[Linux]C언어 Gateway 정보 가져오기

출처 : http://www.linuxquestions.org/questions/linux-networking-3/howto-find-gateway-address-through-code-397078/

Gateway의 정보는 ioctl을 사용하지 않고, socket의 send(), recv()함수를 이용하여, 구할 수 있다.

#include <stdio.h>

#include <stdlib.h>

#include <string.h>

#include <sys/socket.h>

#include <net/if.h>

#include <linux/rtnetlink.h>

#include <unistd.h>

#include <arpa/inet.h>

#define BUFSIZE 8192

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

{

/* 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;

}

/* parse the route info returned */

void parseRoutes(struct nlmsghdr *nlHdr, struct route_info *rtInfo)

{

struct rtmsg *rtMsg;

struct rtattr *rtAttr;

int rtLen;

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:

memcpy(&rtInfo->gateWay, RTA_DATA(rtAttr), sizeof(rtInfo->gateWay));

break;

case RTA_PREFSRC:

memcpy(&rtInfo->srcAddr, RTA_DATA(rtAttr), sizeof(rtInfo->srcAddr));

break;

case RTA_DST:

memcpy(&rtInfo->dstAddr, RTA_DATA(rtAttr), sizeof(rtInfo->dstAddr));

break;

}

}

return;

}

// meat

int get_gatewayip(char *gatewayip, socklen_t size)

{

int found_gatewayip = 0;

struct nlmsghdr *nlMsg;

struct rtmsg *rtMsg;

struct route_info *rtInfo;

char msgBuf[BUFSIZE]; // pretty large buffer

int sock, len, msgSeq = 0;

/* Create Socket */

if((sock = socket(PF_NETLINK, SOCK_DGRAM, NETLINK_ROUTE)) < 0)

{

perror("Socket Creation: ");

return(-1);

}

/* 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)

{

fprintf(stderr, "Write To Socket Failed...\n");

return -1;

}

/* Read the response */

if((len = readNlSock(sock, msgBuf, msgSeq, getpid())) < 0) 

{

fprintf(stderr, "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);

// Check if default gateway

if (strstr((char *)inet_ntoa(rtInfo->dstAddr), "0.0.0.0"))

{

// copy it over

inet_ntop(AF_INET, &rtInfo->gateWay, gatewayip, size);

found_gatewayip = 1;

break;

}

}

free(rtInfo);

close(sock);

return found_gatewayip;

}

int main(void)

{

    char gateway[20];

get_gatewayip(gateway, 20);

fprintf(stderr, "gateway: %s", gateway);

return 0;

}