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Date:      Thu, 17 May 2012 15:07:51 -0700
From:      prabhakar lakhera <prabhakar.lakhera@gmail.com>
To:        freebsd-net@freebsd.org
Subject:   How to set the IPv6 linklocal scope id for an interface?
Message-ID:  <CALg%2BrhVakDqBQdnvwYOsHFNKOo-6dsM2-nk5fGztt7E004hAMA@mail.gmail.com>
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Hi,

Is there any way for the administrator to set an interface's scope if for
link local scope?

I see that in6_domifattach calls scope6_ifattach which sets the interface
local scope and link local scope for the interface equal to the ifp index.

scope6_ifattach(struct ifnet *ifp)
{
      .....
      .....
        /*
         * XXX: IPV6_ADDR_SCOPE_xxx macros are not standard.
         * Should we rather hardcode here?
         */
        sid->s6id_list[IPV6_ADDR_SCOPE_INTFACELOCAL] = ifp->if_index;
        sid->s6id_list[IPV6_ADDR_SCOPE_LINKLOCAL] = ifp->if_index;
      .....
      .....

}


I am trying to avoid the following:

Here's the problem. I have two interfaces A and B on same link. Both learn
RA for the link. One of these is used to add a default route entry in the
routing table. Lets say default router entry for A is picked, we have the
following default route entry:

destination: ::/0
gw:             fe80:<A's link local scope id embedded>::<identifier>
route_ifp:     A

I used IPv6 address hosted on interface B to send a packet to destination X
and I got back a neighbor redirect from gw destined for B for X.
However the redirect is dropped. Below is the code from
icmp6_redirect_input that fails a check:

 2346 <http://fxr.watson.org/fxr/source/netinet6/icmp6.c?v=FREEBSD8#L2346>;
 2347 <http://fxr.watson.org/fxr/source/netinet6/icmp6.c?v=FREEBSD8#L2347>;
                gw6 = &(((struct sockaddr_in6
<http://fxr.watson.org/fxr/ident?v=FREEBSD8;im=bigexcerpts;i=sockaddr_in6>;
*)rt <http://fxr.watson.org/fxr/ident?v=FREEBSD8;im=bigexcerpts;i=rt>->rt_gateway)->sin6_addr);
 2348 <http://fxr.watson.org/fxr/source/netinet6/icmp6.c?v=FREEBSD8#L2348>;
                if (bcmp
<http://fxr.watson.org/fxr/ident?v=FREEBSD8;im=bigexcerpts;i=bcmp>(&src6,
gw6, sizeof(struct in6_addr
<http://fxr.watson.org/fxr/ident?v=FREEBSD8;im=bigexcerpts;i=in6_addr>))
!= 0) {
 2349 <http://fxr.watson.org/fxr/source/netinet6/icmp6.c?v=FREEBSD8#L2349>;
                        nd6log
<http://fxr.watson.org/fxr/ident?v=FREEBSD8;im=bigexcerpts;i=nd6log>((LOG_ERR
<http://fxr.watson.org/fxr/ident?v=FREEBSD8;im=bigexcerpts;i=LOG_ERR>,
 2350 <http://fxr.watson.org/fxr/source/netinet6/icmp6.c?v=FREEBSD8#L2350>;
                            *"ICMP6 redirect rejected; "*
 2351 <http://fxr.watson.org/fxr/source/netinet6/icmp6.c?v=FREEBSD8#L2351>;
                            *"not equal to gw-for-src=%s (must be
same): "*
 2352 <http://fxr.watson.org/fxr/source/netinet6/icmp6.c?v=FREEBSD8#L2352>;
                            *"%s\n"*,
 2353 <http://fxr.watson.org/fxr/source/netinet6/icmp6.c?v=FREEBSD8#L2353>;
                            ip6_sprintf
<http://fxr.watson.org/fxr/ident?v=FREEBSD8;im=bigexcerpts;i=ip6_sprintf>(ip6buf,
gw6),
 2354 <http://fxr.watson.org/fxr/source/netinet6/icmp6.c?v=FREEBSD8#L2354>;
                            icmp6_redirect_diag
<http://fxr.watson.org/fxr/ident?v=FREEBSD8;im=bigexcerpts;i=icmp6_redirect_diag>(&src6,
&reddst6, &redtgt6)));
 2355 <http://fxr.watson.org/fxr/source/netinet6/icmp6.c?v=FREEBSD8#L2355>;
                        RTFREE_LOCKED
<http://fxr.watson.org/fxr/ident?v=FREEBSD8;im=bigexcerpts;i=RTFREE_LOCKED>(rt
<http://fxr.watson.org/fxr/ident?v=FREEBSD8;im=bigexcerpts;i=rt>);
 2356 <http://fxr.watson.org/fxr/source/netinet6/icmp6.c?v=FREEBSD8#L2356>;
                        goto bad;
 2357 <http://fxr.watson.org/fxr/source/netinet6/icmp6.c?v=FREEBSD8#L2357>;
                }


src6 is the router's LLA but since it's received on B, it's embedded
with B's link scope embedded id. So we end up comparing:

  fe80:<A's link local scope id embedded>::<identifier> with
fe80:<B's link local scope id embedded>::<identifier>

Since ids are different the comparison fails and the redirect is dropped.

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