os.system
устарел. Используйте подпроцесс .Popen :
import subprocess
import shlex
cmd='dig @ns1.netnames.net www.rac.co.uk +short'
# cmd='dig @ns1.netnames.net www.rac.co.uk CNAME'
proc=subprocess.Popen(shlex.split(cmd),stdout=subprocess.PIPE)
out,err=proc.communicate()
print(out)
# rac-secure.gslb.norwichunion.com.