第十五夜：网络编程3-常见的应用层协议

C语言的30个夜晚 14 分钟 读完 (约 2089 字)

应用层常见协议及其实现库

DNS
利用C语言实现DNS查询和DNS响应包的解析。

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/*
* 源码出处：https://www.cnblogs.com/qrxqrx/articles/8034781.html
*/
#include <stdio.h> 
#include <string.h>    
#include <stdlib.h>    
#include <sys/socket.h>    
#include <arpa/inet.h> 
#include <netinet/in.h>
#include <unistd.h>    

char dns_servers[1][16];//存放DNS服务器的IP
int dns_server_count = 0;

/*
**DNS报文中查询区域的查询类型 
*/
#define A 1 //查询类型，表示由域名获得IPv4地址 

void ngethostbyname(unsigned char*, int);
void ChangetoDnsNameFormat(unsigned char*, unsigned char*);

/* 
**DNS报文首部
**这里使用了位域
*/
struct DNS_HEADER {
    unsigned short id; //会话标识
    unsigned char rd :1; // 表示期望递归
    unsigned char tc :1; // 表示可截断的 
    unsigned char aa :1; //  表示授权回答
    unsigned char opcode :4; 
    unsigned char qr :1; //  查询/响应标志，0为查询，1为响应
    unsigned char rcode :4; //应答码
    unsigned char cd :1; 
    unsigned char ad :1; 
    unsigned char z :1; //保留值
    unsigned char ra :1; // 表示可用递归
    unsigned short q_count; // 表示查询问题区域节的数量 
    unsigned short ans_count; // 表示回答区域的数量
    unsigned short auth_count; // 表示授权区域的数量
    unsigned short add_count; // 表示附加区域的数量
};

/*
**DNS报文中查询问题区域
*/
struct QUESTION {
    unsigned short qtype;//查询类型
    unsigned short qclass;//查询类
};
typedef struct {
    unsigned char *name;
    struct QUESTION *ques;
} QUERY;

/*
**DNS报文中回答区域的常量字段 
*/
//编译制导命令
#pragma pack(push, 1)//保存对齐状态，设定为1字节对齐
struct R_DATA {
    unsigned short type; //表示资源记录的类型
    unsigned short _class; //类
    unsigned int ttl; //表示资源记录可以缓存的时间
    unsigned short data_len; //数据长度
};
#pragma pack(pop) //恢复对齐状态

/*
**DNS报文中回答区域的资源数据字段
*/
struct RES_RECORD {
    unsigned char *name;//资源记录包含的域名
    struct R_DATA *resource;//资源数据
    unsigned char *rdata;
};

int main(int argc, char *argv[]) {
    unsigned char hostname[100];
    unsigned char dns_servername[100];
    printf("请输入DNS服务器的IP：");
    scanf("%s", dns_servername);
    strcpy(dns_servers[0], dns_servername);
    printf("请输入要查询IP的主机名：");
    scanf("%s", hostname);

    //由域名获得IPv4地址，A是查询类型
    ngethostbyname(hostname, A);

    return 0;
}

/*
**实现DNS查询功能
*/
void ngethostbyname(unsigned char *host, int query_type) {
    unsigned char buf[65536], *qname, *reader;
    int i, j, stop, s;

    struct sockaddr_in a;//地址

    struct RES_RECORD answers[20], auth[20], addit[20];//回答区域、授权区域、附加区域中的资源数据字段
    struct sockaddr_in dest;//地址

    struct DNS_HEADER *dns = NULL;
    struct QUESTION *qinfo = NULL;

    printf("\n所需解析域名：%s", host);

    s = socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP); //建立分配UDP套结字

    dest.sin_family = AF_INET;//IPv4
    dest.sin_port = htons(53);//53号端口
    dest.sin_addr.s_addr = inet_addr(dns_servers[0]);//DNS服务器IP

    dns = (struct DNS_HEADER *) &buf;
    /*设置DNS报文首部*/
    dns->id = (unsigned short) htons(getpid());//id设为进程标识符
    dns->qr = 0; //查询
    dns->opcode = 0; //标准查询
    dns->aa = 0; //不授权回答
    dns->tc = 0; //不可截断
    dns->rd = 1; //期望递归
    dns->ra = 0; //不可用递归
    dns->z = 0; //必须为0
    dns->ad = 0;
    dns->cd = 0;
    dns->rcode = 0;//没有差错
    dns->q_count = htons(1); //1个问题
    dns->ans_count = 0; 
    dns->auth_count = 0;
    dns->add_count = 0;

