Re: Bug with IPv6-UDP address binding

[Eric Dumazet <eric.dumazet@gmail.com>]

On Thu, 2012-08-09 at 11:40 +0200, Jesper Dangaard Brouer wrote:
> On Wed, 2012-08-08 at 22:59 +0200, Eric Dumazet wrote:
> > On Wed, 2012-08-08 at 22:37 +0200, Jesper Dangaard Brouer wrote:

> > Then nc should bind a new socket on this address, then do the connect()
> 
> Yes, after the difficult extraction of the dest IP of the UDP packet.
> 

Thats 10 lines of code. The hard part is in kernel actually ;)

> 
> Now I better understand, why the DNS server named/bind is so annoying,
> that is requires a restart after adding IPs.  I guess they didn't
> implement this recvmsg(), and instead chooses to bind to all avail IPs
> on init/start.

Thats an implementation choice, no more no less.

Here is an IPv4 sample UDP application, able to echo packets with the IP
source set to original DST address of the ping packet.

Doing the same on IPv6 is probably trivial as well

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

#define PORT 4040

int pktinfo_get(struct msghdr *my_hdr, struct in_pktinfo *pktinfo)
{
	int res = -1;

	if (my_hdr->msg_controllen > 0) {
		struct cmsghdr *get_cmsg;
		for (get_cmsg = CMSG_FIRSTHDR(my_hdr); get_cmsg;
			get_cmsg = CMSG_NXTHDR(my_hdr, get_cmsg)) {
			if (get_cmsg->cmsg_type == IP_PKTINFO) {
				struct in_pktinfo *get_pktinfo = (struct in_pktinfo *)CMSG_DATA(get_cmsg);
				memcpy(pktinfo, get_pktinfo, sizeof(*pktinfo));
				res = 0;
			}
		}
	}
	return res;
}

int main(int argc, char *argv[])
{
	int fd = socket(AF_INET, SOCK_DGRAM, 0);
	struct sockaddr_in addr, rem_addr;
	int res, on = 1;
	struct msghdr msghdr;
	struct iovec vec[1];
	char cbuf[512];
	char frame[4096];
	struct in_pktinfo pktinfo;
	int c, count = 1000000;

	while ((c = getopt(argc, argv, "c:")) != -1) {
		if (c == 'c') count = atoi(optarg);
		}
	memset(&addr, 0, sizeof(addr));
	addr.sin_family = AF_INET;
	addr.sin_port = htons(PORT);
	if (bind(fd, (struct sockaddr *)&addr, sizeof(addr)) == -1) {
		perror("bind");
		return 1;
	}
	setsockopt(fd, SOL_IP, IP_PKTINFO, &on, sizeof(on));

	while (1) {
		memset(&msghdr, 0, sizeof(msghdr));
		msghdr.msg_control = cbuf;
		msghdr.msg_controllen = sizeof(cbuf);
		msghdr.msg_iov = vec;
		msghdr.msg_iovlen = 1;
		vec[0].iov_base = frame;
		vec[0].iov_len = sizeof(frame);
		msghdr.msg_name = &rem_addr;
		msghdr.msg_namelen = sizeof(rem_addr);
		res = recvmsg(fd, &msghdr, 0);
		if (res == -1)
			break;
		if (pktinfo_get(&msghdr, &pktinfo) == 0)
			printf("Got IP_PKTINFO dst addr=%s\n", inet_ntoa(pktinfo.ipi_spec_dst));

		/* ok, just echo reply this frame.
		 * Using sendmsg() will provide IP_PKTINFO back to kernel
		 * to let it use the 'right' source address
		 * (destination address of the incoming packet)
		 */
		vec[0].iov_len = res;
		sendmsg(fd, &msghdr, 0);
		if (--count == 0)
			break;
	}
	return 0;
}