Simple TELNET Server：编写Telnet服务端客户端后无响应问题求助

Hey Nickolas, let's dig into why your Telnet server-client setup isn't sending/receiving responses properly. Here are the most common culprits to check, organized by priority:

Telnet runs over TCP, which is a stream-based protocol—no built-in message boundaries. This trips up a lot of new developers:

• If your server uses fixed-length read() calls (e.g., read(buffer, 1024)), it might only get partial data or wait indefinitely if the client's message doesn't fill the buffer. Instead, parse data until you hit the Telnet-standard line terminator (\r\n) to know when a complete command is received.
• Double-check that your server doesn't accidentally close the client socket before sending a response. It's easy to slip up with early close() calls right after accept(), especially if you're nesting logic.

Telnet isn't just raw TCP—it has control codes and negotiation rules that can block communication if ignored:

• Control Code Negotiation: Clients often send Telnet control sequences (like IAC DO ECHO to request echo functionality). If your server doesn't respond with the appropriate IAC WILL ECHO or IAC WONT ECHO, the client might hang waiting for a negotiation reply, never processing your actual response.
• Line Endings: Telnet requires \r\n (carriage return + newline) as the line terminator. If your server sends responses with just \n, many Telnet clients won't recognize it as a complete message and will keep waiting for more data.

This is one of the most overlooked issues in socket programming:

• The send() and recv() functions return the actual number of bytes sent/received, not the number you intended to send. For example, if you call send(sock, response, strlen(response), 0), network buffer limits might mean only part of the response is sent. You need to loop until all bytes are transmitted.
• If recv() returns -1 (or SOCKET_ERROR on Windows), don't ignore it! Check errno (Linux) or WSAGetLastError() (Windows) to find out why—common issues include a dropped connection, timeout, or invalid socket.

• Packet Capture: Use tcpdump (Linux) or Wireshark to sniff the Telnet traffic. You'll see exactly if the server is sending a response, what's in it, and if the client is receiving it. This is the fastest way to rule out network-level issues.
• Add Logs: Stick simple print statements (or proper logging) in your code: log when the server receives a command, when it sends a response, and how many bytes were sent. On the client side, log when it connects, sends data, and receives bytes. This will tell you if the problem is in sending, receiving, or something in between.

Here's a quick example of a robust response-sending function for a Telnet server (C language) that handles partial sends and correct line endings:

void send_telnet_response(int client_sock, const char* message) {
    // Append required Telnet line ending
    char full_response[1024];
    snprintf(full_response, sizeof(full_response), "%s\r\n", message);
    
    size_t total_len = strlen(full_response);
    size_t sent = 0;
    
    while (sent < total_len) {
        ssize_t bytes_sent = send(client_sock, full_response + sent, total_len - sent, 0);
        if (bytes_sent == -1) {
            perror("Failed to send response");
            break;
        }
        sent += bytes_sent;
    }
}

Start with these checks—chances are one of them is the root cause of your missing responses!

内容的提问来源于stack exchange，提问作者Nickolas Daniel