问题
I have seen this question: Linux C: how to know the default interface for internet access?, but it suggested parsing the output of a command, something which I do NOT want to do. I have also checked the source code of it and got completely lost in it.
Could anyone just give me a snippet that would do this? All I want is the interface name.
Here is my current (non-working) source code for the function, using 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;
}
As you can tell, I am new to this.
回答1:
This ought to do. I have taken it from "Bob" here, and have taken the liberty of adding a couple of missing includes and modifying the output. Basically you open a socket to query the routes, and the default interface is the one associated with the null destination address.
The code works on my 64-bit 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;
}
回答2:
Well the good thing about Linux is that almost everything is open source. which includes the source code of ifConfig. which you can easily integrate with your project if you don't want to call remote processes.
ifConfig Usage
来源：https://stackoverflow.com/questions/15668653/how-to-find-the-default-networking-interface-in-linux