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Date:      Thu, 26 Apr 2012 14:06:39 -0700
From:      prabhakar lakhera <prabhakar.lakhera@gmail.com>
To:        freebsd-net@freebsd.org
Subject:   About static route retargetting in IPv6
Message-ID:  <CALg%2BrhVPCOY30xz8GpVz5vEvBmad1_k%2BYVyGKvdfV=jWFmro3g@mail.gmail.com>

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Hi,

In RFC 4861 Section: 8.1. Validation of Redirect
Messages<http://tools.ietf.org/html/rfc4861#page-74>we have the
following bullets:

A host *MUST *silently discard any received Redirect message that does
not satisfy all of the following validity checks:
     - IP Source Address is a link-local address. Routers *MUST *use
their link-local address as the source for Router Advertisement and
Redirect messages so that hosts can uniquely identify routers.
     ...
     ...
     - The IP source address of the Redirect is the same as the
current first-hop router for the specified ICMP Destination Address.

The above two bullets implicitly imply that the gateways installed in
routing tables MUST have LLA scope only for the node to be able to process
redirects.

For static routes when ifa gets deleted or when a port goes down, routes
get re-targeted. For static indirect routes we look for other ifa in the
same subnet or with same prefix as gateway's ifa. For subnet routes the
existing ones are re-targeted to a different ifa/ifp when possible. I
believe the existing dynamic routes (cloned ones) are purged and are
learned again when required.

For IPv6 dynamically learned default router we have default router
selection which gets triggered from various events (neighbor cache entry
for gateway expiring, RA received which implies router is not more valid,
NA received for a neighbor that was considered to be a router, port
configured down/link down etc etc). That is responsible for installing new
default route when required.

We have an option to disabled RA processing. User then has to install
static default routes. When user does that in order to process redirects
user MUST add indirect static routes with LLA scoped addresses. Also
default router selection mechanism would not work for these default route
entries (since defrouter list does not include static routers).

The question is how do we re-target these routes? The route re-targeting
mechanisms in place a based on prefix comparison.

Currently LLA addresses can get auto-configured since they are well defined
(fe80) and don't depend on learning prefixes via received RAs. However they
are currently all limited to the interface they are configured on.

Has there been any effort to dynamically learn what local ports are on-link
and using that information to embed the scope in the LLA rather than
interface ids? That would help in getting these static default routes ,with
LLA gateways, re-targeted.

If people are already working on it, I would appreciate if they could
comment on it and get in touch.


Best,


Prabhakar



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