侦听UDP单播对UDP多播消息的答复

Question

我试图在linux上实现一个非常简单的upnp控制器，这样我就可以控制一个需要专有软件的设备。

文档说，我需要向特定的地址和端口发送一个特定形式的UDP多播请求(参见下面代码中的“M”字符串)，并且UDP单播将响应我发送的地址和端口。

我做不到这件事。tcpdump显示UDP多播请求到正确的地址和端口，格式看起来正确，但我看不到答复。

我正在发送和监听回送接口(设备在同一台机器上)。

另一个upnp控制器(即不是我的控制器)在回送接口上正常工作。

有人能告诉我我做错了什么吗？

以下是代码：

 #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 <netinet/udp.h>
 #include <unistd.h>
 #include <fcntl.h>

 #define MAXBUFSIZE 65536

int main(int argc, char ** argv ) {

unsigned char loop;
loop = 0;
unsigned char ttl;
ttl = 4;
int bcast;
bcast = 1;

int sock;

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

struct sockaddr_in destadd;
memset(&destadd, 0, sizeof(destadd));
destadd.sin_family = AF_INET;
destadd.sin_port = htons((uint16_t)1900);
if (inet_pton(AF_INET, "239.255.255.250", &destadd.sin_addr) < 1) {
    perror("inet_pton dest");
    exit(EXIT_FAILURE);
}

struct sockaddr_in interface_addr;
memset(&interface_addr, 0, sizeof(interface_addr));
interface_addr.sin_family = AF_INET;
interface_addr.sin_port = htons(0);
if (inet_pton(AF_INET, "127.0.0.1", &interface_addr.sin_addr) < 1) {
    perror("inet_pton interface");
    exit(EXIT_FAILURE);
}

if (setsockopt(sock, IPPROTO_IP, IP_MULTICAST_LOOP, &loop, sizeof(loop)) < 0){
    perror("setsockopt loop");
    exit(EXIT_FAILURE);
}

if (setsockopt(sock, IPPROTO_IP, IP_MULTICAST_TTL, &ttl, sizeof(ttl)) < 0){
    perror("setsockopt ttl");
    exit(EXIT_FAILURE);
}

if (setsockopt(sock, IPPROTO_IP, IP_MULTICAST_IF,
               (struct in_addr *)&interface_addr.sin_addr,
               sizeof(interface_addr.sin_addr)) < 0) {
    perror("setsockopt if");
    exit(EXIT_FAILURE);
}

if (setsockopt(sock, SOL_SOCKET, SO_BROADCAST, &bcast, sizeof(bcast)) < 0) {
    perror("setsockopt bcast");
    exit(EXIT_FAILURE);
}

struct ip_mreqn imr;
memset(&imr, 0, sizeof(imr));
if (inet_pton(AF_INET, "239.255.255.250", &imr.imr_multiaddr.s_addr) < 1) {
    perror("inet_pton");
    exit(EXIT_FAILURE);
}
inet_pton(AF_INET, "127.0.0.1", (struct in_addr *)&imr.imr_address);
imr.imr_ifindex = 0;
if (setsockopt(sock, IPPROTO_IP, IP_ADD_MEMBERSHIP,
               (void *)&imr, sizeof(imr)) < 0) {
    perror("setsockopt addmem");
    exit(EXIT_FAILURE);
}

if (bind(sock, (struct sockaddr *)&interface_addr,
    sizeof(struct sockaddr_in)) < 0) {
    perror("bind");
    exit(EXIT_FAILURE);
}

char buffer[1024];

strcpy(buffer, "M-SEARCH * HTTP/1.1\r\n"
                   "Host: 239.255.255.250:1900\r\n"
                   "Man: \"ssdp:discover\"\r\n"
                   "ST: upnp:rootdevice\r\n"
                   "MX: 3\r\n"
                   "User-Agent: Test/1.0\r\n"
                   "\r\n");

