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2009-12-04 [長年日記]

Google Public DNS with DNS round robin

以下のようにIPアドレスが踊ってしまいますが、サービス提供側的にはどうなんでしょうか?

>nslookup www.amazon.co.jp 8.8.4.4
Server:  google-public-dns-b.google.com
Address:  8.8.4.4

Non-authoritative answer:
Name:    www.amazon.co.jp
Address:  207.171.187.20


>nslookup www.amazon.co.jp 8.8.4.4
Server:  google-public-dns-b.google.com
Address:  8.8.4.4

Non-authoritative answer:
Name:    www.amazon.co.jp
Address:  207.171.185.225


>nslookup www.amazon.co.jp 8.8.4.4
Server:  google-public-dns-b.google.com
Address:  8.8.4.4

Non-authoritative answer:
Name:    www.amazon.co.jp
Address:  207.171.185.225


>nslookup www.amazon.co.jp 8.8.4.4
Server:  google-public-dns-b.google.com
Address:  8.8.4.4

Non-authoritative answer:
Name:    www.amazon.co.jp
Address:  207.171.185.225


>nslookup www.amazon.co.jp 8.8.4.4
Server:  google-public-dns-b.google.com
Address:  8.8.4.4

Non-authoritative answer:
Name:    www.amazon.co.jp
Address:  207.171.187.20


>nslookup www.amazon.co.jp 8.8.4.4
Server:  google-public-dns-b.google.com
Address:  8.8.4.4

Non-authoritative answer:
Name:    www.amazon.co.jp
Address:  207.171.185.225


>nslookup www.amazon.co.jp 8.8.4.4
Server:  google-public-dns-b.google.com
Address:  8.8.4.4

Non-authoritative answer:
Name:    www.amazon.co.jp
Address:  207.171.185.225


>nslookup www.amazon.co.jp 8.8.4.4
Server:  google-public-dns-b.google.com
Address:  8.8.4.4

Non-authoritative answer:
Name:    www.amazon.co.jp
Address:  207.171.181.135


>nslookup www.amazon.co.jp 8.8.4.4
Server:  google-public-dns-b.google.com
Address:  8.8.4.4

Non-authoritative answer:
Name:    www.amazon.co.jp
Address:  207.171.181.135


>nslookup www.amazon.co.jp 8.8.4.4
Server:  google-public-dns-b.google.com
Address:  8.8.4.4

Non-authoritative answer:
Name:    www.amazon.co.jp
Address:  207.171.181.135


>nslookup www.amazon.co.jp 8.8.4.4
Server:  google-public-dns-b.google.com
Address:  8.8.4.4

Non-authoritative answer:
Name:    www.amazon.co.jp
Address:  207.171.181.135


>nslookup www.amazon.co.jp 8.8.4.4
Server:  google-public-dns-b.google.com
Address:  8.8.4.4

Non-authoritative answer:
Name:    www.amazon.co.jp
Address:  207.171.181.135


>nslookup www.amazon.co.jp 8.8.4.4
Server:  google-public-dns-b.google.com
Address:  8.8.4.4

Non-authoritative answer:
Name:    www.amazon.co.jp
Address:  207.171.187.20


>nslookup www.amazon.co.jp 8.8.4.4
Server:  google-public-dns-b.google.com
Address:  8.8.4.4

Non-authoritative answer:
Name:    www.amazon.co.jp
Address:  207.171.181.135


>nslookup www.amazon.co.jp 8.8.4.4
Server:  google-public-dns-b.google.com
Address:  8.8.4.4

Non-authoritative answer:
Name:    www.amazon.co.jp
Address:  207.171.181.135


>nslookup www.amazon.co.jp 8.8.4.4
Server:  google-public-dns-b.google.com
Address:  8.8.4.4

Non-authoritative answer:
Name:    www.amazon.co.jp
Address:  207.171.181.135


>nslookup www.amazon.co.jp 8.8.4.4
Server:  google-public-dns-b.google.com
Address:  8.8.4.4

Non-authoritative answer:
Name:    www.amazon.co.jp
Address:  207.171.185.225


>nslookup www.amazon.co.jp 8.8.4.4
Server:  google-public-dns-b.google.com
Address:  8.8.4.4

Non-authoritative answer:
Name:    www.amazon.co.jp
Address:  207.171.181.135

まあAmazonのDNS round robinも1レコードしか返さずに、ユーザーサイドのDNSキャッシュに頼っているっぽいちょっと不思議な仕様なんですが。

ちなみに、以下のようにround robinしている全てのアドレス(?)を順番を変えて返すのが一般的な応答のようです。IPが踊るのには変わりありません。

>nslookup www.google.co.jp 8.8.4.4
Server:  google-public-dns-b.google.com
Address:  8.8.4.4

Non-authoritative answer:
Name:    www.l.google.com
Addresses:  66.249.89.104, 66.249.89.147, 66.249.89.103, 66.249.89.99
Aliases:  www.google.co.jp, www.google.com