TTLs on A records?

Barry Margolin barmar at
Thu Apr 30 23:43:52 UTC 2009

In article <gtd8nt$1vdr$1 at>,
 "online-reg" <online-reg at> wrote:

> Hi All:  I'm running Bind 9.5.0-P1 / Fedora on my primary NS.
> Are TTLs on individual A records universally supported?

They're supposed to be.  Many DNS-based load balancing systems and 
services depend on it.

> I have a domain with a TTL of "3h", and I wanted to route traffic between 
> two servers in that domain quickly, so I set the TTL to the A record like:
> www    300    A
> ;www    300    A
> so I could uncomment one and comment the other to manually switch between 
> them.
> I've had that setup for several weeks during testing...and I just reversed 
> the records, incremented the serial, and reloaded BIND.
> On my secondary NS (Bind 9.5.0-P1 / Freebsd 7), when I dig the "www" record, 
> I see the TTL counting down from 300 (Cool!), and after it reaches "0", the 
> IP address resets to the new one....perfect!

A slave server is authoritative, not caching, so it shouldn't count down 
the TTL at all.  Or did you mean something else when you said "secondary 

> On my Windows DC (server2008), the change was also picked up after 5 
> minutes.
> When I use some other lookup services, however (like, the old 
> IP address shows up for much it's caching it and ignoring the 
> TTL for the record. Should I expect that behavior?

No.  Maybe the web site itself is caching.  Try querying your ISP's DNS.

Barry Margolin, barmar at
Arlington, MA
*** PLEASE don't copy me on replies, I'll read them in the group ***

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