[英]Getting gateway to use for a given ip in ANSI C
我像瘋了似的環顧四周,但沒有得到真正的答案。 我有一個例子,但這取決於個人自己的圖書館,所以不太好。
起初我想獲得接口的默認網關,但由於不同的IP可以以不同的方式路由,我很快就明白我想要的是通過使用AF_ROUTE
套接字和rtm_type RTM_GET
來獲得用於給定目標IP的網關。 有沒有人有一個例子,我實際上最終得到一個包含網關IP(或MAC地址)的字符串? 網關條目似乎是十六進制,但也編碼在/ proc / net / route中,我猜AF_ROUTE
套接字得到它的信息(但我通過內核)。
Thanx提前
和ps我剛剛開始使用堆棧溢出,我必須說,你們所有人都很棒! 快速回復和好的回復! 你是我最好的朋友;)
這是特定於操作系統的,沒有統一的(或ANSI C)API。
假設Linux,最好的方法是解析/ proc / net / route,查找Destination為00000000的條目,默認網關在Gateway列中,您可以在其中讀取網關IP地址的十六進制表示(大endian,我相信)
如果你想通過更具體的API調用來做到這一點,你將不得不經歷一些箍,這是一個示例程序:
#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 {
/* 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.s_addr != 0)
strcpy(tempBuf, inet_ntoa(rtInfo->dstAddr));
else
sprintf(tempBuf, "*.*.*.*\t");
fprintf(stdout, "%s\t", tempBuf);
/* Print Gateway address */
if (rtInfo->gateWay.s_addr != 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.s_addr != 0)
strcpy(tempBuf, inet_ntoa(rtInfo->srcAddr));
else
sprintf(tempBuf, "*.*.*.*\t");
fprintf(stdout, "%s\n", tempBuf);
}
void printGateway()
{
printf("%s\n", gateway);
}
/* For parsing 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:
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;
}
}
//printf("%s\n", inet_ntoa(rtInfo->dstAddr));
if (rtInfo->dstAddr.s_addr == 0)
sprintf(gateway, (char *) inet_ntoa(rtInfo->gateWay));
//printRoute(rtInfo);
return;
}
int main()
{
struct nlmsghdr *nlMsg;
struct rtmsg *rtMsg;
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: ");
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));
//fprintf(stdout, "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);
printGateway();
return 0;
}
也許這是一個非常古老的問題,但我有同樣的問題,我找不到更好的結果。 最后,我用這些代碼解決了我的問題,它有一些變化。 所以我決定分享它。
char* GetGatewayForInterface(const char* interface)
{
char* gateway = NULL;
char cmd [1000] = {0x0};
sprintf(cmd,"route -n | grep %s | grep 'UG[ \t]' | awk '{print $2}'", interface);
FILE* fp = popen(cmd, "r");
char line[256]={0x0};
if(fgets(line, sizeof(line), fp) != NULL)
gateway = string(line);
pclose(fp);
}
我決定采用“快速而骯臟”的方式開始,並使用netstat -rm
從/proc/net/route
讀出ip。
我以為我會分享我的功能......但請注意,它有一些錯誤,並且很可能會幫助我找到它,我會編輯它沒有錯誤。 該函數采用像eth0
這樣的iface名稱,並返回該iface使用的網關的ip。
char* GetGatewayForInterface(const char* interface) {
char* gateway = NULL;
FILE* fp = popen("netstat -rn", "r");
char line[256]={0x0};
while(fgets(line, sizeof(line), fp) != NULL)
{
/*
* Get destination.
*/
char* destination;
destination = strndup(line, 15);
/*
* Extract iface to compare with the requested one
* todo: fix for iface names longer than eth0, eth1 etc
*/
char* iface;
iface = strndup(line + 73, 4);
// Find line with the gateway
if(strcmp("0.0.0.0 ", destination) == 0 && strcmp(iface, interface) == 0) {
// Extract gateway
gateway = strndup(line + 16, 15);
}
free(destination);
free(iface);
}
pclose(fp);
return gateway;
}
這個函數的問題是當我將pclose留在那里時會導致內存損壞。 但是如果我刪除了pclose調用它會起作用(但這不是一個好的解決方案,因為流將保持打開狀態..呵呵)。 因此,如果有人能夠發現錯誤,我將使用正確的版本編輯該功能。 我不是C大師,對所有內存擺弄有點困惑;)
聲明:本站的技術帖子網頁,遵循CC BY-SA 4.0協議,如果您需要轉載,請注明本站網址或者原文地址。任何問題請咨詢:yoyou2525@163.com.