获取设备网关的办法

雨点打透心脏的1/2处 2022-12-20 02:09 168阅读 0赞

# 问题 #

今天在写代码的时候需要在Linux下获取设备的网关，但是找了好久发现都是通过linux指令 `route`、`netstat -r`打印路由表，解析路由表获得，或者通过`cat /pro/net/route`查看内核进程文件读取路由信息，基本没有使用C接口获取网关的示例。

# 结果 #

面向搜索引擎编程，在stackoverflow上看到了相关问题[Getting gateway to use for a given ip in ANSI C][]，看到后大喜过往，不过看到了回答者给的示例，如下：

#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;
    }

我还是决定使用管道读取指令结果算了，正好第二回答就是使用管道获取调用指令的结果：

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);
    }

当然重点是这句话`route -n | grep %s | grep 'UG[ \t]' | awk '{print $2}'`, 调用`route`指令不解析其名称（也就是原数字地址显示），结果传入`grep`指令搜索指定的接口名称，然后再将结果传入`grep`搜索符号位为`UG`的行（`“UG[ \t]”`是正则表达式，`[ \t]`表示紧跟着空格或者制表符），再然后调用`awk`指令打印第二个参数；

其中符号(Flags)可能的符号以及含义如下：

>  *  U 表明路由已启动（Up）；
>  *  G 表明该路由是通往网关（Gateway）的；
>  *  H 表明该路由是通向一个完全确定的主机地址（Host address），而不是一个网络.

参考文章：

1.  [Getting gateway to use for a given ip in ANSI C][]
2.  [How to Display Routing Table Status][]

[Getting gateway to use for a given ip in ANSI C]: https://stackoverflow.com/questions/3288065/getting-gateway-to-use-for-a-given-ip-in-ansi-c
[How to Display Routing Table Status]: https://docs.oracle.com/cd/E19683-01/806-4075/ipconfig-proc-58/index.html