netdev - Re: [PATCH net-next v2 2/4] net: ipv6/addrconf: clamp preferred

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Date: Wed, 25 Oct 2023 18:43:22 -0600
From: David Ahern <dsahern@...nel.org>
To: Alex Henrie <alexhenrie24@...il.com>, netdev@...r.kernel.org,
 jbohac@...e.cz, benoit.boissinot@...-lyon.org, davem@...emloft.net,
 hideaki.yoshifuji@...aclelinux.com, pabeni@...hat.com, kuba@...nel.org
Subject: Re: [PATCH net-next v2 2/4] net: ipv6/addrconf: clamp preferred_lft
 to the minimum required

On 10/24/23 3:23 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 1 second, after a few minutes with the
> preferred lifetime set to 4 seconds, and not at all with the preferred
> lifetime set to 5 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 3 or 4 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 also disabling duplicate address detection (setting
> /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@...il.com>
> ---
>  net/ipv6/addrconf.c | 18 +++++++++++++-----
>  1 file changed, 13 insertions(+), 5 deletions(-)
> 

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