How to stop first DNS lookup waiting for 5 seconds for IPV6 to fail?

[Chris Green]

I have two systems running [x]ubuntu 19.10, both of them suffer from
this problem.  The first lookup of a LAN host gets the IPV4 result
quickly but the IPV6 lookup waits for 5 seconds before failing.

Subsequent lookups run quickly because the results are in the local
cache.  Strangely *all* LAN lookups are speeded up after the first
one, not just the first one tried.  If I reboot the client system then
the first lookup is again slow, the slow lookup also happens again
after a time (presumably the DNS cache 'life').

So, for example, I have a hosts called 'esprimo' and '2860n' on my
LAN. Here are some timings for lookups:-

    chris at backup$ time host esprimo
    esprimo.zbmc.eu has address 192.168.1.3

    real    0m5.288s
    user    0m0.000s
    sys     0m0.016s
    chris at backup$ time host esprimo
    esprimo.zbmc.eu has address 192.168.1.3

    real    0m0.020s
    user    0m0.004s
    sys     0m0.013s
    chris at backup$ time host 2860n
    2860n.zbmc.eu has address 192.168.1.1

    real    0m0.017s
    user    0m0.014s
    sys     0m0.000s
    chris at backup$ 


Then I reboot 'backup' and:-

    chris at backup$ time host 2860n
    2860n.zbmc.eu has address 192.168.1.1

    real    0m5.178s
    user    0m0.007s
    sys     0m0.008s
    chris at backup$  time host esprimo
    esprimo.zbmc.eu has address 192.168.1.3

    real    0m0.020s
    user    0m0.004s
    sys     0m0.013s
    chris at backup$ time host 2860n
    2860n.zbmc.eu has address 192.168.1.1

    real    0m0.018s
    user    0m0.007s
    sys     0m0.008s


So, just the first lookup takes 5 seconds, there's a timeout waiting
for something to fail I think and looking in the logs it would appear
that it's a timeout waiting for an IPV6 address.  Is there any way to
prevent this?

-- 
Chris Green