我已经看到了这个问题：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。
有人可以给我一个可以做到这一点的片段吗？我想要的只是界面名称。
这是我当前（非工作）函数的源代码，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;
}
正如你所知，我是新手。
答案 0 :(得分：2)
这应该做的。我从“Bob”here中取出它，并冒昧地添加了几个缺失的包含并修改了输出。基本上你打开一个套接字来查询路由，默认接口是与空目标地址相关联的接口。
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;
}
答案 1 :(得分：0)
Linux的优点是几乎所有东西都是开源的。其中包含ifConfig的源代码。如果您不想调用远程进程，可以轻松地与项目集成。