broadcast oddity

[Daniel Braniss]

Hi Eygene,

> Daniel, good day.
> 
> Mon, Jul 18, 2011 at 05:04:27PM +0300, Daniel Braniss wrote:
> > this code behaves correctly when run from a diskless host which
> > booted via PXE, but fails on a host that was booted from disk.
> > hint: the non working sends a packet with a non ethernet broadcast
> > address and an ip address of 255.255.255.255,
> 
> And that non-broadcast ethernet address is the MAC of your
> default router?
yes.

> 
> > the working version sets the ethernet address to 0xffffffff and the
> > ip to the network broadcast address.
> 
> What's your routing table (netstat -rn) for the PXE-booted host?
it's ok, same in both cases. it's picked up via DHCP, in the diskless
case by the boot/loader in the second via dhcpclient.

> 
> > what am I doing wrong?
> >=20
> > danny
> > PS: what is the correct way to obtain the network broadcast address?
> 
> You nailed it: you should send packets to the network's broadcast,
> not to the 0xffffffff.
> 
but I'm at the user/ip level!, have no way to set mac/ethernet address.

still, the question is why it works in one case, and failes in the other.

> And in order to find the broadcast address for the interface (or
> network at that interface), you should use getifaddrs(), like in the
> attached example.  It is very quick and dirty one and it has some
> limitations (e.g., it takes the first broadcast address from the
> interface), but it should be a good starting point.

thanks,
	danny
> Eygene Ryabinkin                                        ,,,^..^,,,
> [ Life's unfair - but root password helps!           | codelabs.ru ]
> [ 82FE 06BC D497 C0DE 49EC  4FF0 16AF 9EAE 8152 ECFB | freebsd.org ]
> 
> --UeXZ3FjlYZvuln/G
> Content-Type: text/plain; charset=us-ascii
> Content-Disposition: attachment; filename="bcast.c"
> 
> #include <sys/types.h>
> #include <sys/socket.h>
> #include <netinet/in.h>
> #include <net/if.h>
> #include <ifaddrs.h>
> #include <stdlib.h>
> #include <string.h>
> #include <time.h>
> #include <stdio.h>
> 
> void
> bcast(in_addr_t addr)
> {
>      int so, on;
>      char msg[BUFSIZ];
>      struct timespec t2;
>      struct sockaddr_in soin;
> 
>      if((so = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)) < 0) {
>           perror("socket");
>           exit(-1);
>      }
>      on = 1;
>      if(setsockopt(so, SOL_SOCKET, SO_BROADCAST, &on, sizeof(on))) {
>           perror("setsockopt");
>           exit(-1);
>      }
> 
>      bzero(&soin, sizeof(struct sockaddr_in));
>      soin.sin_len = sizeof(struct sockaddr_in);
>      soin.sin_family = AF_INET;
>      soin.sin_addr.s_addr = addr;
>      soin.sin_port = htons(12345);
>      while(1) {
>           clock_gettime(CLOCK_REALTIME, &t2);
>           sprintf(msg, "0x%016x", t2.tv_sec);
>           if(sendto(so, msg, strlen(msg)+1,
>                     0, (struct sockaddr *)&soin, sizeof(struct sockaddr)) < 0) {
>                perror("sendto");
>                break;
>           }
>           sleep(10);
>      }
> }
> 
> main(int argc, char *argv[])
> {
>      struct ifaddrs *ifap, *ifa, *our_if;
>      struct sockaddr_in *sin;
> 
>      if (argc < 2)
>           errx(1, "No arguments");
>      if (getifaddrs(&ifap) != 0)
>           perror("getifaddrs");
>      our_if = NULL;
>      for (ifa = ifap; ifa != NULL; ifa = ifa->ifa_next) {
>          if (strcmp(argv[1], ifa->ifa_name) == 0) {
>              sin = (struct sockaddr_in *)ifa->ifa_broadaddr;
>              if (!(ifa->ifa_flags & IFF_BROADCAST) ||
>                sin == NULL ||
>                sin->sin_addr.s_addr == 0)
>                  continue;
>              our_if = ifa;
>              break;
>          }
>      }
>      if (!our_if)
>          errx(1, "Can't find broadcast-able interface '%s'", argv[1]);
>      bcast(sin->sin_addr.s_addr);
>      freeifaddrs(ifap);
> }
> 
> --UeXZ3FjlYZvuln/G--
> 
> --McpcKDxJRrEJVmOH
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