OS:ubuntu18.04

gcc:gcc (Ubuntu 7.5.0-3ubuntu1~18.04) 7.5.0

I can capture data using both tcpdump and Wireshark, but I cannot receive data using the recvfrom function.

source code：

#include <iostream>
#include <cstring>
#include <arpa/inet.h>
#include <unistd.h>
#include <iomanip>
const int BUFFER_SIZE = 1024;

int main() {
    int sockfd;
    struct sockaddr_in server_addr, client_addr;
    char buffer[BUFFER_SIZE];
    socklen_t addr_len = sizeof(client_addr);

    sockfd = socket(AF_INET, SOCK_DGRAM, 0);
    if (sockfd < 0) {
        perror("socket");
        return 1;
    }

    memset(&server_addr, 0, sizeof(server_addr));
    server_addr.sin_family = AF_INET;
    //server_addr.sin_addr.s_addr = INADDR_ANY; 
    server_addr.sin_port = htons(42867); 
    inet_pton(AF_INET, "172.31.3.81", &server_addr.sin_addr); 

    if (bind(sockfd, (struct sockaddr*)&server_addr, sizeof(server_addr)) < 0) {
        perror("bind");
        close(sockfd);
        return 1;
    }

    // std::cout << "Waiting for UDP packets on port 42867..." << std::endl;
  
    const char *message = "Hello from server!";
    const char *client_ip = "172.31.3.77"; 
    int client_port = 10000;
int opt = 1;
setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &opt, sizeof(opt));
setsockopt(sockfd, SOL_SOCKET, SO_REUSEPORT, &opt, sizeof(opt));

 
    while (true) {
        memset(buffer, 0, BUFFER_SIZE);
        ssize_t recv_len = recvfrom(sockfd, buffer, BUFFER_SIZE - 1, 0,
                                     (struct sockaddr*)&client_addr, &addr_len);
        std::cout << "recv_len: " << recv_len << std::endl;
        if (recv_len < 0) {
            perror("recvfrom");
            break;
        }

        buffer[recv_len] = '';
        char client_ip[INET_ADDRSTRLEN];
        inet_ntop(AF_INET, &client_addr.sin_addr, client_ip, sizeof(client_ip));
         std::cout << "Received from port: " << ntohs(client_addr.sin_port) << std::endl;
          std::cout << std::endl;
    }

    close(sockfd);
    return 0;
}

output：

recv_len: 99
Received from port: 10001

recv_len: 99
Received from port: 10001

recv_len: 99
Received from port: 10001

recv_len: 99
Received from port: 10001

recv_len: 99
Received from port: 10001

recv_len: 99
Received from port: 10001

Only received data with a length of 99, cannot receive data with a length of 902.
Why？