Socket bound to 0.0.0.0 never receives broadcasts with non-zero IP source address

[John Howie]

Hi Yuri,

Is your machine a router or gateway, or have a firewall? Are you trying to
capture all broadcast packets, or just UDP targeted and broadcast packets
to a particular port?

Regards,

John

On 4/8/15, 5:21 PM, "Yuri" <yuri at rawbw.com> wrote:

>On 04/08/2015 05:32, Daniel Corbe wrote:
>> If nobody answers this question by the time I get home I'll try and
>> help; however, in the mean time I do have a couple of suggestions.
>>
>> Have you tried writing the equivalent program in C using the sockets
>> API?  IE is this a python specific problem or a sockets problem in
>> general?
>>
>> The second thing is you may just want to try using raw sockets instead.
>
>I verified before with ktrace, and now, following your suggestion,
>rewrote it in C, and result is the same.
>When I change SOCK_DGRAM->SOCK_RAW it keeps getting some other packets,
>sin_port doesn't seem to matter. Also, UDP is the practically important
>case.
>
>Unless there is some reasonable explanation why ip source address can
>influence reception, I believe this is a bug in kernel. And pretty
>important one, because it can hurt DHCP servers.
>
>Yuri
>
>
>--- C program exhibiting the problem ---
>
>#include <sys/types.h>
>#include <sys/socket.h>
>#include <stdio.h>
>#include <stdlib.h>
>#include <netinet/in.h>
>
>#define CK(func, call...) if ((call) < 0) {perror(#func); exit(1);}
>
>int main() {
>   int sock, one = 1;
>   ssize_t count;
>   struct sockaddr_in sa;
>   char buffer[4096];
>   struct sockaddr_storage src_addr;
>
>   struct iovec iov[1];
>   iov[0].iov_base=buffer;
>   iov[0].iov_len=sizeof(buffer);
>
>   struct msghdr msg;
>   msg.msg_name=&src_addr;
>   msg.msg_namelen=sizeof(src_addr);
>   msg.msg_iov=iov;
>   msg.msg_iovlen=1;
>   msg.msg_control=0;
>   msg.msg_controllen=0;
>
>   CK(socket, sock = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP))
>   CK(setsockopt, setsockopt(sock, SOL_SOCKET, SO_REUSEADDR, &one,
>sizeof(one)))
>   CK(setsockopt, setsockopt(sock, SOL_SOCKET, SO_BROADCAST, &one,
>sizeof(one)))
>   sa.sin_family = AF_INET;
>   sa.sin_addr.s_addr = htonl(INADDR_ANY);
>   //sa.sin_port = htons(67);
>   sa.sin_port = htons(1767);
>   CK(bind, bind(sock, (const struct sockaddr*)&sa, sizeof(sa)))
>
>   printf("Waiting for broadcast\n");
>   CK(recvmsg, count = recvmsg(sock, &msg, 0))
>   printf("Received broadcast packet: %zd bytes\n", count);
>
>   return 0;
>}
>
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