Re: "message too long" when sending 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?
>
> 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
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Re: "message too long" when sending broadcasts
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 ___ freebsd-stable@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-stable To unsubscribe, send any mail to "[EMAIL PROTECTED]"
"message too long" when sending broadcasts
Hi,
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?
Thanks,
Michael
N.B:
Code snippet is attached in case somebody cares to take a look - it works fine
for me when using "./bcast-test -r -i lnc1 -p 1500" on the receiving box and
"./bcast-test -r -i lnc1 -p XXX" on the sending box for XXX < 1473.
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#define PORT 694
#define MAXBUF 2048
void
usage(int argc, char ** argv)
{
printf("Usage: %s [-s] [-r] -p packetsize -i eth0/eth1/...\n", argv[0]);
printf("#default is to receive\n");
return;
}
int
create_recv_socket(char* ifname)
{
struct sockaddr_in my_addr;
int sockfd;
int one = 1;
struct ifreq i;
memset(&my_addr, 0, sizeof(my_addr));
my_addr.sin_family = AF_INET;
my_addr.sin_port = htons(PORT);
my_addr.sin_addr.s_addr = htonl(INADDR_ANY);
if (( sockfd = socket(AF_INET, SOCK_DGRAM, 0)) == -1){
printf("Error getting socket: %s\n", strerror(errno));
exit(1);
}
if (setsockopt( sockfd, SOL_SOCKET,
SO_REUSEADDR, (const void*)&one, sizeof(one)) < 0){
printf("Error setting socket option SO_REUSEADDR: %s\n",
strerror(errno));
exit(1);
}
strcpy(i.ifr_name, ifname);
if (bind(sockfd, (struct sockaddr *) & my_addr, sizeof(struct sockaddr))
< 0){
printf("Error binding socket: %s\n", strerror(errno));
exit(1);
}
return sockfd;
}
void
recv_packets(int sockfd, int packetsize)
{
int bufsize = packetsize + 100;
char buf[MAXBUF];
int n;
struct sockaddr_in theiraddr;
int len = sizeof(theiraddr);
while(1){
memset(buf, 0, MAXBUF);
if ((n = recvfrom(sockfd, buf, bufsize - 1, 0,
(struct sockaddr*)&theiraddr, &len)) == -1){
printf("Error receiving from socket: %s\n",
strerror(errno));
exit(1);
}
printf("Message received: (%d bytes)\n%s\n", n, buf);
}
}
int
create_send_socket(char* ifname)
{
int sockfd;
int one = 1;
struct ifreq i;
strcpy(i.ifr_name,ifname);
if ((sockfd = socket(AF_INET, SOCK_DGRAM, 0)) <= 0) {
printf("Error in getting socket: %s", strerror(errno));
exit(1);
}
if (setsockopt(sockfd, SOL_SOCKET, SO_BROADCAST, (const void*) &one,
sizeof(one)) == -1)
{
printf("Error setting socket option SO_BROADCAST:
sockfd =%d , %s\n",
sockfd, strerror(errno));
close(sockfd);
exit(1);
}
return sockfd;
}
int
get_baddr(struct sockaddr_in* baddr, char* ifname)
{
struct ifreq ifr;
int fd;
int return_val;
fd = socket (PF_INET, SOCK_DGRAM, 0);
if (fd <0) {
printf("Error opening socket: %s\n", strerror(errno));
exit(1);
}
strcpy (ifr.ifr_name, ifname);
return_val = ioctl(fd, SIOCGIFBRDADDR, &ifr);
if (return_val == 0 )
{
if (ifr.ifr_broadaddr.sa_family == AF_INET)
{
struct sockaddr_in sin_ptr;
memcpy(&sin_ptr, &ifr.ifr_broadaddr,
sizeof(sin
