获取ANSI C中给定ip的网关

Question

我像疯了似的环顾四周,但没有得到真正的答案.我有一个例子,但这取决于个人自己的图书馆,所以不太好.

起初我想得到一个接口的默认网关,但由于不同的IP可以被不同地路由,我很快就明白我想要它通过使用AF_ROUTE套接字来获得用于给定目标IP的网关rtm_type RTM_GET.有没有人有一个例子,我实际上最终得到一个包含网关IP(或MAC地址)的字符串？网关条目似乎是十六进制的,但也编码在/ proc/net/route中,我想AF_ROUTE套接字从它获取信息(但我猜测是通过内核).

Thanx提前

和ps我刚刚开始使用堆栈溢出,我必须说,你们所有人都很棒!快速回复和好的回复!你是我最好的朋友;)

Answer 1

这是特定于操作系统的,没有统一的(或ANSI C)API.

假设Linux,最好的方法是解析/ proc/net/route,查找Destination为00000000的条目,默认网关位于Gateway列中,您可以在其中读取网关IP地址的十六进制表示(大endian,我相信)

如果你想通过更具体的API调用来做到这一点,你将不得不经历一些箍,这是一个示例程序:

#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;
}

Answer 2

也许这是一个很老的问题，但是我遇到了同样的问题，我找不到更好的结果。最后，我用这些代码解决了我的问题，该代码进行了一些更改。所以我决定分享。

char* GetGatewayForInterface(const char* interface) 
{
    char* gateway = NULL;

    char cmd [1000] = {0x0};
    sprintf(cmd,"route -n | grep %s  | grep 'UG[ \t]' | awk '{print $2}'", interface);
    FILE* fp = popen(cmd, "r");
    char line[256]={0x0};

    if(fgets(line, sizeof(line), fp) != NULL)
        gateway = string(line);


    pclose(fp);
}