    //qname指向查询问题区域的查询名字段
    qname = (unsigned char*) &buf[sizeof(struct DNS_HEADER)];

    ChangetoDnsNameFormat(qname, host);//修改域名格式 
    qinfo = (struct QUESTION*) &buf[sizeof(struct DNS_HEADER)
    + (strlen((const char*) qname) + 1)]; //qinfo指向问题查询区域的查询类型字段

    qinfo->qtype = htons(query_type); //查询类型为A
    qinfo->qclass = htons(1); //查询类为1
    
    //向DNS服务器发送DNS请求报文
    printf("\n\n发送报文中...");
    if (sendto(s, (char*) buf,sizeof(struct DNS_HEADER) + (strlen((const char*) qname) + 1)+ sizeof(struct QUESTION), 0, (struct sockaddr*) &dest,sizeof(dest)) < 0) 
    {
        perror("发送失败！");
    }
    printf("发送成功！");

    //从DNS服务器接受DNS响应报文
    i = sizeof dest;
    printf("\n接收报文中...");
    if (recvfrom(s, (char*) buf, 65536, 0, (struct sockaddr*) &dest,(socklen_t*) &i) < 0) {
        perror("接收失败！");
    }
    printf("接收成功！");

    dns = (struct DNS_HEADER*) buf;

    //将reader指向接收报文的回答区域 
    reader = &buf[sizeof(struct DNS_HEADER) + (strlen((const char*) qname) + 1)
            + sizeof(struct QUESTION)];

    printf("\n\n响应报文包含: ");
    printf("\n %d个问题", ntohs(dns->q_count));
    printf("\n %d个回答", ntohs(dns->ans_count));
    printf("\n %d个授权服务", ntohs(dns->auth_count));
    printf("\n %d个附加记录\n\n", ntohs(dns->add_count));

    /*
    **解析接收报文
    */
    reader = reader + sizeof(short);//short类型长度为32为，相当于域名字段长度，这时reader指向回答区域的查询类型字段
    answers[i].resource = (struct R_DATA*) (reader);
    reader = reader + sizeof(struct R_DATA);//指向回答问题区域的资源数据字段
    if (ntohs(answers[i].resource->type) == A) //判断资源类型是否为IPv4地址
    {
        answers[i].rdata = (unsigned char*) malloc(ntohs(answers[i].resource->data_len));//资源数据
    for (j = 0; j < ntohs(answers[i].resource->data_len); j++) 
    {
        answers[i].rdata[j] = reader[j];
    }
    answers[i].rdata[ntohs(answers[i].resource->data_len)] = '\0';
    reader = reader + ntohs(answers[i].resource->data_len);
    }

    //显示查询结果
    if (ntohs(answers[i].resource->type) == A) //判断查询类型IPv4地址
    {
        long *p;
        p = (long*) answers[i].rdata;
        a.sin_addr.s_addr = *p; 
        printf("IPv4地址:%s\n", inet_ntoa(a.sin_addr));
    }
    
    return;
}

/*
**从www.baidu.com转换到3www5baidu3com
*/
void ChangetoDnsNameFormat(unsigned char* dns, unsigned char* host) {
    int lock = 0, i;
    strcat((char*) host, ".");

    for (i = 0; i < strlen((char*) host); i++) {
        if (host[i] == '.') {
            *dns++ = i - lock;
            for (; lock < i; lock++) {
                *dns++ = host[lock];
            }
            lock++; 
        }
    }
    *dns++ = '\0';
}

