[CentOS] how to increase DNS reliability?

Thu Jul 25 15:14:47 UTC 2019
Nataraj <incoming-centos at rjl.com>

On 7/25/19 6:48 AM, rainer at ultra-secure.de wrote:
> Am 2019-07-25 15:41, schrieb hw:
>> On 7/25/19 2:53 PM, rainer at ultra-secure.de wrote:
>>> Am 2019-07-25 14:51, schrieb hw:
>>>> Hi,
>>>> how can DNS reliability, as experienced by clients on the LAN who are
>>>> sending queries, be increased?
>>>> Would I have to set up some sort of cluster consisting of several
>>>> servers all providing DNS services which is reachable under a single
>>>> IP address known to the clients?
>>>> Just setting up several name servers and making them known to the
>>>> clients
>>>> for the clients to automatically switch isn't a good solution because
>>>> the clients take their timeouts and users lacking even the most basic
>>>> knowledge inevitably panic when the first name server does not answer
>>>> queries.
>>> Run a local cache (unbound) and enter all your local resolvers as
>>> upstreams.
>> That can fail just as well --- or be even worse when the clients
>> can't switch
>> over anymore.  I have that and am avoiding to use it for some clients
>> because
>> it takes a while for the cache to get updated when I make changes.
>> However, if that cache fails, chances are that the internet
>> connection is also
>> down in which case it can be troublesome to even get local host names
>> resolved.
>> When that happens, trouble is to be expected.
> Anything else is - IMHO - much more work, much more complicated and
> much more likely to fail, in a more spectacular way.
> Especially all those keepalive "solutions".
> I have found that I need to restart unbound if all upstreams had failed. 

Configure all dns servers as primary slaves (plus 1 primary master) for
your own domains.  I have never seen problems with resolution of local
dns domains when the Internet was down.

Depending on the size of your network, you can run a caching server on
each host (configured as a primary slave for your own domains) and  then
configure that local server to use forwarders.  When you use multiple
forwarders the local server does not have to wait for timeouts before
querying another server.  Then you just run 2 or more servers to use for
forwarding.  Use forward-only to force all local servers to use only
forwarding (for security and caching reasons).  Much simpler than using
keepalived.  In recent years I *have not had any* problems with bind9 or
powerdns crashing.

As far as using the ISC server vs powerdns, you may want to check on
peoples recent experiences.  There was a time when many thought powerdns
had much better performance and fewer security issues.  For various
reasons  I've seen some people including myself, switch back to ISC
bind9.  I switched about 1.5 years ago because I was getting better
performance from bind9.  You may want to check out other peoples
experience before switching to powerdns.