how to get default gateway via rtnetlink (c code example)

this is a example code showing you how to get the ip of the default gateway via netlink socket. I've also attached some useful information if you want to study netlink

further reading:

Sample Code To Learn Netlink Infrastructure

 understanding and programming with netlink sockets(PDF)

reference:
man 3 rtnetlink, man 7 rtnetlink, man 3 netlink, man 7 netlink


example code:

#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

 char gateway[255];

 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);
 }

 void printGateway()
 {
  printf("%s\n", gateway);
 }

 /* For parsing the route info returned */
 void parseRoutes(struct nlmsghdr *nlHdr, struct route_info *rtInfo)
 {

  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.s_addr = *(u_int *)RTA_DATA(rtAttr);

     break;
   case RTA_PREFSRC:
     rtInfo->srcAddr.s_addr = *(u_int *)RTA_DATA(rtAttr);

     break;
   case RTA_DST:
     rtInfo->dstAddr.s_addr = *(u_int *)RTA_DATA(rtAttr);

     break;
   }
  }
  //printf("%s\n", (char *)inet_ntoa(rtInfo->dstAddr));
  if (strstr((char *)inet_ntoa(rtInfo->dstAddr), "0.0.0.0"))

   sprintf(gateway, (char *)inet_ntoa(rtInfo->gateWay));
  //printRoute(rtInfo);

  free(tempBuf);
  return;
 }

 int main()
 {

  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));

  /* 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);
  }

  free(rtInfo);
  close(sock);

  printGateway();

  return 0;
 }