Comment 16 for bug 1668771

Without the patch for LP: #1668771 resolved caches the SERVFAIL answer on disco.

oot@disco:/home/multipass# host www.montemar.cl
Host www.montemar.cl not found: 2(SERVFAIL)
(reverse-i-search)`': ^C
root@disco:/home/multipass# journalctl -u systemd-resolved -f
-- Logs begin at Fri 2019-07-26 13:15:35 -04. --
Jul 26 13:17:56 disco systemd-resolved[3872]: Transaction 43091 for <www.montemar.cl IN A> scope dns on ens3/*.
Jul 26 13:17:56 disco systemd-resolved[3872]: Using feature level UDP for transaction 43091.
Jul 26 13:17:56 disco systemd-resolved[3872]: Sending query packet with id 43091.
Jul 26 13:17:56 disco systemd-resolved[3872]: Processing incoming packet on transaction 43091 (rcode=SERVFAIL).
Jul 26 13:17:56 disco systemd-resolved[3872]: Server returned error: SERVFAIL
Jul 26 13:17:56 disco systemd-resolved[3872]: Verified we get a response at feature level UDP from DNS server 10.91.4.1.
Jul 26 13:17:56 disco systemd-resolved[3872]: Added SERVFAIL cache entry for www.montemar.cl IN A 30s
Jul 26 13:17:56 disco systemd-resolved[3872]: Transaction 43091 for <www.montemar.cl IN A> on scope dns on ens3/* now complete with <rcode-failure> from network (unsigned).
Jul 26 13:17:56 disco systemd-resolved[3872]: Sending response packet with id 40433 on interface 1/AF_INET.
Jul 26 13:17:56 disco systemd-resolved[3872]: Freeing transaction 43091.

With the patch for LP: #1668771 resolved + setting Cache: 'no-negative' doesn't caches the SERVFAIL option on disco.
Logs begin at Fri 2019-07-26 13:15:35 -04. --
Jul 26 13:18:21 disco systemd-resolved[3893]: Transaction 22380 for <www.montemar.cl IN A> scope dns on ens3/*.
Jul 26 13:18:21 disco systemd-resolved[3893]: Using feature level UDP for transaction 22380.
Jul 26 13:18:21 disco systemd-resolved[3893]: Sending query packet with id 22380.
Jul 26 13:18:21 disco systemd-resolved[3893]: Processing incoming packet on transaction 22380 (rcode=SERVFAIL).
Jul 26 13:18:21 disco systemd-resolved[3893]: Server returned error: SERVFAIL
Jul 26 13:18:21 disco systemd-resolved[3893]: Verified we get a response at feature level UDP from DNS server 10.91.4.1.
Jul 26 13:18:21 disco systemd-resolved[3893]: Not caching negative entry for: www.montemar.cl IN A, cache mode set to no-negative
Jul 26 13:18:21 disco systemd-resolved[3893]: Transaction 22380 for <www.montemar.cl IN A> on scope dns on ens3/* now complete with <rcode-failure> from network (unsigned).
Jul 26 13:18:21 disco systemd-resolved[3893]: Sending response packet with id 30418 on interface 1/AF_INET.
Jul 26 13:18:21 disco systemd-resolved[3893]: Freeing transaction 22380.