Sometimes DNS answers change unexpectedly. This shell script will try to log DNS answers. You may change the DNS-server IP addresses on lines 4 and 5.

test-dns.sh

#!/bin/bash
while true; do
  echo $(date +%FT%T%z) $(dig +noall +answer +stats ftp.example.com  | tr '\n' ' ')
  echo $(date +%FT%T%z) $(nslookup ftp.example.com 10.1.2.3 | tr '\n' ' ')
  echo $(date +%FT%T%z) $(nslookup ftp.example.com 10.2.4.6 | tr '\n' ' ')
  sleep 60
done
#[sirkkalap@mymachine ~]$ nohup ./test_dns.sh &
#[1] 16406
#[sirkkalap@mymachine ~]$ tail nohup.out
#2016-03-16T10:40:27+0200 ftp.example.com. 32 IN A 10.0.0.1 ;; Query time: 2 msec ;; SERVER: 10.1.2.3#53(10.1.2.3) ;; WHEN: Wed Mar 16 10:40:27 2016 ;; MSG SIZE rcvd: 52
#2016-03-16T10:40:27+0200 Server: 10.1.2.3 Address: 10.1.2.3#53 Non-authoritative answer: Name: ftp.example.com Address: 10.0.0.1
#2016-03-16T10:40:27+0200 Server: 10.2.4.6 Address: 10.2.4.6#53 Non-authoritative answer: Name: ftp.example.com Address: 10.0.0.1