gateway.c

/*
 * =====================================================================================
 *
 *       Filename:  gateway.c
 *
 *    Description:  print the default gateway. Code has been found
 *                  at http://www.linuxquestions.org/questions/linux-networking-3/howto-find-gateway-address-through-code-397078/
 *
 *        Version:  1.0
 *        Created:  03/17/2015 12:10:21 PM
 *       Revision:  none
 *       Compiler:  gcc
 *
 *         Author:  nikkolasg (mn), 
 *        Company:  
 *
 * =====================================================================================
 */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include <sys/socket.h>
#include <net/if.h>
#include <linux/rtnetlink.h>

#include <unistd.h>
#include <arpa/inet.h>

#define BUFSIZE 8192

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, size_t buf_size, int seqNum, int pId)
{
    struct nlmsghdr *nlHdr;
    int readLen = 0, msgLen = 0;

    do
    {
        /* Recieve response from the kernel */
        if((readLen = recv(sockFd, bufPtr, buf_size - 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;
}

/* parse the route info returned */
int 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 -1;

    /* 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:
                memcpy(&rtInfo->gateWay, RTA_DATA(rtAttr), sizeof(rtInfo->gateWay));
                break;

            case RTA_PREFSRC:
                memcpy(&rtInfo->srcAddr, RTA_DATA(rtAttr), sizeof(rtInfo->srcAddr));
                break;

            case RTA_DST:
                memcpy(&rtInfo->dstAddr, RTA_DATA(rtAttr), sizeof(rtInfo->dstAddr));
                break;
        }
    }

    return 0;
}

// meat
int get_gatewayip(char *gatewayip, socklen_t size)
{
    int found_gatewayip = 0;

    struct nlmsghdr *nlMsg;
    //struct rtmsg *rtMsg;
    struct route_info route_info;
    char msgBuf[BUFSIZE]; // pretty large buffer

    int sock, len, msgSeq = 0;

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

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

    /* 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)
    {
        fprintf(stderr, "Write To Socket Failed...\n");
        return -1;
    }

    /* Read the response */
    if((len = readNlSock(sock, msgBuf, sizeof(msgBuf), msgSeq, getpid())) < 0)
    {
        fprintf(stderr, "Read From Socket Failed...\n");
        return -1;
    }

    /* Parse and print the response */
    for(; NLMSG_OK(nlMsg,len); nlMsg = NLMSG_NEXT(nlMsg,len))
    {
        memset(&route_info, 0, sizeof(route_info));
        if ( parseRoutes(nlMsg, &route_info) < 0 )
            continue;  // don't check route_info if it has not been set up

        // Check if default gateway
        if (strstr((char *)inet_ntoa(route_info.dstAddr), "0.0.0.0"))
        {
            // copy it over
            inet_ntop(AF_INET, &route_info.gateWay, gatewayip, size);
            found_gatewayip = 1;
            break;
        }
    }

    close(sock);

    return found_gatewayip;
}


    int
main ( int argc, char *argv[] )
{
    char gateway[INET_ADDRSTRLEN];
    if ( get_gatewayip(gateway,INET_ADDRSTRLEN) == -1) {
        fprintf(stderr,"exiting application ...\n");
        exit(EXIT_FAILURE);
    } 
    printf("Default Gateaway : %s\n",gateway);
    return EXIT_SUCCESS;
}				/* ----------  end of function main  ---------- */