二、取得本机网关

#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))
    //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)) 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)
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;
}

 

转自：http://blog.163.com/www_baby/blog/static/97325854200891311616663/