Cannot receive multicast packets

Cannot receive multicast packets

[Andrew Athan]

I sent a similar message to LKML, and linux-dev but was directed to this 
list might as most appropriate.  Thanks in advance...

It's likely this is a programming error, but I am at a loss.

The simple UDP multicast receive application below, adapted from source
on IBM's web site, will not receive any data.  While it is blocked on
recv():

hfclinux2:/home/root/UDP/apps/Crosser2# netstat -ang
IPv6/IPv4 Group Memberships
Interface       RefCnt Group
--------------- ------ ---------------------
lo              1      224.0.0.1
eth0            1      224.0.0.1
eth1            1      224.0.17.44
eth1            1      224.0.0.1
lo              1      ff02::1
eth0            1      ff02::1:ffdd:eb2b
eth0            1      ff02::1
eth1            1      ff02::1:ffdd:eb2c
eth1            1      ff02::1


And tcpdump shows traffic arriving at the interface(see below)!  This
leads me to belive the kernel is filtering it, and or not delivering it
to the socket.

The one "strange" thing about this network is that the traffic is
flowing in "dense mode", meaning there is no explicit need for IGMP
messaging (nor any response to such messaging).

Can anyone point me in the right direction regarding why the socket
never receives any data????

Thanks.
Andrew

hfclinux2:/home/root/UDP/apps/Crosser2# uname -a
Linux hfclinux2 2.6.15.1smp #6 SMP PREEMPT Mon Feb 13 11:38:32 EST 2006
i686 GNU/Linux

hfclinux2:/home/root/UDP/apps/Crosser2# tcpdump -i eth1 port 55300
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth1, link-type EN10MB (Ethernet), capture size 96 bytes
12:54:06.980000 IP 206.200.5.144.2232 > 224.0.17.44.55300: UDP, length 91
12:54:07.079675 IP 206.200.5.144.2232 > 224.0.17.44.55300: UDP, length 46
12:54:07.179869 IP 206.200.5.144.2232 > 224.0.17.44.55300: UDP, length 181
12:54:07.280437 IP 206.200.5.144.2232 > 224.0.17.44.55300: UDP, length 181
12:54:07.380005 IP 206.200.5.144.2232 > 224.0.17.44.55300: UDP, length 181
12:54:07.480073 IP 206.200.5.144.2232 > 224.0.17.44.55300: UDP, length 136
12:54:07.680459 IP 206.200.5.144.2232 > 224.0.17.44.55300: UDP, length 91
12:54:07.780277 IP 206.200.5.144.2232 > 224.0.17.44.55300: UDP, length 136


hfclinux2:/home/root/UDP/apps/Crosser2# ifconfig
eth0      Link encap:Ethernet  HWaddr 00:11:43:DD:EB:2B
         inet addr:172.22.71.128  Bcast:172.22.71.255  Mask:255.255.255.0
         inet6 addr: fe80::211:43ff:fedd:eb2b/64 Scope:Link
         UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
         RX packets:19540 errors:0 dropped:0 overruns:0 frame:0
         TX packets:7838 errors:0 dropped:0 overruns:0 carrier:0
         collisions:0 txqueuelen:1000
         RX bytes:1563190 (1.4 MiB)  TX bytes:1943895 (1.8 MiB)
         Base address:0xdcc0 Memory:dfae0000-dfb00000

eth0:1    Link encap:Ethernet  HWaddr 00:11:43:DD:EB:2B
         inet addr:10.61.6.128  Bcast:10.61.6.255  Mask:255.255.255.0
         UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
         Base address:0xdcc0 Memory:dfae0000-dfb00000

eth0:2    Link encap:Ethernet  HWaddr 00:11:43:DD:EB:2B
         inet addr:196.0.34.11  Bcast:196.0.34.255  Mask:255.255.255.0
         UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
         Base address:0xdcc0 Memory:dfae0000-dfb00000

eth0:3    Link encap:Ethernet  HWaddr 00:11:43:DD:EB:2B
         inet addr:192.168.254.128  Bcast:192.168.254.255
Mask:255.255.255.0
         UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
         Base address:0xdcc0 Memory:dfae0000-dfb00000

eth1      Link encap:Ethernet  HWaddr 00:11:43:DD:EB:2C
         inet addr:192.168.250.128  Bcast:192.168.250.255
Mask:255.255.255.0
         inet6 addr: fe80::211:43ff:fedd:eb2c/64 Scope:Link
         UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
         RX packets:137717 errors:0 dropped:0 overruns:0 frame:0
         TX packets:24 errors:0 dropped:0 overruns:0 carrier:0
         collisions:0 txqueuelen:1000
         RX bytes:58314262 (55.6 MiB)  TX bytes:1596 (1.5 KiB)
         Base address:0xccc0 Memory:df8e0000-df900000

lo        Link encap:Local Loopback
         inet addr:127.0.0.1  Mask:255.0.0.0
         inet6 addr: ::1/128 Scope:Host
         UP LOOPBACK RUNNING  MTU:16436  Metric:1
         RX packets:0 errors:0 dropped:0 overruns:0 frame:0
         TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
         collisions:0 txqueuelen:0
         RX bytes:0 (0.0 b)  TX bytes:0 (0.0 b)



