UDP广播接收-绑定到多个NIC

Question

I am writing a program for Linux that should receive UDP broadcast packets from the specified port on any of the network interfaces that exist in the system.

However, if the system has multiple network interfaces of the same subnet address, the packets are accepted only by the first interface.

For example, if eth0 has 192.168.225.107 and eth1 has 192.168.225.108, packets accepted only from eth0.

NIC's are attached to different physical networks. According to the tcpdump, the packets are present in both networks.

Code (error checking skipped):

sock = socket(PF_INET, SOCK_DGRAM, 0);
setsockopt(sock, SOL_SOCKET, SO_BROADCAST, (void *) &on, sizeof(on));
setsockopt(sock, SOL_SOCKET, SO_RCVBUF, (void *) &s, sizeof(s));
bzero(&serv_addr_ip, sizeof(serv_addr_ip));
serv_addr_ip.sin_family = AF_INET;
serv_addr_ip.sin_port = htons(port);
serv_addr_ip.sin_addr.s_addr = htonl(INADDR_ANY);
serv_addr = (struct sockaddr *) &serv_addr_ip;
addr_len = sizeof(struct sockaddr_in);
bind(sock, serv_addr, addr_len);

while (1) {
    if ((chars = recvfrom(sock, var.buf, MSG_MAX, 0, serv_addr, &addr_len)) < 0) {
        ...
    }
...
}

Answer 1

If I understand you correctly, you have two NICs, connected to two physical networks (ie network cables, hubs), with each having a separate IP address from the same subnet address range?

The short answer is that your network configuration is wrong. If they really are separate physical networks, then they should have different subnet addresses. It depends on what you mean with separate physical networks, separate hardware? You can NOT have two separate subnets with the same subnet address. Thats why I am saying your network configuration is wrong.

However, The impression I get is that you are trying to bridge the two networks, so that the two NICs belong to the same subnet (not separate). Well, then you should bridge them. You bridge the two NICs together and assign ONE IP address to the bridge. Then you will be able to receive your packets on both NICs.

In linux:

brctl addbr br0
ifconfig eth0 0.0.0.0 down
ifconfig eth1 0.0.0.0 down
brctl addif br0 eth0
brctl addif br0 eth1
ifconfig eth0 up
ifconfig eth1 up
ifconfig br0 up
ifconfig br0 192.168.225.107 (or 192.168.225.108, whatever you prefer)