Binding to 0.0.0.0 as src addr for UDP Broadcast Message

lcamp · 01-28-2011, 09:43 AM

How do you bind a UDP socket to 0.0.0.0 not INADDR_ANY
I am trying to send a DHCPDISCOVER packet on udp but the address src is INADDR_ANY and is grabbing the address from another interface.

Currently I am broadcasting on a selected interface but still the SRC Address comes up 192.168.*.* when I need the src to be 0.0.0.0 in accordance with RFC.

Code:

	snprintf(ifr.ifr_name,sizeof(ifr.ifr_name),"tap0");


	bcast.sin_family = AF_INET;
	bcast.sin_port = htons(67);
	bcast.sin_addr.s_addr = INADDR_BROADCAST;

	src.sin_family = AF_INET;
	src.sin_port = htons(68);

	memcpy(&ifr.ifr_addr,&src,sizeof(src));



	sockfd = Socket(AF_INET,SOCK_DGRAM,0);
	Bind(sockfd,(SA *) &src,sizeof(src));
	if(ioctl(sockfd,SIOCSIFADDR,&ifr)<0)
		err_quit("ioctl error\n");
	if(setsockopt(sockfd,SOL_SOCKET,SO_BINDTODEVICE,(void *) &ifr,sizeof(ifr)) < 0)
		err_quit("setsockopt\n");
	if(setsockopt(sockfd,SOL_SOCKET,SO_BROADCAST,(char *) &flags, sizeof(flags)) < 0)
		err_quit("setsockopt\n");

as a note both bcast and src are both struct sockaddr_in and are initialized to zero.

jefro · 01-28-2011, 03:46 PM

This??

http://stackoverflow.com/questions/8...cket-in-python

lcamp · 01-29-2011, 07:45 AM

So the problem is that I want to mimic a DHCPDISCOVER packet and when I bind to INADDR_ANY (0.0.0.0) the other interfaces that are up interfere for example:
eth0: 192.168.1.8
eth1: down
and I want to send the packet on eth1, the packet goes out from a source of 192.168.1.8 even though I binded to 0.0.0.0.

The solution I am using (it's not good) is to bring all interfaces down first

Code:

int sockfd = Socket(AF_INET,SOCK_DGRAM,0); //wrapper
struct ifreq ifr;
bzero(&ifr,sizeof(ifr));
/* specify interface to bring down */
strncpy(ifr.ifr_name,"eth1",sizeof(ifr.ifr_name));
ifr.ifr_flags = 0; //forces IFF_UP to be false
if(ioctl(sockfd,SIOCSIFFLAGS,&ifr)<0)
  err_quit("ioctl error\n");

I do this for the interfaces that are up then send the packet and it successfully transmits from 0.0.0.0 to 255.255.255.255 in accordance with DHCP RFC.

Then I have to bring up the interfaces and they're settings are messed up so I have to save all the interfaces' settings and it becomes tedious I would have thought there would be a more simple way to bind to 0.0.0.0 and not have it switch to the INADDR_ANY in the syscall to bind.