Re: [PATCH] net: ipv6: don't generate link-local address in any addr_gen_mode

Hi Lorenzo,

On Mon, 13 Sept 2021 at 19:38, Lorenzo Colitti [off-list ref] wrote:

On Mon, Sep 13, 2021 at 12:47 AM Mark Smith [off-list ref] wrote:

quoted

This is all going in the wrong direction. Link-local addresses are not
optional on an interface, all IPv6 enabled interfaces are required to
have one:

The original patch did indeed disable the generation of the link-local
address, but that patch was rejected. It sounds like the right
approach here is to provide two new addressing modes:

IN6_ADDR_GEN_MODE_RANDOM_LL_TOKEN
IN6_ADDR_GEN_MODE_STABLE_PRIVACY_LL_TOKEN

which would form the link-local address from the token passed in via
IFLA_INET6_TOKEN, but would form non-link-local addresses (e.g.,
global addresses) via the specified means (either random or stable
privacy). I haven't looked at how to do that yet though.

I think there is a broader issue here.

If RFC7217 is used to generate the Link-Local address, then the
likelihood of the Link-Local address being a duplicate with the GGSN
is very low, because RFC7217 uses a function such as SHA-1 or SHA-256.
RFC7217 also performs DAD just in case, and triggers RFC7217 again if
a duplicate does occur.

RFC8064 recommends RFC7217 for all stable IPv6 addresses by default
now, which includes Link-Local addresses.

Following RFC8064 by Implementing RFC7217 for all stable IPv6
addresses the Linux kernel generates would solve both the 3GPP's
duplicate Link-Local address concern, and also make the Linux kernel
compliant with the latest SLAAC recommendation for default IIDs and
stable IPv6 addresses.

Regards,
Mark.