Date: Sun, 20 Nov 2005 13:04:11 +0000 (GMT) From: Robert Watson <rwatson@FreeBSD.org> To: Michael Voucko <voucko@gmx.li> Cc: freebsd-stable@freebsd.org Subject: Re: "message too long" when sending broadcasts Message-ID: <20051120130304.T8182@fledge.watson.org> In-Reply-To: <437FCEAB.80000@gmx.li> References: <437B1466.8030202@gmx.li> <20051119165212.O88861@carver.gumbysoft.com> <437FCEAB.80000@gmx.li>
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On Sun, 20 Nov 2005, Michael Voucko wrote:
>
> 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.
Robert N M Watson
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