if (sendto(sock, buffer, strlen(buffer), 0, (struct sockaddr*)&destadd,
       sizeof(destadd)) < 0) {
    perror("sendto");
    exit(EXIT_FAILURE);
}

if (recvfrom(sock, &buffer, sizeof(buffer)-1, 0, NULL, NULL) < 0) {
    perror("recvfrom");
    exit(EXIT_FAILURE);
}


if (close(sock) < 0) {
    perror("close");
    exit(EXIT_FAILURE);
}

}

Answer 1

最后对我起作用的代码如下。

方法摘要：

• 创建UDP套接字
• 启用IF_MULTICAST_LOOP (允许与同一主机上的客户端通信)
• 设置IF_MULTICAST_TTL
• 绑定到INADDR_ANY和端口8201 (端口号)。(任意选择)
• 发送多播消息至239.255.255.250:1900
• 使用相同的套接字接收回复

我们开始：

#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 <netinet/udp.h>
#include <unistd.h>
#include <fcntl.h>

int main(int argc, char ** argv ) {

unsigned char loop;
loop = 1; // Needs to be on to get replies from clients on the same host
unsigned char ttl;
ttl = 4;
int bcast;
bcast = 1;

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

// Multicast message will be sent to 239.255.255.250:1900
struct sockaddr_in destadd;
memset(&destadd, 0, sizeof(destadd));
destadd.sin_family = AF_INET;
destadd.sin_port = htons((uint16_t)1900);
if (inet_pton(AF_INET, "239.255.255.250", &destadd.sin_addr) < 1) {
    perror("inet_pton dest");
    exit(EXIT_FAILURE);
}

// Listen on all interfaces on port 8201 
struct sockaddr_in interface_addr;
memset(&interface_addr, 0, sizeof(interface_addr));
interface_addr.sin_family = AF_INET;
interface_addr.sin_port = htons(8201);
interface_addr.sin_addr.s_addr = htonl(INADDR_ANY);

// Got to have this to get replies from clients on same machine
if (setsockopt(sock, IPPROTO_IP, IP_MULTICAST_LOOP, &loop, sizeof(loop)) < 0){
    perror("setsockopt loop");
    exit(EXIT_FAILURE);
}

if (setsockopt(sock, IPPROTO_IP, IP_MULTICAST_TTL, &ttl, sizeof(ttl)) < 0){
    perror("setsockopt ttl");
    exit(EXIT_FAILURE);
}

// Bind to port 8201 on all interfaces
if (bind(sock, (struct sockaddr *)&interface_addr,
    sizeof(struct sockaddr_in)) < 0) {
    perror("bind");
    exit(EXIT_FAILURE);
}

char buffer[1024];

strcpy(buffer, "M-SEARCH * HTTP/1.1\r\n"
               "Host: 239.255.255.250:1900\r\n"
               "Man: \"ssdp:discover\"\r\n"
               "ST: upnp:rootdevice\r\n"
               "MX: 3\r\n"
               "User-Agent: Test/1.0\r\n"
               "\r\n");

if (sendto(sock, buffer, strlen(buffer), 0, (struct sockaddr*)&destadd,
       sizeof(destadd)) < 0) {
    perror("sendto");
    exit(EXIT_FAILURE);
}

if (recvfrom(sock, &buffer, sizeof(buffer)-1, 0, NULL, NULL) < 0) {
    perror("recvfrom");
    exit(EXIT_FAILURE);
}


printf("%s\n", buffer);

if (close(sock) < 0) {
    perror("close");
    exit(EXIT_FAILURE);
}
}

若要从外部客户端获得答复，请确保端口8201不被阻塞。

Answer 2

如注释中所示，route -n的输出

0.0.0.0 192.168.1.1 0.0.0.0 UG 0 0 0 eth0 192.168.1.0 0.0.0 255.255.0 U 1 0 0 eth0

建议发送到239.x.x.x的数据包将通过eth0发送，而不是回送。

因此，要么添加一个静态路由来强制传出数据包进入回送接口，要么确保您可以通过IF_MULTICAST_LOOP标志通过回送接收。