This is OS specific, there's no unified(or ANSI C) API for this.
Assuming Linux, the best way is to just parse /proc/net/route , look for the entry where Destination is 00000000 , the default gateway is in the Gateway column , where you can read the hex representation of the gateway IP address (in big endian , I believe)
If you want to do this via more specific API calls, you'll have to go through quite some hoops, here's an example program:
#include <netinet/in.h>
#include <net/if.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/socket.h>
#include <sys/ioctl.h>
#include <linux/netlink.h>
#include <linux/rtnetlink.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#define BUFSIZE 8192
char gateway[255];
struct route_info {
struct in_addr dstAddr;
struct in_addr srcAddr;
struct in_addr 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.s_addr != 0)
strcpy(tempBuf, inet_ntoa(rtInfo->dstAddr));
else
sprintf(tempBuf, "*.*.*.*\t");
fprintf(stdout, "%s\t", tempBuf);
/* Print Gateway address */
if (rtInfo->gateWay.s_addr != 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.s_addr != 0)
strcpy(tempBuf, 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;
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", inet_ntoa(rtInfo->dstAddr));
if (rtInfo->dstAddr.s_addr == 0)
sprintf(gateway, (char *) inet_ntoa(rtInfo->gateWay));
//printRoute(rtInfo);
return;
}
int main()
{
struct nlmsghdr *nlMsg;
struct rtmsg *rtMsg;
struct route_info *rtInfo;
char msgBuf[BUFSIZE];
int sock, len, msgSeq = 0;
/* Create Socket */
if ((sock = socket(PF_NETLINK, SOCK_DGRAM, NETLINK_ROUTE)) < 0)
perror("Socket Creation: ");
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));
//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;
}