Michael S. Keller via Dng <dng@???> wrote:
>
> On 2023-11-16 16:49, Gregory Nowak via Dng wrote:
>> I see from output you posted previously your router supports stateful
>> address configuration, but you didn't post any output regarding
>> autonomous address configuration. I don't expect the following to make
>> a difference, but just in case, can you change in
>> /etc/network/interfaces:
>> iface eth0 inet6 dhcp
>> to:
>> iface eth0 inet6 auto
>
> With this setting, eth0 does not get a routeable IPv6 address.
>

> eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
>        inet 192.168.1.12  netmask 255.255.255.0  broadcast 192.168.1.255
>        inet6 fe80::16da:e9ff:fef9:aa5c  prefixlen 64  scopeid 0x20<link>
>        ether 14:da:e9:f9:aa:5c  txqueuelen 1000  (Ethernet)
>        RX packets 943  bytes 119440 (116.6 KiB)

>
> My router is set to its defaults for IPv6:
>
> Dynamic IP (with options Dynamic, PPPoE, 6to4, Bridge)
> Get IPv6: Auto (with options Stateful, Stateless, Non-Address, Auto)
> Prefix Delegation: On
> DNS Address: Auto
> Assigned Type: DHCPv6 (with options ND Proxy, SLAAC+Stateless DHCP, DHCPv6, SLAAC+RDNSS)

That’s ... “interesting”.

The default for pretty much all CPEs is to support SLAAC (StateLess Address Auto Configuration), and using DHCPv6 is something that needs specific action*.
By default, your router should be sending RAs (Router Advertisements) with (from memory, I’ve very much not an IPv6 guru) the A flag set to indicate that the prefix can be used for autoconfig. And unless you’ve disabled it, Linux is set to use autoconfig.

As the router supports PD (Prefix Delegation), this means it must be running a DHCPv6 server, and it looks like your client is using this instead of autoconfig.

Can you use something like Wireshark (or it’s command line sibling, tshark) to capture RA packets and decode what flags it is setting for each prefix advertised ?


Simon



* Mostly because a certain vendor of a mobile OS is so determined that no-one will use DHCP on a mobile (and yes, there are valid arguments for this) that they not only don’t include a DHCPv6 client in the OS, but also coerce the hardware manufacturers to filter DHCPv6 packets at the hardware layer in the mobile chipsets to make sure no third party can fill the gap. The fact that there are genuine reasons, including legal requirements, for many to use DHCPv6 is apparently not a good enough reason to not block it entirely for everyone to prevent “misuse”.
As a result, if you run a managed network with DHCP and no autoconfig, then a large chunk of devices will not work on it.