Proxmox Host IPv6 DHCP not working

[spirit]

oh, maybe this one:
https://github.com/CumulusNetworks/ifupdown2/issues/174

"
having a mix of both "static" and "dhcp" stanza is currently not supported.
The work around is to add your static ip using pre-up / up / post-up attributes and use iproute2 to set your static ip.

Not ideal I know but to support both type some adjustment are needed to a core object (ifaceobj) - basically the addr_method need to become a list, but this attribute is used in many places so it's won't be a trivial change.

Feel free to have a look and maybe write a patch for it if you have free cycles. I think the workaround would be enough for now for your use case.
"

 

[enginefeeder101]

Sorry for necroposting, but this is one of the few threads that my search led me too.
For reference: I was facing the same issue and this is how I got IPv6 SLAAC auto-configuration working on the Proxmox VE host:

Removed all references to 'inet6' in /etc/network/interfaces

Added to /etc/sysctl.conf

net.ipv6.conf.vmbr0.accept_ra = 2
# Overrule forwarding behaviour. Accept Router Advertisements even if forwarding is enabled.

You can add other interfaces or properties (eg. use_tempaddr = 2) here as required.

Source:
[1] https://strugglers.net/~andy/blog/2011/09/04/linux-ipv6-router-advertisements-and-forwarding/
[2] https://www.kernel.org/doc/Documentation/networking/ip-sysctl.txt (CTRL+F accept_ra)