Amazon provides instructions on how to enable IPv6 on RHEL 7 and Centos 7: http://docs.aws.amazon.com/AmazonVPC/latest/UserGuide/vpc-migrate-ipv6.html#ipv6-dhcpv6-rhel
These instructions work on RHEL 7-7.3 and Centos 7-7.3, but do not work on RHEL 7.4 (and presumably neither on Centos 7.4, when it will become available).
The problem is that the file
/etc/sysconfig/network-scripts/ifcfg-eth0 is user-writable on the older versions, but on RHEL 7.4 this file becomes managed by cloud-init instead.
$ sed 1q /etc/sysconfig/network-scripts/ifcfg-eth0 # Created by cloud-init on instance boot automatically, do not edit.
I find it very unexpected that merely upgrading from 7.3 to 7.4 would require such a fundamental change to managing networking. I am not sure if this change is specific to Amazon EC2 or not, I can't find anything in the RHEL 7.4 release notes, so it might be specific to Amazon.
So how are you supposed to enable IPv6 on RHEL 7.4 and Centos 7.4 in the cloud-init model?