Linux系统获取主机网关IP 代码(C/C++)

#include <arpa/inet.h>  //for in_addr
#include <linux/rtnetlink.h>    //for rtnetlink
#include <net/if.h> //for IF_NAMESIZ, route_info
#include <stdio.h>
#include <stdlib.h> //for malloc(), free()
#include <string.h> //for strstr(), memset()
#include <string>
using namespace std;

 

#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)) < 0)
        {
            perror("SOCK READ: ");
            return -1;
        }
        nlHdr = (struct nlmsghdr *)bufPtr;
        //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;
        }
        if(nlHdr->nlmsg_type == NLMSG_DONE)
        {
            break;
        }
        else
        {
            bufPtr += readLen;
            msgLen += readLen;
        }
        if((nlHdr->nlmsg_flags & NLM_F_MULTI) == 0)
        {
            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, char *ifName)
{
    struct rtmsg *rtMsg;
    struct rtattr *rtAttr;
    int rtLen;
    char *tempBuf = NULL;
    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;

 

    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;
        }
    }
    dst.s_addr = rtInfo->dstAddr;
    if (strstr((char *)inet_ntoa(dst), "0.0.0.0"))
    {
        sprintf(ifName, "%s", rtInfo->ifName);
        //printf("oif:%s",rtInfo->ifName);
        gate.s_addr = rtInfo->gateWay;
        sprintf(gateway, "%s", (char *)inet_ntoa(gate));
        //printf("%s\n",gateway);
        gate.s_addr = rtInfo->srcAddr;
        //printf("src:%s\n",(char *)inet_ntoa(gate));
        gate.s_addr = rtInfo->dstAddr;
        //printf("dst:%s\n",(char *)inet_ntoa(gate));
    }
    free(tempBuf);
    return;
}

 

int get_gateway(char *gateway, char *ifName)
{
    struct nlmsghdr *nlMsg;
    struct rtmsg *rtMsg;
    struct route_info *rtInfo;
    char msgBuf[BUFSIZE];

 

    int sock, len, msgSeq = 0;

 

    if((sock = socket(PF_NETLINK, SOCK_DGRAM, NETLINK_ROUTE)) < 0)
    {
        perror("Socket Creation: ");
        return -1;
    }
    memset(msgBuf, 0, BUFSIZE);

 

    nlMsg = (struct nlmsghdr *)msgBuf;
    rtMsg = (struct rtmsg *)NLMSG_DATA(nlMsg);

 

    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.

 

    if(send(sock, nlMsg, nlMsg->nlmsg_len, 0) < 0){
        printf("Write To Socket Failed…\n");
        return -1;
    }

 

    if((len = readNlSock(sock, msgBuf, msgSeq, getpid())) < 0) {
        printf("Read From Socket Failed…\n");
        return -1;
    }
    rtInfo = (struct route_info *)malloc(sizeof(struct route_info));
    for(;NLMSG_OK(nlMsg,len);nlMsg = NLMSG_NEXT(nlMsg,len)){
        memset(rtInfo, 0, sizeof(struct route_info));
        parseRoutes(nlMsg, rtInfo, gateway, ifName);
    }
    free(rtInfo);
    close(sock);
    return 0;
}

 

int main()
{
    char buff[256], ifName[12];
    get_gateway(buff, ifName);
    printf("interface:%s\ngetway: %s\n",ifName, buff);
    return 0;
}