I'm trying to install the latest version of heartbeat (linux-ha), which
unfortunately is not available from the ports tree yet.
Setup is two boxes running (5.4-STABLE FreeBSD from Oct 17 2005) both connected
to a hub with lnc NICs on a private 10.0.0.0/24 subnet (actually its only a
'simulated hub' provided by VMWare if this matters).
After some initial struggles configuration, build and installations works as
expected - but now the most basic parts of this application won't work anymore.
When trying to use UDP broadcast messages for the heartbeat it would error out
with "Message too long" for packets larger than 1472 bytes, in other words as
soon as fragmentation of the package would be necessary (MTU is 1500).
I received a code snippet from the heartbeat developers to isolate the problem
(create sender and receiver socket, send broadcast packets of a certain size).
To rule out VMware as the basic problem I tried real boxes but the problem
persists.
Is there any size restriction for UDP broadcast messages?
Is there anything which prevents UDP broadcast from being fragmented?
I searched RFCs, man pages for socket, setsockopt, ioctl, sendto and other
places but did not find anything that could explain the behaviour (which seems
to be no problem on other OS).
Any pointers, comments?
From sendmsg(2):
The address of the target is given by to with tolen specifying its size.
The length of the message is given by len. If the message is too long to
pass atomically through the underlying protocol, the error EMSGSIZE is
returned, and the message is not transmitted.
I remember debugging some funky behavior regarding fragmenting packets and
UDPv6 some time ago. I don't remember if a fix was committed or not.
In the meantime I found the explaination.
Taken from
W. Richard Stevens
Unix Network Programming - Vol 1 - 3rd Edition: The Sockets Networking API
20.4 dg_cli Function Using Broadcasting
..
IP Fragmentation and Broadcasts
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?
Michael
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