e.p*_*kov 8 c sockets linux udp multicast
我有一个包含多台计算机的网络，每台计算机都有多个网络接口和一个目标。我正在开发一个积极使用多播的应用程序。总的来说，一切都按预期进行。除了我无法在通过第二个网络接口发送的同一台计算机上接收多播的那一刻之外。网络上的其他计算机可以通过任何网络接口接收多播。是否可以通过一个接口发送多播并通过同一网络中的另一个接口接收多播？如果是这样，我在哪里犯了错误：在客户端代码中，在收件人代码中，还是在系统设置中？
典型机器配置：
$ ifconfig
enp0s31f6: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
inet 192.168.88.230 netmask 255.255.255.0 broadcast 192.168.88.255
enp10s0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
inet 192.168.88.229 netmask 255.255.255.0 broadcast 192.168.88.255
wlp6s0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
inet 192.168.88.48 netmask 255.255.255.0 broadcast 192.168.88.255
lo: flags=73<UP,LOOPBACK,RUNNING> mtu 65536
inet 127.0.0.1 netmask 255.0.0.0
路由表：
$ route -n
Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
0.0.0.0 192.168.88.1 0.0.0.0 UG 100 0 0 enp0s31f6
0.0.0.0 192.168.88.1 0.0.0.0 UG 101 0 0 enp10s0
0.0.0.0 192.168.88.1 0.0.0.0 UG 600 0 0 wlp6s0
169.254.0.0 0.0.0.0 255.255.0.0 U 1000 0 0 enp0s31f6
192.168.88.0 0.0.0.0 255.255.255.0 U 100 0 0 enp0s31f6
192.168.88.0 0.0.0.0 255.255.255.0 U 101 0 0 enp10s0
192.168.88.0 0.0.0.0 255.255.255.0 U 600 0 0 wlp6s0
这些组合按预期工作
./sender 239.255.255.251 27335 192.168.88.48
./listener 239.255.255.251 27335 192.168.88.48
或者
./sender 239.255.255.251 27335 192.168.88.229
./listener 239.255.255.251 27335 192.168.88.229
或者
./sender 239.255.255.251 27335 192.168.88.230
./listener 239.255.255.251 27335 192.168.88.230
但这些不起作用：
./sender 239.255.255.251 27335 192.168.88.48
./listener 239.255.255.251 27335 192.168.88.229
或者
./sender 239.255.255.251 27335 192.168.88.48
./listener 239.255.255.251 27335 192.168.88.230
或者
./sender 239.255.255.251 27335 192.168.88.229
./listener 239.255.255.251 27335 192.168.88.230
对于我的任务，我改编了https://tldp.org/HOWTO/Multicast-HOWTO-6.html中的几个示例中的代码
发送者.c
//
// Simple sender.c program for UDP
//
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char *argv[])
{
if (argc < 3) {
printf("Command line args should be multicast group and port\n");
printf("(e.g. for SSDP, `sender 239.255.255.250 1900 [interface_ip]`)\n");
return 1;
}
const char* group = argv[1]; // e.g. 239.255.255.250 for SSDP
const int port = atoi(argv[2]); // 0 if error, which is an invalid port
const char* source_iface = (argc == 4 ? argv[3] : NULL);
//
// create what looks like an ordinary UDP socket
//
const int fd = socket(AF_INET, SOCK_DGRAM, 0);
if (fd < 0) {
perror("socket");
return 1;
}
struct ip_mreq mreq;
memset(&mreq, 0, sizeof(mreq));
mreq.imr_interface.s_addr = source_iface ? inet_addr(source_iface) : htonl(INADDR_ANY);
if (
setsockopt(
fd, IPPROTO_IP, IP_MULTICAST_IF, (char*) &mreq, sizeof(mreq)
) < 0
){
perror("setsockopt");
return 1;
}
//
// set up destination address
//
struct sockaddr_in addr;
memset(&addr, 0, sizeof(addr));
addr.sin_family = AF_INET;
addr.sin_addr.s_addr = inet_addr(group);
addr.sin_port = htons(port);
//
// now just sendto() our destination
//
for (unsigned i = 0; ; i++) {
char buffer[64];
memset(buffer, '\0', sizeof(buffer));
snprintf(buffer, sizeof(buffer), "Hello, World! Sequence: %u", i & 0xFF);
const int nbytes = sendto(
fd,
buffer,
sizeof(buffer),
0,
(struct sockaddr*) &addr,
sizeof(addr)
);
if (nbytes < 0) {
perror("sendto");
return 1;
}
const int delay_secs = 1;
sleep(delay_secs);
}
return 0;
}
监听器.c
//
// Simple listener.c program for UDP multicast
//
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <time.h>
#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#define MSGBUFSIZE 4096
