协议不匹配。 C，套接字，sendall / recvall

Question

我只想编写一个应用程序，它连接到我的SSH服务器，发送消息并接收响应。我不想登录，或者类似的东西，只是发送消息并获得响应。我有一个openSSH服务器在端口192.168.0.1上监听22。

然而，我的代码（当然发送并获取消息），打印：

SSH-1.99-OpenSSH_4.4
Protocol mismatch.

代码：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>

int sendAll(int socket, char *buffer, int numBytes)
{
    int ret;           // Return value for 'send'
    int sentBytes;     // Total number of bytes sent

    // Send the given number of bytes.
    sentBytes = 0;

    while (sentBytes < numBytes)
    {
        ret = send(socket, buffer + sentBytes, numBytes - sentBytes, 0);

        // Error encountered.
        if (ret == -1)
        {
            close(socket);
            perror("send");
            exit(1);
        }
        // Data sent.
        else
        {
            sentBytes += ret;
        }
    }
    return sentBytes;
}

int recvAll(int socket, char *buffer, int numBytes)
{
    int ret;               // Return value for 'recv'
    int receivedBytes;     // Total number of bytes received

    // Retrieve the given number of bytes.
    receivedBytes = 0;
    while ((receivedBytes < numBytes) && ret != -1)
    {
        ret = recv(socket, buffer + receivedBytes, numBytes - receivedBytes, 0);

        // Error encountered.
        if (ret == -1)
        {
            close(socket);
            perror("recv");
            exit(1);
        }
        else if(ret == 0)
        {
            receivedBytes += ret;
            break;
        }
        // Data received.
        else
        {
            receivedBytes += ret;
        }
    }


    buffer[receivedBytes] = '\0';
    return receivedBytes;
}

int main(int argc, char **argv)
{
    char * ip_addr = "192.168.0.1";
    int port = 22;

    int sockfd;
    struct sockaddr_in server_addr;

    if ((sockfd = socket(AF_INET, SOCK_STREAM, 0)) == -1)
    {
        perror("socket");
        exit(1);
    }

    server_addr.sin_family = AF_INET;
    server_addr.sin_port = htons(port);
    inet_aton(ip_addr, &server_addr.sin_addr);

    if (connect(sockfd, (struct sockaddr *)&server_addr, sizeof(struct sockaddr)) == -1)
    {
        perror("connect");
        close(sockfd);
        exit(1);
    }

    char msg[] = "abc\n";
    char buffer[255];
    int recv_size, send_size;

    send_size = sendAll(sockfd, msg, strlen(msg));
    recv_size = recvAll(sockfd, buffer, 254);

    buffer[recv_size] = '\0';
    printf("%s\n", buffer);

    return 0;
}

另一方面，当我用recvall替换recv时，它只会显示：SSH-1.99-OpenSSH_4.4。但为什么？

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>

int sendAll(int socket, char *buffer, int numBytes)
{
    int ret;           // Return value for 'send'
    int sentBytes;     // Total number of bytes sent

    // Send the given number of bytes.
    sentBytes = 0;

    while (sentBytes < numBytes)
    {
        ret = send(socket, buffer + sentBytes, numBytes - sentBytes, 0);

        // Error encountered.
        if (ret == -1)
        {
            close(socket);
            perror("send");
            exit(1);
        }
        // Data sent.
        else
        {
            sentBytes += ret;
        }
    }
    return sentBytes;
}

int recvAll(int socket, char *buffer, int numBytes)
{
    int ret;               // Return value for 'recv'
    int receivedBytes;     // Total number of bytes received

    // Retrieve the given number of bytes.
    receivedBytes = 0;
    while ((receivedBytes < numBytes) && ret != -1)
    {
        ret = recv(socket, buffer + receivedBytes, numBytes - receivedBytes, 0);

        // Error encountered.
        if (ret == -1)
        {
            close(socket);
            perror("recv");
            exit(1);
        }
        else if(ret == 0)
        {
            receivedBytes += ret;
            break;
        }
        // Data received.
        else
        {
            receivedBytes += ret;
        }
    }


    buffer[receivedBytes] = '\0';
    return receivedBytes;
}

int main(int argc, char **argv)
{
    char * ip_addr = "192.168.0.1";
    int port = 22;

    int sockfd;
    struct sockaddr_in server_addr;

    if ((sockfd = socket(AF_INET, SOCK_STREAM, 0)) == -1)
    {
        perror("socket");
        exit(1);
    }

    server_addr.sin_family = AF_INET;
    server_addr.sin_port = htons(port);
    inet_aton(ip_addr, &server_addr.sin_addr);

    if (connect(sockfd, (struct sockaddr *)&server_addr, sizeof(struct sockaddr)) == -1)
    {
        perror("connect");
        close(sockfd);
        exit(1);
    }

    char msg[] = "abc\n";
    char buffer[255];
    int recv_size, send_size;

    send_size = sendAll(sockfd, msg, strlen(msg));

    if ((recv_size = recv(sockfd, buffer, 254, 0)) == 0)
    {
        close(sockfd);
        if (errno != 0)
        {
            perror("recv");
            exit(1);
        }
    }

    buffer[recv_size] = '\0';
    printf("%s\n", buffer);

    return 0;
}