UDP 소켓 프로그래밍

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <arpa/inet.h>

int main() {
    int sockfd;
    struct sockaddr_in server_addr, client_addr;
    socklen_t client_len = sizeof(client_addr);
    char buffer[1024];

    sockfd = socket(AF_INET, SOCK_DGRAM, 0);
    if (sockfd < 0) {
        perror("socket failed");
        exit(EXIT_FAILURE);
    }

    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(8080);

    if (bind(sockfd, (struct sockaddr *)&server_addr,
             sizeof(server_addr)) < 0) {
        perror("bind failed");
        close(sockfd);
        exit(EXIT_FAILURE);
    }
    printf("UDP Echo Server on port 8080\n");

    while (1) {
        ssize_t n = recvfrom(sockfd, buffer, sizeof(buffer) - 1, 0,
                             (struct sockaddr *)&client_addr, &client_len);
        if (n < 0) continue;
        buffer[n] = '\0';
        printf("Received from %s:%d: %s\n",
               inet_ntoa(client_addr.sin_addr),
               ntohs(client_addr.sin_port), buffer);
        sendto(sockfd, buffer, n, 0,
               (struct sockaddr *)&client_addr, client_len);
    }

    close(sockfd);
    return 0;
}

import socket

server = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
server.bind(("0.0.0.0", 8080))
print("UDP Echo Server on port 8080")

while True:
    data, addr = server.recvfrom(1024)
    print(f"Received from {addr}: {data.decode()}")
    server.sendto(data, addr)

import socket

client = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
client.settimeout(3.0)  # 3초 타임아웃 (UDP는 응답 보장 없음)

try:
    client.sendto(b"Hello, UDP!", ("127.0.0.1", 8080))
    data, addr = client.recvfrom(1024)
    print(f"Received: {data.decode()}")
except socket.timeout:
    print("No response (timeout)")
finally:
    client.close()

import socket
import struct

def build_dns_query(domain):
    # DNS 헤더 (12바이트)
    tx_id = 0x1234
    flags = 0x0100  # 표준 질의, 재귀 요청
    header = struct.pack("!HHHHHH", tx_id, flags, 1, 0, 0, 0)

    # 질의 섹션
    question = b""
    for label in domain.split("."):
        question += struct.pack("!B", len(label)) + label.encode()
    question += b"\x00"  # 종료
    question += struct.pack("!HH", 1, 1)  # Type A, Class IN

    return header + question

def parse_dns_response(data):
    def skip_name(offset):
        while True:
            length = data[offset]
            if length & 0xC0 == 0xC0:  # 압축 포인터
                return offset + 2
            if length == 0:
                return offset + 1
            offset += 1 + length

    _, _, qdcount, ancount, _, _ = struct.unpack("!HHHHHH", data[:12])
    offset = 12

    for _ in range(qdcount):
        offset = skip_name(offset)
        offset += 4  # QTYPE, QCLASS

    for _ in range(ancount):
        offset = skip_name(offset)
        rtype, rclass, _, rdlength = struct.unpack(
            "!HHIH", data[offset:offset + 10]
        )
        offset += 10
        rdata = data[offset:offset + rdlength]
        offset += rdlength

        if rtype == 1 and rclass == 1 and rdlength == 4:  # A, IN
            return ".".join(str(b) for b in rdata)

    raise ValueError("A record not found")

sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
sock.settimeout(5.0)

query = build_dns_query("example.com")
sock.sendto(query, ("8.8.8.8", 53))
response, _ = sock.recvfrom(512)
ip = parse_dns_response(response)
print(f"example.com → {ip}")
sock.close()