Re: [PATCH net-next v2] ipv6: allow userspace to add IFA_F_OPTIMISTIC addresses

[David Miller <davem@davemloft.net>]

From: Sabrina Dubroca <sd@queasysnail.net>
Date: Wed, 28 Feb 2018 16:40:08 +0100

> According to RFC 4429 (section 3.1), adding new IPv6 addresses as
> optimistic addresses is acceptable, as long as the implementation
> follows some rules:
> 
>    * Optimistic DAD SHOULD only be used when the implementation is aware
>         that the address is based on a most likely unique interface
>         identifier (such as in [RFC2464]), generated randomly [RFC3041],
>         or by a well-distributed hash function [RFC3972] or assigned by
>         Dynamic Host Configuration Protocol for IPv6 (DHCPv6) [RFC3315].
>         Optimistic DAD SHOULD NOT be used for manually entered
>         addresses.
> 
> Thus, it seems reasonable to allow userspace to set the optimistic flag
> when adding new addresses.
> 
> We must not let userspace set NODAD + OPTIMISTIC, since if the kernel is
> not performing DAD we would never clear the optimistic flag. We must
> also ignore userspace's request to add OPTIMISTIC flag to addresses that
> have already completed DAD (addresses that don't have the TENTATIVE
> flag, or that have the DADFAILED flag).
> 
> Then we also need to clear the OPTIMISTIC flag on permanent addresses
> when DAD fails. Otherwise, IFA_F_OPTIMISTIC addresses added by userspace
> can still be used after DAD has failed, because in
> ipv6_chk_addr_and_flags(), IFA_F_OPTIMISTIC overrides IFA_F_TENTATIVE.
> 
> Setting IFA_F_OPTIMISTIC from userspace is conditional on
> CONFIG_IPV6_OPTIMISTIC_DAD and the optimistic_dad sysctl.
> 
> Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
> Reviewed-by: Stefano Brivio <sbrivio@redhat.com>

Applied, thank you.