netdev - [Patch][IPv6] Fix wrong routing mechanism for Link Local IPv6 packets

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Date:   Tue, 16 May 2017 15:37:39 +0000
From:   Michael Ulmer <mulmer@...dlepoint.com>
To:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: [Patch][IPv6] Fix wrong routing mechanism for Link Local IPv6 packets

Blast from the past. 10 years back Wei Dong submitted the patch found (amongst several places) here:
http://lists.openwall.net/netdev/2007/01/30/20

Problem:
I have a firewall rule that DNATs ipv6 traffic from a destination address to ::1. The route lookup gives me the Main table & forwards that DNAT'd traffic instead of sending it to local process.

Example:
Looking at this from a netfilter point of view, a client (IP of fd00::5) requests a web page at [2000::25:0:0:1]:8080. The firewall rule DNATs it to ::1 (note that I threw a -j TRACE in raw's REROUTING).
TRACE: nat:PREROUTING:rule:2 SRC=fd00::5 DST=2000::25:0:0:1
TRACE: mangle:FORWARD:rule:1 SRC=fd00::5 DST=::1

The patch is verbatim (as is the subject line for this email). Traffic DNAT'd to ::1 now goes to mangle's INPUT chain after routing decision. I'm not sure why it was removed--I'm assuming it was an accident--as I can't find a record in the mailing list archive.

diff --git a/gpl/kernel/linux/net/ipv6/route.c b/gpl/kernel/linux/net/ipv6/route.c
index 3809ca2..2a2563f 100644
--- a/gpl/kernel/linux/net/ipv6/route.c
+++ b/gpl/kernel/linux/net/ipv6/route.c
@@ -611,7 +611,7 @@ static int rt6_score_route(struct rt6_info *rt, int oif,
        int m;

        m = rt6_check_dev(rt, oif);
-       if (!m && (strict & RT6_LOOKUP_F_IFACE))
+       if (!m && (rt->rt6i_flags & RTF_CACHE) && (strict & RT6_LOOKUP_F_IFACE))
                return RT6_NUD_FAIL_HARD;
 #ifdef CONFIG_IPV6_ROUTER_PREF
        m |= IPV6_DECODE_PREF(IPV6_EXTRACT_PREF(rt->rt6i_flags)) << 2;

__
Mike Ulmer