linux C 获取网关代码实现

源码：

#include <stdio.h>

#include <arpa/inet.h>   
#include <linux/rtnetlink.h>     
#include <net/if.h> 
#include <stdlib.h> 
#include <string.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
{   
//收到内核的应答         
if((readLen = recv(sockFd, bufPtr, BUFSIZE - msgLen, 0)) < 0)         
{           
perror("SOCK READ: ");           
return -1;         
}   

nlHdr = (struct nlmsghdr *)bufPtr;

//检查header是否有效         
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;    
}     


//分析返回的路由信息     
void parseRoutes(struct nlmsghdr *nlHdr, struct route_info *rtInfo,char *gateway)     
{       
struct rtmsg *rtMsg;       
struct rtattr *rtAttr;       
int rtLen;       
char *tempBuf = NULL;       
struct in_addr dst;       
struct in_addr gate;              
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;   
}

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;        
}       
} 

dst.s_addr = rtInfo->dstAddr;       
if(strstr((char *)inet_ntoa(dst), "0.0.0.0"))       
{         
printf("oif:%s",rtInfo->ifName);         
gate.s_addr = rtInfo->gateWay;         
sprintf(gateway, (char *)inet_ntoa(gate));         
printf("gw%s\n",gateway);         
gate.s_addr = rtInfo->srcAddr;         
printf("src:%s\n",(char *)inet_ntoa(gate));         
gate.s_addr = rtInfo->dstAddr;        
printf("dst:%s\n",(char *)inet_ntoa(gate));        
}   

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;   

if((sock = socket(PF_NETLINK, SOCK_DGRAM, NETLINK_ROUTE)) < 0)      
{       
perror("Socket Creation: ");       
return -1;      
}                 

memset(msgBuf, 0, BUFSIZE);                 
nlMsg = (struct nlmsghdr *)msgBuf;      
rtMsg = (struct rtmsg *)NLMSG_DATA(nlMsg);                 
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.     

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,gateway);     
}

free(rtInfo);      
close(sock);      
return 0;     
}  


int main()     
{         
char buff[256];         
get_gateway(buff);         
printf("gw = %s\n",buff);    
return 0;     

}  

测试如下：

oif:enp0s3
gw:192.168.2.1
src:0.0.0.0
dst:0.0.0.0
gw = 192.168.2.1