Speeding up DNS change propagation

[John Miller]

The .com nameservers don't know anything about ftp.example.com; they
just know the nameservers for example.com.  So have no fear -- BIND
will not cache an upstream response for ftp.example.com: you'll only
hear about ftp.example.com from the example.com nameservers.

Pretty much all upstream nameservers: root NSs, .com NSs, example.com
NSs--are authoritative-only.  They don't cache or offer cached
responses.  (Not 100% accurate, but nearly always so.)

John

On Fri, Sep 18, 2015 at 2:58 PM, Danny Sinang <d.sinang at gmail.com> wrote:
> As a follow-up to your answer for question #2, after my clearing the cache
> or restarting BIND, won't BIND find an old cache of "ftp.example.com" in the
> ".com" top level DNS server ?
>
> Regards,
> Danny
>
> On Fri, Sep 18, 2015 at 2:51 PM, John Miller <johnmill at brandeis.edu> wrote:
>>
>> On Fri, Sep 18, 2015 at 2:35 PM, Danny Sinang <d.sinang at gmail.com> wrote:
>> > Hi,
>> >
>> > Our vendor is changing their FTP server's IP address tomorrow.
>> >
>> > 1. How can I tell how long their DNS change will propagate to us ?
>>
>> Whatever TTL you have cached when the vendor makes the switch is how
>> long it'll take for your caching servers to pick up the change.
>>
>> >      a. Do I just run dig a "ftp.example.com" and look for the TTL for
>> > that
>> > DNS entry ?
>> >      b. Every time I run that command, the TTL is shrinking. How do I
>> > find
>> > out the full TTL for it ?
>>
>> If you want to know the full TTL, ask the company's NSs directly -
>> authoritative servers only give out the full TTL.
>>
>> > 2. Can I just restart BIND tomorrow to clear its cache and force it to
>> > query
>> > the "example.com" name server for "ftp.example.com" (so as not to wait
>> > for
>> > the propagation to reach us) ?
>>
>> Sure can.  Depending on your BIND version, you can also run rndc
>> flushname <name> and it'll clear just that name from your cache.
>>
>> If the TTL is very long, don't forget about client-side caching as
>> well.  Windows and OS X cache DNS lookups by default.
>>
>> John
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>



-- 
John Miller
Systems Engineer
Brandeis University
johnmill at brandeis.edu
(781) 736-4619