HTTP

利用C语言实现HTTP请求，可以采用原生Socket往端口发包，根据HTTP协议自行构造请求的方式。

但更方便的方式是使用libcurl库。自行构造Socket请求的方式既不能保证性能，又无法保证安全。

libcurl库至少是相对完善的第三方库。

详细用法，参考libcurl官方文档以及libcurl官方样例。

SMTP

SMTP一例

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/*
* 源码出处：https://blog.csdn.net/zx824/article/details/7387418
*/
#ifdef WIN32
#include <windows.h>
#include <stdio.h>
#else
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <sys/types.h>
#include <arpa/inet.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <errno.h>
#include <unistd.h>
#include <sys/time.h>
#endif
struct data6{
    unsigned int d4:6;
    unsigned int d3:6;
    unsigned int d2:6;
    unsigned int d1:6;
};
// 协议中加密部分使用的是base64方法
char con628(char c6);
void base64(char *dbuf,char *buf128,int len);
void sendemail(char *email,char *body);
int open_socket(struct sockaddr *addr);

int main()
{
    char email[] = "[email protected]";
    char body[] = "From: \"yyy\"<[email protected]>\r\n"
    "To: \"xxx\"<[email protected]>\r\n"
    "Subject: Hello\r\n\r\n"
    "Hello World, Hello Email!";
    sendemail(email, body);
    return 0;
}
 
char con628(char c6)
{
    char rtn = '\0';
    if (c6 < 26) rtn = c6 + 65;
    else if (c6 < 52) rtn = c6 + 71;
    else if (c6 < 62) rtn = c6 - 4;
    else if (c6 == 62) rtn = 43;
    else rtn = 47;
    return rtn;
}
 
// base64的实现
void base64(char *dbuf, char *buf128, int len)
{
    struct data6 *ddd = NULL;
    int i = 0;
    char buf[256] = {0};
    char *tmp = NULL;
    char cc = '\0';
    memset(buf, 0, 256);
    strcpy(buf, buf128);
    for(i = 1; i <= len/3; i++)
    {
    tmp = buf+(i-1)*3;
    cc = tmp[2];
    tmp[2] = tmp[0];
    tmp[0] = cc;
    ddd = (struct data6 *)tmp;
    dbuf[(i-1)*4+0] = con628((unsigned int)ddd->d1);
    dbuf[(i-1)*4+1] = con628((unsigned int)ddd->d2);
    dbuf[(i-1)*4+2] = con628((unsigned int)ddd->d3);
    dbuf[(i-1)*4+3] = con628((unsigned int)ddd->d4);
    }
    if(len%3 == 1)
    {
    tmp = buf+(i-1)*3;
    cc = tmp[2];
    tmp[2] = tmp[0];
    tmp[0] = cc;
    ddd = (struct data6 *)tmp;
    dbuf[(i-1)*4+0] = con628((unsigned int)ddd->d1);
    dbuf[(i-1)*4+1] = con628((unsigned int)ddd->d2);
    dbuf[(i-1)*4+2] = '=';
    dbuf[(i-1)*4+3] = '=';
    }
    if(len%3 == 2)
    {
    tmp = buf+(i-1)*3;
    cc = tmp[2];
    tmp[2] = tmp[0];
    tmp[0] = cc;
    ddd = (struct data6 *)tmp;
    dbuf[(i-1)*4+0] = con628((unsigned int)ddd->d1);
    dbuf[(i-1)*4+1] = con628((unsigned int)ddd->d2);
    dbuf[(i-1)*4+2] = con628((unsigned int)ddd->d3);
    dbuf[(i-1)*4+3] = '=';
    }
    return;
}
// 发送邮件
void sendemail(char *email, char *body)
{
    int sockfd = 0;
    struct sockaddr_in their_addr = {0};
    char buf[1500] = {0};
    char rbuf[1500] = {0};
    char login[128] = {0};
    char pass[128] = {0};
    #ifdef WIN32
    WSADATA WSAData;
    WSAStartup(MAKEWORD(2, 2), &WSAData);
    #endif
    memset(&their_addr, 0, sizeof(their_addr));
    their_addr.sin_family = AF_INET;
    their_addr.sin_port = htons(25);
    their_addr.sin_addr.s_addr = inet_addr("123.58.177.49");//qq smtp 服务器
    // 连接邮件服务器，如果连接后没有响应，则2 秒后重新连接
    sockfd = open_socket((struct sockaddr *)&their_addr);
    memset(rbuf,0,1500);
    while(recv(sockfd, rbuf, 1500, 0) == 0)
    {
        printf("reconnect...\n");
        //Sleep(2);
        //close(sockfd);
        sockfd = open_socket((struct sockaddr *)&their_addr);
 
        memset(rbuf,0,1500);
    }
 
    printf("%s\n", rbuf);
 
