A problem about NETLINK

["xiaoyu Du" <tingsrain@xxxxxxxxx>]

Does the NETLINK interface have changed a lot from 2.6.17 - 2.6.20.
if not. Why my code which runs fine on 2.6.17 cannot run on my FC 6 with 2.6.20
version kernel(Both official or my compiled).
This is the code (just get the gateway IP address ):

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<unistd.h>
#include<sys/socket.h>
#include<netinet/in.h>
#include<sys/types.h>
#include<linux/netlink.h>
#include<linux/rtnetlink.h>

struct req {
 	struct nlmsghdr nlmsg;
 	struct rtmsg rtm;
};

static struct rtattr* get_gw_attr(struct nlmsghdr *nlmsghdr);
int print_route(struct rtattr *rtp);

int get_gateway_ip(struct sockaddr *gwp);

int main(void)
{
	int n;
	struct sockaddr gw;
	if (get_gateway_ip(&gw)); {
		perror("inet_ntop failed !");
		exit (1);
	}
}

int get_gateway_ip(struct sockaddr *gwp){
	int fd;
	int n;
	char *c;
	char buff[BUFSIZ];
	char str[22];
	struct req req;
	struct rtattr *rtp;
 	struct nlmsghdr nlmsg, *nlp;
 	struct rtmsg rtm;
	struct sockaddr so;
	struct sockaddr_nl nl;
	struct in_addr addr;

	fd = socket(AF_NETLINK, SOCK_RAW, NETLINK_ROUTE);
	if (fd < 0) {
		perror("open error");
		exit (1);
	}
	nl.nl_family = AF_NETLINK;
	nl.nl_pad = 0;
	nl.nl_pid = getpid();
	nl.nl_groups = 0;

	n = bind(fd, (struct sockaddr*)&nl, sizeof(nl));
	if (n < 0) {
		perror(" bind  error");
		exit (1);
	}

	memset(&nlmsg, 0, sizeof(struct nlmsghdr));
	req.nlmsg.nlmsg_len = NLMSG_LENGTH(sizeof(struct rtmsg));
	req.nlmsg.nlmsg_type = RTM_GETROUTE;

	/** In kernel header comments NLM_F_ROOT
		 "specify tree root", but On somebooks
	   said this means return the entire table
		 not just one entry **/
	req.nlmsg.nlmsg_flags = NLM_F_ROOT|NLM_F_REQUEST;

	 /** this is not required! **/
//	req.nlmsg.nlmsg_pid = nl.pid;

	memset(&rtm, 0, sizeof(struct rtmsg));
	req.rtm.rtm_family = AF_NETLINK;
	
	/** we need change here **/
	req.rtm.rtm_dst_len = 4;
	req.rtm.rtm_src_len = 4;

	req.rtm.rtm_table = RT_TABLE_MAIN;

	/** Route installed during boot **/
	req.rtm.rtm_protocol = RTPROT_BOOT;
	/** located on directly LAN **/
	req.rtm.rtm_scope = RT_SCOPE_LINK;

	/** Gateway or direct route **/
	req.rtm.rtm_type = RTN_UNICAST;

	n = send(fd, &req, sizeof(req), 0);
	if (n < 0)
	{
		perror("send error!");
		exit (-1);
	}
	else
		printf("%d bytes send!\n",n);

	n = recv(fd, buff, BUFSIZ, 0);
	if (n < 0)
		perror("received failed!");

	printf("%d bytes received!\n", n);
	for (nlp = (struct nlmsghdr*)buff; \
		(nlp->nlmsg_type != NLMSG_DONE)&& NLMSG_OK(nlp, n);
			nlp = NLMSG_NEXT(nlp, n)) {
				if ((rtp = get_gw_attr(nlp)) != NULL) {
					memcpy((void* )(gwp->sa_data), RTA_DATA(rtp),\
						 sizeof(struct in_addr));
					print_route(rtp);
					return 0;
				}
	}
	return -1;
}
static struct rtattr* get_gw_attr(struct nlmsghdr *nlmsghdr)
{
	struct rtattr *rta;
	int len;
	int gw;
	char str[16];

	len = nlmsghdr->nlmsg_len - NLMSG_LENGTH(sizeof(struct rtmsg));

	/** NLMSG_DATA(nlmsghdr) return the rtmsg
		 pointer following, and RTM_RTA return
		the rtattr pointer following the rtmsg.	**/
	rta = RTM_RTA(NLMSG_DATA(nlmsghdr));
	while (RTA_OK(rta, len)) {
		if (rta->rta_type >= RTA_MAX)
			break;
		/** We check if the address is INADDR_ANY.I
			 don't know whethher this is needed **/
		if(rta->rta_type == RTA_GATEWAY &&\
			 *(int *)RTA_DATA(rta) != INADDR_ANY)
				return rta;
		rta = RTA_NEXT(rta, len);
	}
	return NULL;
};
int print_route(struct rtattr *rtp)
{
	char str[16];
	char *c;
	char buff[16];
	c = inet_ntop (AF_INET, RTA_DATA(rtp), buff, INET_ADDRSTRLEN);
	if (!c) {
		perror("inet_ntop failed !");
		exit (1);
	}
	printf("The gateway IP address is %s\n",c);
}
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