-------------------------
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>

using namespace std;


struct sockaddr_in    localSock;
struct ip_mreqn        group;
int                   sd;
int                   datalen;
char                  databuf[10];

int main (int argc, char *argv[])
{
sd = socket(AF_INET, SOCK_DGRAM, 0);
if (sd < 0) {
   perror("opening datagram socket");
   exit(1);
}

{
   int reuse=1;

   if (setsockopt(sd, SOL_SOCKET, SO_REUSEADDR,
                  (char *)&reuse, sizeof(reuse)) < 0) {
     perror("setting SO_REUSEADDR");
     close(sd);
     exit(1);
   }
}

memset((char *) &localSock, 0, sizeof(localSock));
localSock.sin_family = AF_INET;
localSock.sin_port = htons(55300);;
localSock.sin_addr.s_addr  = INADDR_ANY;

if (bind(sd, (struct sockaddr*)&localSock, sizeof(localSock))) {
   perror("binding datagram socket");
   close(sd);
   exit(1);
}


group.imr_multiaddr.s_addr = inet_addr("224.0.17.44");
printf("%d\n",group.imr_multiaddr.s_addr);
group.imr_address.s_addr = inet_addr("192.168.250.128");
if (setsockopt(sd, IPPROTO_IP, IP_ADD_MEMBERSHIP,
                (char *)&group, sizeof(group)) < 0) {
   perror("adding multicast group");
   close(sd);
   exit(1);
}
/*
  * Read from the socket.
  */
datalen = sizeof(databuf);
if (recv(sd, databuf, datalen,0) < 0) {
   perror("reading datagram message");
   close(sd);
   exit(1);
} else {
   puts("success reading");
}
}

Re: Cannot receive multicast packets

[David Stevens]

I've run your test program and it receives fine for me.

I note that the source address is not on the same subnet as
(any of) the receiver's addresses. Are the packets being
routed? The default multicasting TTL is 1, though I don't
know if it'll be checked or dropped on the receiver, seeing
as we aren't forwarding it.

Also, you might want to run "netstat -s" to see if any of the
drop counters are being incremented (e.g., checksum error).

Finally, I'm assuming you don't have any firewall rules that
are matching, right?

                                        +-DLS

Re: Cannot receive multicast packets

[Andrew Athan]

David:

Thank you for taking the time to respond.

The packets are arriving via a switched network composed of Cisco 
devices in PIM dense mode.  The packets pass through several switch 
hops, but no routing hops that have been documented to me.  I did not 
think the source IP was relevant to the matching code in linux, since 
there are no source squelching socket options.

There are no firewall rules active on this machine, and the packets are 
definitely visible at the interface (see tcpdump output in my email).

I am going to try upgrading the kernel, and turning off the multicast 
router kernel options as a next step.  But if you have any other ideas 
at all, I'm all ears.

This seems too much like Mr. Murphy's in the room.

A.

David Stevens wrote:
> I've run your test program and it receives fine for me.
>
> I note that the source address is not on the same subnet as
> (any of) the receiver's addresses. Are the packets being
> routed? The default multicasting TTL is 1, though I don't
> know if it'll be checked or dropped on the receiver, seeing
> as we aren't forwarding it.
>
> Also, you might want to run "netstat -s" to see if any of the
> drop counters are being incremented (e.g., checksum error).
>
> Finally, I'm assuming you don't have any firewall rules that
> are matching, right?
>
>                                         +-DLS
>
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>   

Re: Cannot receive multicast packets

[David Stevens]

Andrew,

>  I did not 
> think the source IP was relevant to the matching code in linux, since 
> there are no source squelching socket options. 
> 
> There are no firewall rules active on this machine, and the packets are 
> definitely visible at the interface (see tcpdump output in my email).

        The source address is not relevant (other than potentially
for firewall rules), and I understand from your original mail that
they are arriving at the machine. The IP TTL is what I wanted to
know there; but "netstat -s" will normally tell you why a packet
was dropped, if it's arriving but not making it through the UDP/IP
stack (as is your case).

> I am going to try upgrading the kernel, and turning off the multicast 
> router kernel options as a next step.  But if you have any other ideas 
> at all, I'm all ears.

        "netstat -s" would be a good start. :-) tcpdump receiving a copy
of the packet does not mean UDP or IP won't drop it, but those drops
are counted.

                                                        +-DLS