通过C套接字编程的邮件格式

Question

我正在学习 C 语言的套接字编程，最好的学习方法是通过实现它。所以我写了一个简单的程序来通过 C 套接字编程发送带有附件和一些文本 body 的邮件。到目前为止，代码是成功的，只有一个例外——格式化。 MIME headers 出现在邮件 body 中（附上屏幕截图），邮件主题全乱了。

#ifndef WIN32_LEAN_AND_MEAN
#define WIN32_LEAN_AND_MEAN
#endif

#include <windows.h>
#include <winsock2.h>
#include <ws2tcpip.h>
#include <iphlpapi.h>
#include <stdio.h>

#pragma comment(lib, "Ws2_32.lib")

#define SMTP_PORT "25"
#define DEFAULT_BUFLEN 1024

int sendData(SOCKET* socket, const char* data)
{
    int iResult;
    printf("%s", data);
    iResult = send(*socket, data, (int)strlen(data), 0);
    if (iResult == SOCKET_ERROR) {
        printf("send failed (msg: %s): %d\n", data, WSAGetLastError());
    }
    return iResult;
}

void recvData(SOCKET* socket)
{
    int recvbuflen = DEFAULT_BUFLEN;
    char recvbuf[DEFAULT_BUFLEN];
    int iResult;
    iResult = recv(*socket, recvbuf, recvbuflen - 1, 0);
    printf("In recvData function: %d\n", iResult);
    if (iResult > 0) {
        recvbuf[iResult] = '[=10=]';
        printf("%s", recvbuf);
    }
    else if (iResult == 0)
        printf("Connection closed\n");
    else
        printf("recv failed: %d\n", WSAGetLastError());
}

int main(int argc, char** argv)
{
    WSADATA wsaData;

    int iResult;
    iResult = WSAStartup(MAKEWORD(2, 2), &wsaData);
    if (iResult != 0) {
        printf("WSAStartup failed: %d\n", iResult);
        return 1;
    }

    struct addrinfo *result = NULL;
    struct addrinfo *ptr = NULL;
    struct addrinfo hints;

    ZeroMemory(&hints, sizeof(hints));
    hints.ai_family = AF_UNSPEC;
    hints.ai_socktype = SOCK_STREAM;
    hints.ai_protocol = IPPROTO_TCP;

    iResult = getaddrinfo("mail.sharklasers.com", SMTP_PORT, &hints, &result);
    if (iResult != 0) {
        printf("getaddrinfo failed: %d\n", iResult);
        WSACleanup();
        return 1;
    }

    SOCKET ConnectSocket = INVALID_SOCKET;
    for (ptr = result; ptr != NULL; ptr = ptr->ai_next) {
        ConnectSocket = socket(ptr->ai_family, ptr->ai_socktype, ptr->ai_protocol);
        if (ConnectSocket == INVALID_SOCKET) {
            printf("socket failed: %ld\n", WSAGetLastError());
            WSACleanup();
            return 1;
        }

        iResult = connect(ConnectSocket, ptr->ai_addr, (int)ptr->ai_addrlen);
        if (iResult == SOCKET_ERROR) {
            closesocket(ConnectSocket);
            ConnectSocket = INVALID_SOCKET;
            continue;
        }
        break;
    }
    freeaddrinfo(result);

    if (ConnectSocket == INVALID_SOCKET) {
        printf("Unable to connect to server!\n");
        WSACleanup();
        return 1;
    }

    sendData(&ConnectSocket, "HELO mail.sharklasers.com\r\n");
    recvData(&ConnectSocket);
    sendData(&ConnectSocket, "MAIL FROM: psgibrxp@sharklasers.com\r\n");
    recvData(&ConnectSocket);
    sendData(&ConnectSocket, "RCPT TO: psgibrxp@sharklasers.com\r\n");
    recvData(&ConnectSocket);
    sendData(&ConnectSocket, "DATA\r\n");
    recvData(&ConnectSocket);
    sendData(&ConnectSocket, "Subject:This is subject of my mail\r\n");
    sendData(&ConnectSocket, "And this is text\r\n");
    sendData(&ConnectSocket, "MIME-Version: 1.0\r\n");
    sendData(&ConnectSocket, "Content-Type:multipart/mixed;boundary=\"KkK170891tpbkKk__FV_KKKkkkjjwq\"\r\n");
    sendData(&ConnectSocket, "--KkK170891tpbkKk__FV_KKKkkkjjwq\r\n");
    sendData(&ConnectSocket, "Content-Transfer-Encoding: quoted-printable\r\n");
    sendData(&ConnectSocket, "Content-Type: text/plain;name=\"details.txt\"\r\n");
    sendData(&ConnectSocket, "Content-Disposition: attachment; filename=\"details.txt\"\r\n");
    sendData(&ConnectSocket, "\r\n\r\n");

