Having used Linode and CentOS for years I have never had a problem quite
like this. Sure sounds like the IPv6 is misconfigured in the DHCP server
or is in use somewhere. Some things I would try are:
1. Set "Auto configure networking" in your config profile and reboot.
2. Try to assign the adddress static.
3. Ask Linode to assign you a new IPv6
4. Wait for Linode to fix the problem.
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