參考：
https://support.google.com/a/answer/60764?hl=zh-Hant
nslookup -q=TXT _spf.google.com 8.8.8.8
nslookup -q=TXT _netblocks.google.com 8.8.8.8
nslookup -q=TXT _netblocks2.google.com 8.8.8.8
nslookup -q=TXT _netblocks3.google.com 8.8.8.8
即可拿到IP清單
