Linux 程序中获取网关

#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>
#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{
/* 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 != 0)
strcpy(tempBuf, (char *)inet_ntoa(rtInfo->dstAddr));
else
sprintf(tempBuf,"*.*.*.*\t");
fprintf(stdout,"%s\t", tempBuf);
/* Print Gateway address */
if(rtInfo->gateWay != 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 != 0)
strcpy(tempBuf, (char *)inet_ntoa(rtInfo->srcAddr));
else
sprintf(tempBuf,"*.*.*.*\t");
fprintf(stdout,"%s\n", tempBuf);
}

/* For parsing the route info returned */
void parseRoutes(struct nlmsghdr *nlHdr, struct route_info *rtInfo,char *gateway)
{
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;
}
}
if (strstr((char *)inet_ntoa(rtInfo->dstAddr), "0.0.0.0"))
sprintf(gateway, (char *)inet_ntoa(rtInfo->gateWay));
//printRoute(rtInfo);
free(tempBuf);
return;
}
int get_gateway(char *gateway)
{
struct nlmsghdr *nlMsg;
struct rtmsg *rtMsg;
struct route_info *rtInfo;
char msgBuf[BUFSIZE];
int sock, len, msgSeq = 0;
char buff[1024];

/* 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;
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));
for(;NLMSG_OK(nlMsg,len);nlMsg = NLMSG_NEXT(nlMsg,len))
{
memset(rtInfo, 0, sizeof(struct route_info));
parseRoutes(nlMsg, rtInfo,gateway);
}
free(rtInfo);
close(sock);
return 0;
}
int main()
{
char gateway[255]={0};
get_gateway(gateway);
printf("Gateway:%s\n", gateway);
}