int main(int argc, char *argv[])
{
if (argc < 3) {
printf("Command line args should be multicast group and port and [interface] optional\n");
printf("(e.g. for SSDP, `listener 239.255.255.250 1900 [192.168.1.1]`)\n");
return 1;
}
const char* group = argv[1]; // e.g. 239.255.255.250 for SSDP
const int port = atoi(argv[2]); // 0 if error, which is an invalid port
const char* source_iface = (argc == 4) ? argv[3] : NULL;
//
// create what looks like an ordinary UDP socket
//
const int fd = socket(AF_INET, SOCK_DGRAM, 0);
if (fd < 0) {
perror("socket");
return 1;
}
//
// allow multiple sockets to use the same PORT number
//
const u_int yes = 1;
if (
setsockopt(
fd, SOL_SOCKET, SO_REUSEADDR, (char*) &yes, sizeof(yes)
) < 0
){
perror("Reusing ADDR failed");
return 1;
}
//
// set up destination address
//
struct sockaddr_in addr;
memset(&addr, 0, sizeof(addr));
addr.sin_family = AF_INET;
addr.sin_addr.s_addr = inet_addr(group);
addr.sin_port = htons(port);
//
// bind to receive address
//
if (bind(fd, (struct sockaddr*) &addr, sizeof(addr)) < 0) {
perror("bind");
return 1;
}
//
// use setsockopt() to request that the kernel join a multicast group
//
struct ip_mreq mreq;
memset(&mreq, 0, sizeof(mreq));
mreq.imr_multiaddr.s_addr = inet_addr(group);
mreq.imr_interface.s_addr = source_iface ? inet_addr(source_iface) : htonl(INADDR_ANY);
if (
setsockopt(
fd, IPPROTO_IP, IP_ADD_MEMBERSHIP, (char*) &mreq, sizeof(mreq)
) < 0
){
perror("setsockopt");
return 1;
}
//
// now just enter a read-print loop
//
while (1) {
char msgbuf[MSGBUFSIZE];
unsigned addrlen = sizeof(addr);
int const nbytes = recvfrom(
fd,
msgbuf,
MSGBUFSIZE,
0,
(struct sockaddr *) &addr,
&addrlen
);
if (nbytes < 0) {
perror("recvfrom");
return 1;
}
msgbuf[nbytes] = '\0';
printf("from: %s message: %s\n", inet_ntoa(addr.sin_addr), msgbuf);
}
return 0;
}
tcpdump 看起来适合所有接口：
$ sudo tcpdump -i wlp6s0 -s0 -vv host 239.255.255.251
tcpdump: listening on wlp6s0, link-type EN10MB (Ethernet), capture size 262144 bytes
18:03:40.547226 IP (tos 0x0, ttl 1, id 55512, offset 0, flags [DF], proto UDP (17),
length 92)
i7-6700k-system.48397 > 239.255.255.251.27335: [udp sum ok] UDP, length 64
18:03:41.547602 IP (tos 0x0, ttl 1, id 55691, offset 0, flags [DF], proto UDP (17),
length 92)
i7-6700k-system.48397 > 239.255.255.251.27335: [udp sum ok] UDP, length 64
$ sudo tcpdump -i enp0s31f6 -s0 -vv host 239.255.255.251
tcpdump: listening on enp0s31f6, link-type EN10MB (Ethernet), capture size 262144 bytes
18:07:42.639153 IP (tos 0x0, ttl 1, id 20849, offset 0, flags [DF], proto UDP (17),
length 92)
i7-6700k-system.48397 > 239.255.255.251.27335: [udp sum ok] UDP, length 64
18:07:43.639911 IP (tos 0x0, ttl 1, id 20997, offset 0, flags [DF], proto UDP (17),
length 92)
i7-6700k-system.48397 > 239.255.255.251.27335: [udp sum ok] UDP, length 64
$ sudo tcpdump -i enp10s0 -s0 -vv host 239.255.255.251
tcpdump: listening on enp10s0, link-type EN10MB (Ethernet), capture size 262144 bytes
18:08:57.666159 IP (tos 0x0, ttl 1, id 30039, offset 0, flags [DF], proto UDP (17),
length 92)
i7-6700k-system.48397 > 239.255.255.251.27335: [udp sum ok] UDP, length 64
18:08:58.666518 IP (tos 0x0, ttl 1, id 30171, offset 0, flags [DF], proto UDP (17),
length 92)
ci7-6700k-system.48397 > 239.255.255.251.27335: [udp sum ok] UDP, length 64
我重新审视了这个问题，希望找到答案，看来我做到了。这个问题的答案是配置问题。这个内核参数可以解决这个问题：
net.ipv4.conf.all.accept_local=1
接受具有本地源地址的数据包。与适当的路由相结合，这可用于通过线路在两个本地接口之间引导数据包并正确接受它们。默认为假
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