受信したNICアドレスから返すUDPエコーサーバ

.gitignore

*.swp
a.out
*.o
*.pyc

echoserver.cpp

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <net/if.h>
#include <string.h> // memcpy

#include <iostream>
#include <stdio.h>
#include <stdlib.h>

int main() {

  int sock;
  sock = socket(AF_INET, SOCK_DGRAM, 0);
  if (sock < 0) {
    perror("socket");
    exit(1);
  }

  int one = 1;
  if (setsockopt(sock, IPPROTO_IP, IP_PKTINFO, &one, sizeof (one)) < 0) {
    perror("setsockopt(IP_PKTINFO)");
    exit(1);
  }

  sockaddr_in sin;
  sin.sin_family = AF_INET;
  sin.sin_addr.s_addr = htonl(INADDR_ANY);
  sin.sin_port = htons(3000);
  if (bind(sock, (sockaddr*)&sin, sizeof (sin)) < 0) {
    perror("bind");
    exit(1);
  }

  for (;;) {
    char buf[4096]; // 受信データ
    iovec iov[1];
    iov[0].iov_base = buf;
    iov[0].iov_len = sizeof (buf);

    char cbuf[1024]; // 制御データ
    msghdr msg;
    msg.msg_name = &sin; // 送信元アドレスが返る
    msg.msg_namelen = sizeof (sin);
    msg.msg_iov = iov; // 受信データが返る
    msg.msg_iovlen = sizeof (iov) / sizeof (iovec);
    msg.msg_control = cbuf; // 制御データが返る
    msg.msg_controllen = sizeof (cbuf);

    // 受信
    int length = recvmsg(sock, &msg, 0);
    if (length < 0) {
      perror("recvmsg");
      continue;
    }

    // 解釈
    in_addr localaddr;
    cmsghdr* cm;
    for (cm = CMSG_FIRSTHDR(&msg); cm != 0; cm = CMSG_NXTHDR(&msg, cm)) {
      if (cm->cmsg_level == IPPROTO_IP && cm->cmsg_type == IP_PKTINFO) {
	localaddr = ((in_pktinfo*)CMSG_DATA(cm))->ipi_spec_dst;
	break;
      }
    }

    if (cm == 0) {
      std::cerr << "IP_PKTINFO not found" << std::endl;
      continue;
    }

    // 応答を構築
    iov[0].iov_len = length;
    msg.msg_name = &sin; // 宛先
    msg.msg_namelen = sizeof (sin);
    msg.msg_iov = iov; // 送信データ
    msg.msg_iovlen = sizeof (iov) / sizeof (iovec);
    msg.msg_control = cbuf; // 制御データをこのあと構築
    msg.msg_controllen = sizeof (cbuf);

    // 制御データを構築
    in_pktinfo pktinfo;
    pktinfo.ipi_ifindex = 0;
    pktinfo.ipi_spec_dst = localaddr;
    pktinfo.ipi_addr.s_addr = INADDR_ANY;

    cm = CMSG_FIRSTHDR(&msg);
    cm->cmsg_level = IPPROTO_IP;
    cm->cmsg_type = IP_PKTINFO;
    cm->cmsg_len = CMSG_LEN(sizeof (pktinfo));
    memcpy(CMSG_DATA(cm), &pktinfo, sizeof (pktinfo));
    msg.msg_controllen = cm->cmsg_len;

    // 応答
    if (sendmsg(sock, &msg, 0) < 0) {
      perror("sendmsg");
      continue;
    }
  }
}

sender.py

#!/usr/bin/python

import socket
import sys

# command message dst

message = 'test message'
dst = '127.0.0.1'

if len(sys.argv) > 1:
  message = sys.argv[1]
if len(sys.argv) > 2:
  dst = sys.argv[2]

sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
sock.sendto(message, (dst, 3000))

(message, source) = sock.recvfrom(1024)
print "%s: '%s'" % (source, message)