DNS resolution

[Doug Hardie]

> On 27 February 2020, at 22:22, Dewayne Geraghty <dewayne at heuristicsystems.com.au> wrote:
> 
> On 28/02/2020 11:34 am, Doug Hardie wrote:
>> I have encountered an inconsistent result in DNS resolution that I am unable to explain.  The situation is three servers with completely different IP addresses on three different internet connections.  All three are via different companies.  In the DNS records for the domain I have an A record for each server with the name A.domain, B.domain, and C.domain.  Each points to the appropriate IP address.  The name www.domain (and also just domain) have 3 A records: one for each of the 3 servers.  The goal is to have a fail over such that A is the primary server, and if it fails, then try B, and finally C.  Load balancing is not desirable because the web transactions require numerous exchanges and need to all use the same server.
>> 
>> On Frontier, Spectrum, and Charter connections, host www.domain lists the 3 servers in the order they are defined in the DNS records.  Every time I run host I get the same result.  However, there is one PC running windows 10 that connects via the charter supplied modem and is configured for DNS at the modem address that gets the addresses in random order.  It appears to be trying to do load balancing.  When I use Charter's official DNS servers, I don't see that.  But if I use the modem then even FreeBSD 11.1 gets the random results.
>> 
>> The spectrum interface has the same issue.  The official DNS servers are consistent, the modem as a DNS server is random.  I thought the TTL for the domain would be honored by the DNS servers.  That doesn't always appear to be the case, or the modem DNS servers are rotating their responses from their cache.  However, shouldn't the client computers cache the DNS response?  Windows does not appear to be doing that.
>> 
>> 
> Yes, Windows boxes cache the results, to check, open cmd prompt
> ipconfig /displaydns
> 
> I'd suggest that the modem isn't caching and rotating dns responses as
> expected.

I can't see what DNS servers the modem is using, but I suspect they are the "official" ones per the ISP.  When I use those directly, there is no rotating.  When I go to the modem with either windows or FreeBSD then I get rotation.  I don't see where else the rotation could be occurring.  

-- Doug