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Accepted
Sean
Asked: 2010-03-18 17:02:23 +0800 CST

CNAME and A record have different TTLs. Which one will be cached?

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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?

domain-name-system cname-record ttl a-record

3 Answers

  1. 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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  2. 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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  3. Best Answer

    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  [email protected].....
        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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