Linux C编程获取网关

#include <asm/types.h>
#include <netinet/ether.h>
#include <netinet/in.h>
#include <net/if.h>
#include <stdio.h>
#include <sys/socket.h>
#include <sys/ioctl.h>
#include <linux/netlink.h>
#include <linux/rtnetlink.h>
#include <sys/types.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{
  /* Recieve response from the kernel */
  if((readLen = recv(sockFd, bufPtr, BUFSIZE - msgLen, 0)) < 0){
   perror("SOCK READ: ");
   return -1;
  }
 
  nlHdr = (struct nlmsghdr *)bufPtr;

  /* Check if the header is valid */
  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;
}

/* For printing the routes. */
void printRoute(struct route_info *rtInfo)
{
 char tempBuf[512];
 
 /* Print Destination address */
 if(rtInfo->dstAddr != 0)
  strcpy(tempBuf, (char *)inet_ntoa(rtInfo->dstAddr));
 else
  sprintf(tempBuf,"*.*.*.*\t");
 fprintf(stdout,"%s\t", tempBuf);
 
 /* Print Gateway address */
 if(rtInfo->gateWay != 0)
  strcpy(tempBuf, (char *)inet_ntoa(rtInfo->gateWay));
 else
  sprintf(tempBuf,"*.*.*.*\t");
 fprintf(stdout,"%s\t", tempBuf);
 
 /* Print Interface Name*/
 fprintf(stdout,"%s\t", rtInfo->ifName);
 
 /* Print Source address */
 if(rtInfo->srcAddr != 0)
  strcpy(tempBuf, (char *)inet_ntoa(rtInfo->srcAddr));
 else
  sprintf(tempBuf,"*.*.*.*\t");
 fprintf(stdout,"%s\n", tempBuf);
}

/* For parsing the route info returned */
void parseRoutes(struct nlmsghdr *nlHdr, struct route_info *rtInfo,char *gateway)
{
 struct rtmsg *rtMsg;
 struct rtattr *rtAttr;
 int rtLen;
 char *tempBuf = NULL;

 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;
  }
 }
 //printf("%s\n", (char *)inet_ntoa(rtInfo->dstAddr));

 // ADDED BY BOB - ALSO COMMENTED printRoute

 if (strstr((char *)inet_ntoa(rtInfo->dstAddr), "0.0.0.0"))
  sprintf(gateway, (char *)inet_ntoa(rtInfo->gateWay));
  //printRoute(rtInfo);

 free(tempBuf);
 return;
}

int get_gateway(char *gateway)
{
 struct nlmsghdr *nlMsg;
 struct rtmsg *rtMsg;
 struct route_info *rtInfo;
 char msgBuf[BUFSIZE];
 
 int sock, len, msgSeq = 0;
 char buff[1024];
 
 
 /* Create Socket */
 if((sock = socket(PF_NETLINK, SOCK_DGRAM, NETLINK_ROUTE)) < 0)
 perror("Socket Creation: ");
 
 /* 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));
 // ADDED BY BOB

 /* THIS IS THE NETTSTAT -RL code I commented out the printing here and in parse routes */
 //fprintf(stdout, "Destination\tGateway\tInterface\tSource\n");

 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;
}


int main()
{
 char gateway[255]={0};
 get_gateway(gateway);
 printf("Gateway:%s\n", gateway);
}