Re: [RFC] The Linux kernel IPv6 stack don't follow the RFC 4942 recommendation

[Eric Dumazet <eric.dumazet@gmail.com>]

Le vendredi 04 novembre 2011 à 07:46 -0700, François-Xavier Le Bail a
écrit :
> Hi,
> 
> I do some tests on a Linux 3.0 kernel with IPv6 forwarding mode enabled.
> 
> When I ping (ICMPv6 echo request) on one of its Subnet-Router anycast addresses
> (SRAA, http://tools.ietf.org/html/rfc4291#section-2.6.1),
> the Linux kernel reply with an unicast source address, not the anycast one.
> 
> When I send an IPv6 UDP packet to a server on Linux on one of its SRAA,
> the Linux kernel build a reply with an unicast source address, not the anycast one.
> 

Nothing in the kernel builds a reply to an UDP packet.

I would say the user application is responsible to build an answer, and
chose appropriate source address.

If your application uses a ANY_ADDR bind, then it must appropriate
action so that a good source address is used in answers.

In case of IPv6 socket, I advise you take a look at IPV6_PKTINFO /
IPV6_RECVPKTINFO options.


> The RFC 4942 states (http://tools.ietf.org/html/rfc4942#section-2.1.6) :
> 2.1.6. Anycast Traffic Identification and Security
> [. . .]
>    To avoid exposing knowledge about the internal structure of the
>    network, it is recommended that anycast servers now take advantage of
>    the ability to return responses with the anycast address as the
>    source address if possible.
> 
> Also, If the source address of reply differs from destination address of the request, many applications are broken.
> Please let me know your feedback.
> 

'anycast servers' are a combination of kernel and userland parts.