netdev - Re: [PATCH iproute2-next 2/3] ipmroute: don't complain about unicast routes

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Date:   Wed, 7 Mar 2018 08:51:48 -0800
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     dsahern@...il.com
Cc:     netdev@...r.kernel.org, Stephen Hemminger <sthemmin@...rosoft.com>
Subject: Re: [PATCH iproute2-next 2/3] ipmroute: don't complain about
 unicast routes

On Tue,  6 Mar 2018 17:03:54 -0800
Stephen Hemminger <stephen@...workplumber.org> wrote:

> From: Stephen Hemminger <sthemmin@...rosoft.com>
> 
> Every non-multicast route prints an error message.
> Kernel doesn't filter out unicast routes, it is up to filter function
> to do this.
> 
> Signed-off-by: Stephen Hemminger <stephen@...workplumber.org>

I found the issue (in kernel) but not sure how to deal with it.
If kernel is built without multicast routing configured !CONFIG_IP_MROUTE
then the netlink request to return multicast routes will return all routes!

This is because in the kernel the way route dump works is that each
address family registers a callback to dump routes for a specific address
family.  If that address family is not registered then the fall back
is to address family PF_UNSPEC which has a handler that dumps all
routes.

Unfortunately, changing that behavior in kernel will certainly break
some user. And there is no direct way to determine multicast routing
is enabled in ip mroute code.

Maybe just change the message in ip mroute to do:

diff --git a/ip/ipmroute.c b/ip/ipmroute.c
index aa5029b44f41..31b9bfe95596 100644
--- a/ip/ipmroute.c
+++ b/ip/ipmroute.c
@@ -76,9 +76,8 @@ int print_mroute(const struct sockaddr_nl *who, struct nlmsghdr *n, void *arg)
                return -1;
        }
        if (r->rtm_type != RTN_MULTICAST) {
-               fprintf(stderr, "Not a multicast route (type: %s)\n",
-                       rtnl_rtntype_n2a(r->rtm_type, b1, sizeof(b1)));
-               return 0;
+               fprintf(stderr, "Multicast routing does not appear to be enabled\n");
+               return -1;
        }

        parse_rtattr(tb, RTA_MAX, RTM_RTA(r), len);