    // EHLO
    memset(buf, 0, 1500);
    sprintf(buf, "EHLO abcdefg-PC\r\n");
    send(sockfd, buf, strlen(buf), 0);
    memset(rbuf, 0, 1500);
    recv(sockfd, rbuf, 1500, 0);
    printf("%s\n", rbuf);
 
    // AUTH LOGIN
    memset(buf, 0, 1500);
    sprintf(buf, "AUTH LOGIN\r\n");
    send(sockfd, buf, strlen(buf), 0);
    printf("%s\n", buf);
    memset(rbuf, 0, 1500);
    recv(sockfd, rbuf, 1500, 0);
    printf("%s\n", rbuf);
 
    // USER
    memset(buf, 0, 1500);
    sprintf(buf,"[email protected]");
    memset(login, 0, 128);
    base64(login, buf, strlen(buf));
    sprintf(buf, "%s\r\n", login);
    send(sockfd, buf, strlen(buf), 0);
    printf("%s\n", buf);
    memset(rbuf, 0, 1500);
    recv(sockfd, rbuf, 1500, 0);
    printf("%s\n", rbuf);
 
    // PASSWORD
    sprintf(buf, "password");
    memset(pass, 0, 128);
    base64(pass, buf, strlen(buf));
    sprintf(buf, "%s\r\n", pass);
    send(sockfd, buf, strlen(buf), 0);
    printf("%s\n", buf);
 
    memset(rbuf, 0, 1500);
    recv(sockfd, rbuf, 1500, 0);
    printf("%s\n", rbuf);
 
    // MAIL FROM
    memset(buf, 0, 1500);
    sprintf(buf, "MAIL FROM: <[email protected]>\r\n");
    send(sockfd, buf, strlen(buf), 0);
    memset(rbuf, 0, 1500);
    recv(sockfd, rbuf, 1500, 0);
    printf("%s\n", rbuf);
 
    // RCPT TO 第一个收件人
    sprintf(buf, "RCPT TO:<%s>\r\n", email);
    send(sockfd, buf, strlen(buf), 0);
    memset(rbuf, 0, 1500);
    recv(sockfd, rbuf, 1500, 0);
    printf("%s\n", rbuf);
 
    // DATA 准备开始发送邮件内容
    sprintf(buf, "DATA\r\n");
    send(sockfd, buf, strlen(buf), 0);
    memset(rbuf, 0, 1500);
    recv(sockfd, rbuf, 1500, 0);
    printf("%s\n", rbuf);
 
    // 发送邮件内容，\r\n.\r\n内容结束标记
    sprintf(buf, "%s\r\n.\r\n", body);
    send(sockfd, buf, strlen(buf), 0);
    memset(rbuf, 0, 1500);
    recv(sockfd, rbuf, 1500, 0);
    printf("%s\n", rbuf);
 
    // QUIT
    sprintf(buf, "QUIT\r\n");
    send(sockfd, buf, strlen(buf), 0);
    memset(rbuf, 0, 1500);
    recv(sockfd, rbuf, 1500, 0);
    printf("%s\n", rbuf);
 
// VC2005 需要使用
    //closesocket(sockfd);
    close(sockfd);
    #ifdef WIN32
    WSACleanup();
    #endif
    return;
 
}
    // 打开TCP Socket连接
int open_socket(struct sockaddr *addr)
{
        int sockfd = 0;
        sockfd=socket(PF_INET, SOCK_STREAM, 0);
        if(sockfd < 0)
        {
            fprintf(stderr, "Open sockfd(TCP) error!\n");
            exit(-1);
        }
        if(connect(sockfd, addr, sizeof(struct sockaddr)) < 0)
        {
            fprintf(stderr, "Connect sockfd(TCP) error!\n");
            exit(-1);
        }
        return sockfd;
}

其他协议，后续学到再补充。

#编程 #C语言
第十六夜：编写一个Linux的服务 第十四夜：网络编程2-Socket通信
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