Re: [PATCH net-next v2 3/3] net: ipv6/addrconf: clamp preferred_lft to the minimum required

[David Ahern <dsahern@kernel.org>]

On 2/13/24 11:26 PM, Alex Henrie wrote:
> If the preferred lifetime was less than the minimum required lifetime,
> ipv6_create_tempaddr would error out without creating any new address.
> On my machine and network, this error happened immediately with the
> preferred lifetime set to 5 seconds or less, after a few minutes with
> the preferred lifetime set to 6 seconds, and not at all with the
> preferred lifetime set to 7 seconds. During my investigation, I found a
> Stack Exchange post from another person who seems to have had the same
> problem: They stopped getting new addresses if they lowered the
> preferred lifetime below 3 seconds, and they didn't really know why.
> 
> The preferred lifetime is a preference, not a hard requirement. The
> kernel does not strictly forbid new connections on a deprecated address,
> nor does it guarantee that the address will be disposed of the instant
> its total valid lifetime expires. So rather than disable IPv6 privacy
> extensions altogether if the minimum required lifetime swells above the
> preferred lifetime, it is more in keeping with the user's intent to
> increase the temporary address's lifetime to the minimum necessary for
> the current network conditions.
> 
> With these fixes, setting the preferred lifetime to 5 or 6 seconds "just
> works" because the extra fraction of a second is practically
> unnoticeable. It's even possible to reduce the time before deprecation
> to 1 or 2 seconds by setting /proc/sys/net/ipv6/conf/*/regen_min_advance
> and /proc/sys/net/ipv6/conf/*/dad_transmits to 0. I realize that that is
> a pretty niche use case, but I know at least one person who would gladly
> sacrifice performance and convenience to be sure that they are getting
> the maximum possible level of privacy.
> 
> Link: https://serverfault.com/a/1031168/310447
> Signed-off-by: Alex Henrie <alexhenrie24@gmail.com>
> ---
>  Documentation/networking/ip-sysctl.rst |  2 +-
>  net/ipv6/addrconf.c                    | 43 ++++++++++++++++++++------
>  2 files changed, 35 insertions(+), 10 deletions(-)
> 

Reviewed-by: David Ahern <dsahern@kernel.org>