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.