    FILE* MailFilePtr = fopen("details.txt", "r");
    if (MailFilePtr == NULL)
        printf("Error opening attachment\n");

    char FileBuffer[1024];
    char buf[1024];

    memset(FileBuffer, 0, sizeof(FileBuffer));
    while (fgets(FileBuffer, sizeof(FileBuffer), MailFilePtr))
    {
        sprintf(buf, "%s", FileBuffer);
        buf[strlen(buf) - 1] = 0;
        sendData(&ConnectSocket, buf);
        memset(FileBuffer, 0, sizeof(FileBuffer));
        memset(buf, 0, sizeof(buf));
    }

    fclose(MailFilePtr);

    sendData(&ConnectSocket, "\r\n\r\n--KkK170891tpbkKk__FV_KKKkkkjjwq--\r\n\r\n");
    sendData(&ConnectSocket, ".\r\n");
    recvData(&ConnectSocket);

    sendData(&ConnectSocket, "QUIT\r\n");

    iResult = shutdown(ConnectSocket, SD_SEND);
    if (iResult == SOCKET_ERROR) {
        printf("shutdown failed: %d\n", WSAGetLastError());
        closesocket(ConnectSocket);
        WSACleanup();
        return 1;
    }

    closesocket(ConnectSocket);
    WSACleanup();
    return 0;
}

问题： 我怎样才能得到正确格式的邮件body？我想我缺少一些 SMTP 命令或命令序列，但我不确定。

注意

我目前正在使用 Guerrilla Mail，因为 Gmail 需要 SSL\TLS 进行通信（如果我错了请纠正我）。我将为此使用 openssl 库。
这不是最终代码。我确实需要清理一些代码。但是，也欢迎提出代码审查建议。
我是来学习的，欢迎任何与C语言和网络编程相关的链接，书籍参考。

编辑

已编辑的代码基于目前的评论和答案。不幸的是问题仍然存在。我没有读取文件，而是直接传递了 base64 编码的附件（这是有效的）。此编辑代码的输出显示在下面的屏幕截图中

    sendData(&ConnectSocket, "MIME-Version: 1.0\r\n");
    sendData(&ConnectSocket, "Content-Type:multipart/mixed;boundary=\"977d81ff9d852ab2a0cad646f8058349\"\r\n");
    sendData(&ConnectSocket, "Subject:This is subject of my mail\r\n");
    sendData(&ConnectSocket, "--977d81ff9d852ab2a0cad646f8058349\r\n");
    sendData(&ConnectSocket, "Content-Type: text/plain; charset=\"utf-8\"\r\n");
    sendData(&ConnectSocket, "Content-Transfer-Encoding: quoted-printable\r\n\r\n");
    sendData(&ConnectSocket, "Hi Me,=0A=0AThis is an empty file.=0A=0ARegards,=0A<ME>=0A=0A---- =0ASent using Guerrillamail.com =0ABlock or report abuse : https://www.guerrillamail.com//abuse/?a=3DUVJzDA8SW6Q1mwa14nUTcwfCX9ne0dhd=0A \r\n\r\n");
    sendData(&ConnectSocket, "--977d81ff9d852ab2a0cad646f8058349\r\n");
    sendData(&ConnectSocket, "Content-Type: text/plain\r\n");
    sendData(&ConnectSocket, "Content-Transfer-Encoding: base64\r\n");
    sendData(&ConnectSocket, "Content-Disposition: attachment; filename=\"details.txt\"\r\n\r\n");
    sendData(&ConnectSocket, "U2FtcGxlIFRleHQu");
    sendData(&ConnectSocket, "\r\n\r\n--977d81ff9d852ab2a0cad646f8058349--\r\n\r\n");
    sendData(&ConnectSocket, ".\r\n");
    recvData(&ConnectSocket);
    sendData(&ConnectSocket, "QUIT\r\n");

