这是与操作系统相关的内容,没有统一(或ANSI C)API。
假设在Linux上,最好的方法是解析/proc/net/route,查找目标为00000000的条目,在网关列中可以读取网关IP地址的十六进制表示(我认为是大端)
如果您想通过更具体的API调用来完成此操作,则需要进行相当多的操作,以下是一个示例程序:
#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 {
if ((readLen = recv(sockFd, bufPtr, BUFSIZE - msgLen, 0)) < 0) {
perror("SOCK READ: ");
return -1;
}
nlHdr = (struct nlmsghdr *) bufPtr;
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;
}
void printRoute(struct route_info *rtInfo)
{
char tempBuf[512];
if (rtInfo->dstAddr.s_addr != 0)
strcpy(tempBuf, inet_ntoa(rtInfo->dstAddr));
else
sprintf(tempBuf, "*.*.*.*\t");
fprintf(stdout, "%s\t", tempBuf);
if (rtInfo->gateWay.s_addr != 0)
strcpy(tempBuf, (char *) inet_ntoa(rtInfo->gateWay));
else
sprintf(tempBuf, "*.*.*.*\t");
fprintf(stdout, "%s\t", tempBuf);
fprintf(stdout, "%s\t", rtInfo->ifName);
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);
}
void parseRoutes(struct nlmsghdr *nlHdr, struct route_info *rtInfo)
{
struct rtmsg *rtMsg;
struct rtattr *rtAttr;
int rtLen;
rtMsg = (struct rtmsg *) NLMSG_DATA(nlHdr);
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.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;
}
}
if (rtInfo->dstAddr.s_addr == 0)
sprintf(gateway, (char *) inet_ntoa(rtInfo->gateWay));
return;
}
int main()
{
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: ");
memset(msgBuf, 0, BUFSIZE);
nlMsg = (struct nlmsghdr *) msgBuf;
rtMsg = (struct rtmsg *) NLMSG_DATA(nlMsg);
nlMsg->nlmsg_len = NLMSG_LENGTH(sizeof(struct rtmsg));
nlMsg->nlmsg_type = RTM_GETROUTE;
nlMsg->nlmsg_flags = NLM_F_DUMP | NLM_F_REQUEST;
nlMsg->nlmsg_seq = msgSeq++;
nlMsg->nlmsg_pid = getpid();
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);
}
free(rtInfo);
close(sock);
printGateway();
return 0;
}