How to configure IPv6 on Amazon EC2 instance running CentOS 7

Problem

According to the IPv6 migration guide:

RHEL 7.4 and CentOS 7 and later use cloud-init to configure your network interface and generate the /etc/sysconfig/network-scripts/ifcfg-eth0 file. You can create a custom cloud-init configuration file to enable DHCPv6, which generates an ifcfg-eth0 file with settings that enable DHCPv6 after each reboot.

However after rebooting the instance it looses IPv6 address and IPv6 connectivity.

It appears that this is caused by cloud-init package, see this bug report for details. There are two similar bugs reported in CentOS bug tracker: #14760 and #15833.

According to EC2 cloud-init guide, cloud-init only runs during first launch of the instance. But every time I reboot the instance, I see the following messages in the /var/log/cloud-init.log.

2019-10-27 14:18:18,575 - __init__.py[DEBUG]: Update datasource metadata and network config due to events: New instance first boot
2019-10-27 14:18:20,515 - __init__.py[DEBUG]: Update datasource metadata and network config due to events: System boot

That indicates that cloud-init doesn’t detect first launch properly and runs on every reboot. Because of that it regenerates /etc/sysconfig/network-scripts/ifcfg-eth0 and setting IPV6_AUTOCONF=yes is not preserved or generated.

Solution

1. Follow until step 5 in Migrating to IPv6 guide.
2. Add cloud-init configuration script according to RHEL 7.4 / CentOS 7 instructions.
3. Connect to your instance via SSH.
4. Edit /etc/rc.local

 file and add the lines below.

   
   ###############################################################################
   # Workaround for issue with 'cloud-init' and IPv6 connectivity
   # See https://bugs.centos.org/view.php?id=15833
   # -----------------------------------------------------------------------------
   echo IPV6_AUTOCONF=yes >> /etc/sysconfig/network-scripts/ifcfg-eth0
   systemctl restart network
   ###############################################################################
   

5. Save (:wq) and exit.
6. Add executable permissions to rc.local file.

   
   $ chmod +x /etc/rc.local
   

7. Reboot the instance.
8. Ping ipv6.google.com.

$ ping6 ipv6.google.com

PING ipv6.google.com(fra16s12-in-x0e.1e100.net (2a00:1450:4001:818::200e)) 56 data bytes
64 bytes from fra16s12-in-x0e.1e100.net (2a00:1450:4001:818::200e): icmp_seq=1 ttl=51 time=0.449 ms
64 bytes from fra16s12-in-x0e.1e100.net (2a00:1450:4001:818::200e): icmp_seq=2 ttl=51 time=0.464 ms

9. Ping your instance IPv6 address from outside, for example from IPv6now website.

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