 0000 00    ..&...)........
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The CNAME should cache for an hour (the alias value), but when the corresponding A is looked up, it will only cache for a total of 1 minute. You're talking about two independent records that are handled separately.

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+1 - Spot on. There will be two trips to the DNS server, one for the CNAME and one for the A, and they will be cached appropriately –  Mark Henderson Mar 18 '10 at 1:30
2  
@Farseeker, that isn't entire true. Typically recursive DNS server will return the both the CNAME, and and the results of the lookup of the CNAME. –  Zoredache Mar 18 '10 at 1:42
    
Well that's lame. I was hoping that since the CNAME was cached for an hour, every request (from one person) for that domain would be cached for an hour - even if the A record it points to is only 1 minute. Because their computer is caching the result for an hour. It just seems strange that it would be behave this way. Are you 100% certain? –  Sean Mar 18 '10 at 16:57
    
While the aliases can be packed into the same DNS response, they're cached independently. Consider the implications if that wasn't the case. Admin updates A record and expects it to take effect right away (perhaps for a migration), and they're forced to wait the duration of the CNAME TTL. That said, it MAY be possible to override that with certain caching NS software and just set your own TTL on your entire cache. –  McJeff Mar 18 '10 at 17:04
    
Ok. That makes sense. Thanks for your help :) –  Sean Mar 18 '10 at 17:57
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Using Google's public DNS servers, the expiration of the A record causes a query of the CNAME record as well, even if the CNAME has a longer TTL.

We painfully experienced that because the DNS provider charged us for DNS queries. The CNAME hosted by the DNS provider had a TTL of several days. The TTL of the A record was hosted on Windows Azure with a TTL of 10 seconds. The DNS provider charged us for 7.5 million queries.

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