"message too long" when sending broadcasts

[Michael Voucko]

 >> Berkeley-derived kernels do not allow a broadcast datagram to be
 >> fragmented. If the size of an IP datagram that is being sent to a
 >> broadcast address exceeds the outgoing interface MTU, EMSGSIZE is
 >> returned (pp. 233?234 of TCPv2). This is a policy decision that has
 >> existed since 4.2BSD. There is nothing that prevents a kernel from
 >> fragmenting a broadcast datagram, but the feeling is that broadcasting
 >> puts enough load on the network as it is, so there is no need to
 >> multiply this load by the number of fragments. .... AIX, FreeBSD, and
 >> MacOS implement this limitation. Linux, Solaris, and HP-UX fragment
 >> datagrams sent to a broadcast address.
 >>
 >> Maybe it's worth adding this information somewhere to make it easier
 >> to find?
 >
 > It would also be quite easy to make this configurable -- the code in
 > ip_output is fairly straight forward:
 >
 >         /*
 >          * Look for broadcast address and
 >          * verify user is allowed to send
 >          * such a packet.
 >          */
 >         if (isbroadcast) {
 >                 if ((ifp->if_flags & IFF_BROADCAST) == 0) {
 >                         error = EADDRNOTAVAIL;
 >                         goto bad;
 >                 }
 >                 if ((flags & IP_ALLOWBROADCAST) == 0) {
 >                         error = EACCES;
 >                         goto bad;
 >                 }
 >                 /* don't allow broadcast messages to be fragmented */
 >                 if (ip->ip_len > ifp->if_mtu) {
 >                         error = EMSGSIZE;
 >                         goto bad;
 >                 }
 >                 if (flags & IP_SENDONES)
 >                         ip->ip_dst.s_addr = INADDR_BROADCAST;
 >                 m->m_flags |= M_BCAST;
 >         } else {
 >                 m->m_flags &= ~M_BCAST;
 >         }
 >
 > We could add a global option, or maybe just an IP socket option.

I had quite a hard time to find this peace of information which makes me think 
that there aren't too many other application suffering from the inability to 
fragment?!
But if there will be setsockopt option for this feature, I'm happy to test it 
with heartbeat.

  Michael