已编辑输出邮件屏幕截图：

Answer 1

名称在 Content-Type 中看起来没有必要，请将其更改为

sendData(&ConnectSocket, "Content-Type: text/plain");

编辑：

请改成这个（将邮件消息移到 text/plain 下方，并进行其他更改）

sendData(&ConnectSocket, "MIME-Version: 1.0\r\n");
sendData(&ConnectSocket, "Content-Type:multipart/mixed;boundary=\"KkK170891tpbkKk__FV_KKKkkkjjwq\"\r\n");
sendData(&ConnectSocket, "--KkK170891tpbkKk__FV_KKKkkkjjwq\r\n");
sendData(&ConnectSocket, "Content-Type: text/plain\r\n");
sendData(&ConnectSocket, "And this is text\r\n");
sendData(&ConnectSocket, "--KkK170891tpbkKk__FV_KKKkkkjjwq\r\n");
sendData(&ConnectSocket, "Content-Type: text/plain;name=\"details.txt\"\r\n");
sendData(&ConnectSocket, "Content-Transfer-Encoding: base64\r\n");
sendData(&ConnectSocket, "Content-Disposition: attachment; filename=\"details.txt\"\r\n");

Answer 2

body需要用空行与header分开。

sendData(&ConnectSocket, "MIME-Version: 1.0\r\n");
sendData(&ConnectSocket, "Content-Type:multipart/mixed;boundary=\"977d81ff9d852ab2a0cad646f8058349\"\r\n");
sendData(&ConnectSocket, "Subject:This is subject of my mail\r\n");
sendData(%ConnectSocket, "\r\n"); /* added */
sendData(&ConnectSocket, "--977d81ff9d852ab2a0cad646f8058349\r\n");
sendData(&ConnectSocket, "Content-Type: text/plain; charset=\"utf-8\"\r\n");
sendData(&ConnectSocket, "Content-Transfer-Encoding: quoted-printable\r\n\r\n");
sendData(&ConnectSocket, "Hi Me,=0A=0AThis is an empty file.=0A=0ARegards,=0A<ME>=0A=0A---- =0ASent using Guerrillamail.com =0ABlock or report abuse : https://www.guerrillamail.com//abuse/?a=3DUVJzDA8SW6Q1mwa14nUTcwfCX9ne0dhd=0A \r\n\r\n");
sendData(&ConnectSocket, "--977d81ff9d852ab2a0cad646f8058349\r\n");
sendData(&ConnectSocket, "Content-Type: text/plain\r\n");
sendData(&ConnectSocket, "Content-Transfer-Encoding: base64\r\n");
sendData(&ConnectSocket, "Content-Disposition: attachment; filename=\"details.txt\"\r\n\r\n");
sendData(&ConnectSocket, "U2FtcGxlIFRleHQu");
sendData(&ConnectSocket, "\r\n\r\n--977d81ff9d852ab2a0cad646f8058349--\r\n\r\n");
sendData(&ConnectSocket, ".\r\n");

当您向 SMTP 发送内容时，您应该正确检查结果代码是否符合预期（大多数命令为 2xx，DATA 为 345）。

随着时间的推移，使用静态边界字符串将无法正常工作（尽管我想如果您发现边界字符串嵌入到内容中，您可以通过对边界字符串进行编码来替换或混淆）。

当消息超过大约 1,000 个字符时，将整个消息作为一个长行发送将开始中断。 quoted-printable 有用的原因之一是它允许您透明地在您的内容中嵌入换行符（尽管在这种情况下仅发送包含换行符的消息中的文字换行符已经让您走得很远了）。

well-formed 消息还应包含 From: 和 To: header。

通过C套接字编程的邮件格式

Mail format via C socket programming

c

email

smtp

mime

winsock2