c++ - 如何在 linux 中找到默认网络接口(interface)？

Question

我看过这个问题:Linux C: how to know the default interface for internet access? ，但它建议解析命令的输出，这是我不想做的事情。我还检查了它的源代码并完全迷失了它。

谁能给我一个可以做到这一点的片段？我只需要接口(interface)名称。

---

这是我当前的(非工作)函数源代码，使用 libmnl:

bool get_default_interface(std::string* interface_name) {
    char buf[MNL_SOCKET_BUFFER_SIZE];
    struct nlmsghdr* nlh = mnl_nlmsg_put_header(buf);
    struct ifinfomsg* ifm = (ifinfomsg*) mnl_nlmsg_get_payload(nlh);
    int len = nlh->nlmsg_len;
    struct rtmsg* r = (rtmsg*) NLMSG_DATA(nlh);
    struct rtattr* rta = RTM_RTA(r);
    struct nlattr* tb[IFLA_MAX + 1] = {};
    mnl_attr_parse(nlh, sizeof(*ifm), data_attr_cb, tb);
    //struct rtnexthop* nh = (rtnexthop*) RTA_DATA(tb[RTA_MULTIPATH]);
    if (r->rtm_dst_len) {
            return false;
    }
    std::cout << mnl_attr_get_str(tb[IFLA_IFNAME]) << std::endl;
    *interface_name = mnl_attr_get_str(tb[IFLA_IFNAME]);
    return true;
}

如您所知，我是新手。

Answer 1

这应该可以。我从“鲍勃”那里拿走了它here ，并冒昧地添加了几个缺失的包含并修改了输出。基本上，您打开一个套接字来查询路由，默认接口(interface)是与空目标地址关联的接口(interface)。

该代码适用于我的 64 位 Linux。

> Destination     Gateway         Interface       Source
> 0.0.0.0         192.168.0.1     eth1            *.*.*.*
>                                 ^^^^^ here it is!
> 127.0.0.0       *.*.*.*         lo              *.*.*.*
> ...


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

#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 {
                /* Receive 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((0 == NLMSG_OK(nlHdr, readLen)) || (NLMSG_ERROR == nlHdr->nlmsg_type))
                {
                        perror("Error in received packet");
                        return -1;
                }
                /* Check if it is the last message */
                if (NLMSG_DONE == nlHdr->nlmsg_type)
                {
                        break;
                }
                /* Else move the pointer to buffer appropriately */
                bufPtr += readLen;
                msgLen += readLen;
                /* Check if its a multi part message; return if it is not. */
                if (0 == (nlHdr->nlmsg_flags & NLM_F_MULTI)) {
                        break;
                }
        } while((nlHdr->nlmsg_seq != seqNum) || (nlHdr->nlmsg_pid != pId));
        return msgLen;
}

char *ntoa(int addr)
{
        static char buffer[18];
        sprintf(buffer, "%d.%d.%d.%d",
                (addr & 0x000000FF)      ,
                (addr & 0x0000FF00) >>  8,
                (addr & 0x00FF0000) >> 16,
                (addr & 0xFF000000) >> 24);
        return buffer;
}
/* For printing the routes. */
void printRoute(struct route_info *rtInfo)
{
        /* Print Destination address */
        printf("%s\t", rtInfo->dstAddr ? ntoa(rtInfo->dstAddr) : "0.0.0.0  ");

        /* Print Gateway address */
        printf("%s\t", rtInfo->gateWay ? ntoa(rtInfo->gateWay) : "*.*.*.*");

        /* Print Interface Name */
        printf("%s\t", rtInfo->ifName);

        /* Print Source address */
        printf("%s\n", rtInfo->srcAddr ? ntoa(rtInfo->srcAddr) : "*.*.*.*");

        if (0 == rtInfo->dstAddr) {
                printf("\t\t\t^^^^^ here it is!\n");
        }

}

/* 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 = *(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;
                }
        }
        printRoute(rtInfo);
        free(tempBuf);
}

int main()
{
        struct nlmsghdr *nlMsg;
        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: ");

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

        /* point the header and the msg structure pointers into the buffer */
        nlMsg = (struct nlmsghdr *)msgBuf;

        /* 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));
        // ADDED BY BOB
        /* THIS IS THE NETTSTAT -RL code I commented out the printing here and in parse routes */
        printf("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);
        return 0;
}