netdev - Getting the IPv6 'prefix_len' for DHCP6 assigned addresses.

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Date:   Wed, 2 Feb 2022 16:58:22 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>
CC:     David Ahern <dsahern@...nel.org>
Subject: Getting the IPv6 'prefix_len' for DHCP6 assigned addresses.

I'm trying to work out how DHCP6 is supposed to work.

I've a test network with the ISC dhcp6 server and radvd running.
If I enable 'autoconf' I get a nice address with the prefix from
radvd and the last 8 bytes from my mac address, prefix_len 64.
I get a nice address from dhcp6 (busybox udhcpc6) with the same prefix.

udhcpc6 runs my scripts and 'ip add $ipv6 dev $interface' adds the address.
But the associated prefix_len is /128.

All the documentation for DHCP6 says the prefix_len (and probably the
default route - but I've not got that far) should come from the network
(I think from RA messages).

But I can't get it to work, and google searches just seem to show
everyone else having the same problem.

The only code I've found that looks at the prefix_len from RA messages
is that which adds to 'autoconf' addresses - and that refuses to do
anything unless the prefix_len is 64.

I can't see anything that would change the prefix_len of an address
that dhcp6 added.

Has something fallen down a big crack?

Kernel is 5.10.84 (LTS) - but I don't think anything relevant
will have changed.

	David

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