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    SubjectRe: [PATCH RFC v1 2/3] ipv6: move from sha1 to blake2s in address calculation
    Hi Toke,

    On Thu, Jan 13, 2022 at 2:30 PM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
    > Right, but that implies we need to work on a transition mechanism. For
    > newly deployed systems changing the hash is obviously fine, it's the
    > "reboot and you have a new address" problem.
    >
    > We could introduce new values to the addr_gen_mode? I.e. values of 4 and
    > 5 would be equivalent to 2 and 3 (respectively), but with the new
    > hashing algorithm? And then document that 2 and 3 are considered
    > deprecated to be removed at some point in the future...

    Right, so this is exactly the flow of conversation I anticipated.
    "Let's change it!" "No, we can't." "Okay, let's add a knob."

    The knob I was thinking about, though, was actually a compile-time one
    CONFIG_NET_OBSOLETE_INSECURE_ADDRCONF_HASH, which itself is a `depends
    on CONFIG_OLD_N_CRUSTY` or something. This way we could gate the
    inclusion of sha1.c/sha1.o on that at compile time, and shave down
    vmlinux a bit, which would make Geert happy.

    Then, at some point down the road, we can talk about removing
    CONFIG_NET_OBSOLETE_INSECURE_ADDRCONF_HASH too.

    Jason

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