linux下 获取本机网关

#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <net/if.h>
#include <sys/ioctl.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include <arpa/inet.h>
#include <net/if_arp.h> 
#include <string>

#include <asm/types.h>
#include <netinet/ether.h>

#include <sys/ioctl.h>
#include <linux/netlink.h>
#include <linux/rtnetlink.h>
#include<stdlib.h>

#define BUFSIZE 8192

struct route_info{
u_int dstAddr;
u_int srcAddr;
u_int gateWay;
char ifName[IF_NAMESIZE];
};

int readNlSock(int sockFd, char *bufPtr, int seqNum, int pId)
{
struct nlmsghdr *nlHdr;
int readLen = 0, msgLen = 0;

do{
    //收到内核的应答
    if((readLen = recv(sockFd, bufPtr, BUFSIZE - msgLen, 0)) < 0)
    {
      perror("SOCK READ: ");
      return -1;
    }
   
    nlHdr = (struct nlmsghdr *)bufPtr;

    //检查header是否有效
    if((NLMSG_OK(nlHdr, readLen) == 0) || (nlHdr->nlmsg_type == NLMSG_ERROR))
    {
      perror("Error in recieved packet");
      return -1;
    }
   
    /* Check if the its the last message */
    if(nlHdr->nlmsg_type == NLMSG_DONE) 
    {
     break;
    }
    else
    {
      /* Else move the pointer to buffer appropriately */
      bufPtr += readLen;
      msgLen += readLen;
    }
   
    /* Check if its a multi part message */
    if((nlHdr->nlmsg_flags & NLM_F_MULTI) == 0) 
    {
      /* return if its not */
     break;
    }
} while((nlHdr->nlmsg_seq != seqNum) || (nlHdr->nlmsg_pid != pId));
return msgLen;
}

//分析返回的路由信息
void parseRoutes(struct nlmsghdr *nlHdr, struct route_info *rtInfo,char *gateway)
{
struct rtmsg *rtMsg;
struct rtattr *rtAttr;
int rtLen;
char *tempBuf = NULL;
//2007-12-10
struct in_addr dst;
struct in_addr gate;

tempBuf = (char *)malloc(100);
rtMsg = (struct rtmsg *)NLMSG_DATA(nlHdr);

// If the route is not for AF_INET or does not belong to main routing table
//then return. 
if((rtMsg->rtm_family != AF_INET) || (rtMsg->rtm_table != RT_TABLE_MAIN))
return;

/* get the rtattr field */
rtAttr = (struct rtattr *)RTM_RTA(rtMsg);
rtLen = RTM_PAYLOAD(nlHdr);
for(;RTA_OK(rtAttr,rtLen);rtAttr = RTA_NEXT(rtAttr,rtLen)){
   switch(rtAttr->rta_type) {
   case RTA_OIF:
    if_indextoname(*(int *)RTA_DATA(rtAttr), rtInfo->ifName);
    break;
   case RTA_GATEWAY:
    rtInfo->gateWay = *(u_int *)RTA_DATA(rtAttr);
    break;
   case RTA_PREFSRC:
    rtInfo->srcAddr = *(u_int *)RTA_DATA(rtAttr);
    break;
   case RTA_DST:
    rtInfo->dstAddr = *(u_int *)RTA_DATA(rtAttr);
    break;
   }
}
//2007-12-10
dst.s_addr = rtInfo->dstAddr;
if (strstr((char *)inet_ntoa(dst), "0.0.0.0"))
{
    gate.s_addr = rtInfo->gateWay;
    sprintf(gateway, (char *)inet_ntoa(gate));
}
free(tempBuf);
return;
}

/********************************************************************
* 函数名： get_gateway
* 参数名： gateway(out)   网关
* 返回值： 0            成功
*          -1             失败
* 功 能：获取本地机的网关
********************************************************************/
int get_gateway(char *gateway)
{
struct nlmsghdr *nlMsg;
struct rtmsg *rtMsg;
struct route_info *rtInfo;
char msgBuf[BUFSIZE];

int sock, len, msgSeq = 0;

//创建 Socket 
if((sock = socket(PF_NETLINK, SOCK_DGRAM, NETLINK_ROUTE)) < 0)
{
   perror("Socket Creation: ");
   return -1;
}

/* Initialize the buffer */
memset(msgBuf, 0, BUFSIZE);

/* point the header and the msg structure pointers into the buffer */
nlMsg = (struct nlmsghdr *)msgBuf;
rtMsg = (struct rtmsg *)NLMSG_DATA(nlMsg);

/* Fill in the nlmsg header*/
nlMsg->nlmsg_len = NLMSG_LENGTH(sizeof(struct rtmsg)); // Length of message.
nlMsg->nlmsg_type = RTM_GETROUTE; // Get the routes from kernel routing table .

nlMsg->nlmsg_flags = NLM_F_DUMP | NLM_F_REQUEST; // The message is a request for dump.
nlMsg->nlmsg_seq = msgSeq++; // Sequence of the message packet.
nlMsg->nlmsg_pid = getpid(); // PID of process sending the request.

/* Send the request */
if(send(sock, nlMsg, nlMsg->nlmsg_len, 0) < 0){
printf("Write To Socket Failed.../n");
return -1;
}

/* Read the response */
if((len = readNlSock(sock, msgBuf, msgSeq, getpid())) < 0) {
printf("Read From Socket Failed.../n");
return -1;
}
/* Parse and print the response */
rtInfo = (struct route_info *)malloc(sizeof(struct route_info));

for(;NLMSG_OK(nlMsg,len);nlMsg = NLMSG_NEXT(nlMsg,len)){
memset(rtInfo, 0, sizeof(struct route_info));
parseRoutes(nlMsg, rtInfo,gateway);
}
free(rtInfo);
close(sock);
return 0;
}