[ECOS] Re: Why does sending multicast UDP require a gateway?

Grant Edwards grant.b.edwards@gmail.com
Wed Mar 30 14:14:00 GMT 2011


On 2011-03-29, Grant Edwards <grant.b.edwards@gmail.com> wrote:

> Somebody I work with has spent the last three days trying to send UDP
> multicast packets using eCos and the FreeBSD network stack.

AFAICT, the FreeBSD stack will refuse to send a UDP multicast packet
unless it can find a valid route to the multicast destination IP
address with that IP _treated_as_a_unicast_address_.

The broken code is in sys/netinet/ip_output.c, and the error is
asserted at line 269:

   123	ip_output(m0, opt, ro, flags, imo)
[...]
   239	        /*
   240	         * If routing to interface only,
   241	         * short circuit routing lookup.
   242	         */
   243	#define ifatoia(ifa)    ((struct in_ifaddr *)(ifa))
   244	#define sintosa(sin)    ((struct sockaddr *)(sin))
   245	        if (flags & IP_ROUTETOIF) {
   246	                if ((ia = ifatoia(ifa_ifwithdstaddr(sintosa(dst)))) == 0 &&
   247	                    (ia = ifatoia(ifa_ifwithnet(sintosa(dst)))) == 0) {
   248	                        ipstat.ips_noroute++;
   249	                        error = ENETUNREACH;
   250	                        goto bad;
   251	                }
   252	                ifp = ia->ia_ifp;
   253	                ip->ip_ttl = 1;
   254	                isbroadcast = in_broadcast(dst->sin_addr, ifp);
   255	        } else {
   256	                /*
   257	                 * If this is the case, we probably don't want to allocate
   258	                 * a protocol-cloned route since we didn't get one from the
   259	                 * ULP.  This lets TCP do its thing, while not burdening
   260	                 * forwarding or ICMP with the overhead of cloning a route.
   261	                 * Of course, we still want to do any cloning requested by
   262	                 * the link layer, as this is probably required in all cases
   263	                 * for correct operation (as it is for ARP).
   264	                 */
   265	                if (ro->ro_rt == 0)
   266	                        rtalloc_ign(ro, RTF_PRCLONING);
   267	                if (ro->ro_rt == 0) {
   268	                        ipstat.ips_noroute++;
   269	                        error = EHOSTUNREACH;
   270	                        goto bad;
   271	                }
   272	                ia = ifatoia(ro->ro_rt->rt_ifa);
   273	                ifp = ro->ro_rt->rt_ifp;
   274	                ro->ro_rt->rt_use++;
   275	                if (ro->ro_rt->rt_flags & RTF_GATEWAY)
   276	                        dst = (struct sockaddr_in *)ro->ro_rt->rt_gateway;
   277	                if (ro->ro_rt->rt_flags & RTF_HOST)
   278	                        isbroadcast = (ro->ro_rt->rt_flags & RTF_BROADCAST);
   279	                else
   280	                        isbroadcast = in_broadcast(dst->sin_addr, ifp);
   281	        }
   282	        if (IN_MULTICAST(ntohl(ip->ip_dst.s_addr))) {
   283	                struct in_multi *inm;
   284	
   285	                m->m_flags |= M_MCAST;
   286	                /*
   287	                 * IP destination address is multicast.  Make sure "dst"
   288	                 * still points to the address in "ro".  (It may have been
   289	                 * changed to point to a gateway address, above.)
   290	                 */

As you can see, the output code rejects the packet for lack of a valid
route to the destination IP address before it checks to see if it's a
multicast IP address.

That doesn't look right to me...

If it's a multicast destination, shouldn't that short-circuit the
route-checking stuff?

The docs I can find are pretty clear: if it's a multicast IP
destination you ship it out to the corresponding multicast MAC
address. Period. No messing about with routes and gateway addresses.

-- 
Grant Edwards               grant.b.edwards        Yow! I selected E5 ... but
                                  at               I didn't hear "Sam the Sham
                              gmail.com            and the Pharoahs"!


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