[RndTbl] DNS propagation

Sean Walberg sean at ertw.com
Mon Dec 18 19:03:57 CST 2006

It depends on the nameserver asking the question.

The authoritative nameservers (master/secondaries) are going to instantly
give the correct answer as soon as you change it.  If a remote DNS server
asks, it's first going to consult its cache, and failing that, recursively
resolve the name.  If the record is in the cache, it won't change until the
record times out**.  If it isn't, the correct answer is returned instantly.

So, how to find out if it's cached?  Our friend dig to the rescue:

# dig ertw.com @ns1.ertw.com

; <<>> DiG 9.3.1 <<>> ertw.com @ertw.com
; (1 server found)
;; global options:  printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 8220
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 2, ADDITIONAL: 0

;ertw.com.                      IN      A

ertw.com.               4079    IN      A

ertw.com.               7200    IN      NS      ns14.zoneedit.com.
ertw.com.               7200    IN      NS      ns12.zoneedit.com.

You can see that the A record for ertw.com has been cached locally -- the
ttl is 4079 seconds as of my query.  However the 2 NS records were not
cached, so they are pulled from the authoritative server and now have a 7200
second lifetime.  I'll ask again in a few seconds (just the relevent lines

ertw.com.               3898    IN      A
ertw.com.               7019    IN      NS      ns14.zoneedit.com.
ertw.com.               7019    IN      NS      ns12.zoneedit.com.

As you can see, the server continues to count down.  It's not for another
3898 seconds that my local server would notice that ertw.com has changed.

** Some DNS servers have been known to ignore the TTL on a record and make
up its own value.  I've only run into this when setting absurdly low TTLs (<
300s) but I wouldn't be surprised if there were some that always ignored it.


On 12/18/06, Montana Quiring <montanaq at gmail.com> wrote:
> Hello,
> I'm interested in hearing what people's experiences are.
> When changing the IP of a (web) server on the Internet, how long does
> it take to DNS to propagate?
> -Montana
> _______________________________________________
> Roundtable mailing list
> Roundtable at muug.mb.ca
> http://www.muug.mb.ca/mailman/listinfo/roundtable

Sean Walberg <sean at ertw.com>    http://ertw.com/
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