EC2のPublicDNSの解決の仕組み

結論

• 同じリージョンであればZoneが異なっても、Public DNSはPrivate IPとして解決される
• リージョンが異なるとPublic DNSはPublic IPとして解決される

確認してみた

同じリージョンの異なるゾーンでインスタンスをたてる

1.us-west-2a

Public DNS:ec2-54-203-255-149.us-west-2.compute.amazonaws.com
Public IP:54.203.255.149
Elastic IP:54.203.255.149

Private DNS:ip-10-252-192-161.us-west-2.compute.internal
Private IPs:10.252.192.161

Security groups:launch-wizard-1

2.us-west-2c

Public DNS:ec2-54-245-152-35.us-west-2.compute.amazonaws.com
Public IP:54.245.152.35 

Private DNS:ip-10-217-149-198.us-west-2.compute.internal
Private IPs:10.217.149.198

Security groups:launch-wizard-1

3.ap-northeast-1a
異なるリージョンのサーバーも用意しておく

Public DNS:ec2-54-238-12-24.ap-northeast-1.compute.amazonaws.com
Public IP:54.238.12.24

Private DNS:ip-10-132-73-226.ap-northeast-1.compute.internal
Private IPs:10.132.73.226

IPの解決

us-west-2cからus-west-2aを呼ぶ場合

[ec2-user@ip-10-217-149-198 ~]$ nslookup ec2-54-203-255-149.us-west-2.compute.amazonaws.com
Server:     172.16.0.23
Address:    172.16.0.23#53

Non-authoritative answer:
Name:   ec2-54-203-255-149.us-west-2.compute.amazonaws.com
Address: 10.252.192.161

[ec2-user@ip-10-217-149-198 ~]$ nslookup ip-10-252-192-161.us-west-2.compute.internal
Server:     172.16.0.23
Address:    172.16.0.23#53

Non-authoritative answer:
Name:   ip-10-252-192-161.us-west-2.compute.internal
Address: 10.252.192.161

Public DNSもPrivate DNSもPrivate IPの10.252.192.161となった

ここで、us-west-2aをStop/Startしてみる

http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/elastic-ip-addresses-eip.html

classicの場合は、Elastic IPが外れるので、手動で再度関連づける

Public DNS:ec2-54-203-255-149.us-west-2.compute.amazonaws.com
Public IP:54.203.255.149
Elastic IP:54.203.255.149

Private DNS:ip-10-217-149-198.us-west-2.compute.internal
Private IP:10.232.1.18

Security groups:launch-wizard-1

[ec2-user@ip-10-217-149-198 ~]$ nslookup ec2-54-203-255-149.us-west-2.compute.amazonaws.com
Server:     172.16.0.23
Address:    172.16.0.23#53

Non-authoritative answer:
Name:   ec2-54-203-255-149.us-west-2.compute.amazonaws.com
Address: 10.232.1.18

[ec2-user@ip-10-217-149-198 ~]$ nslookup ip-10-232-1-18.us-west-2.compute.internal
Server:     172.16.0.23
Address:    172.16.0.23#53

Non-authoritative answer:
Name:   ip-10-232-1-18.us-west-2.compute.internal
Address: 10.232.1.18

Elastic IPはかわらないが、nslookup ip-10-217-149-198.us-west-2.compute.internalの結果は、あたらしいPrivate IP(10.232.1.18)になった（期待通り）

us-west-2cからap-northeast-1aを呼ぶ場合

[ec2-user@ip-10-217-149-198 ~]$ nslookup ec2-54-238-12-24.ap-northeast-1.compute.amazonaws.com
Server:     172.16.0.23
Address:    172.16.0.23#53

Non-authoritative answer:
Name:   ec2-54-238-12-24.ap-northeast-1.compute.amazonaws.com
Address: 54.238.12.24

[ec2-user@ip-10-217-149-198 ~]$ nslookup ip-10-132-73-226.ap-northeast-1.compute.internal
Server:     172.16.0.23
Address:    172.16.0.23#53

** server can't find ip-10-132-73-226.ap-northeast-1.compute.internal: NXDOMAIN

Public DNSの解決はできたが、Private DNSは解決できない

まとめ

• 同じリージョンであればZoneが異なっても、Public DNSはPrivate IPとして解決される
• リージョンが異なるとPublic DNSはPublic IPとして解決される