Finding all of the machines available on my local subnet

Question

We have several embedded systems that work on a small network, and these run some physical displays, part of which shows the network status for each embedded system. We are no longer able to get the wireless bridges we have been using up until now, and have switched to new ones, which no longer work with the old code used to detect other machines / wireless bridges on the network. The old code does the following:

char broadcast[27] = { 1, 0, 0, 0, 0, 0, 0, 0, 0, 2, 10, 10, 60, 213, 69, 120, 1, 0, 192, 1, 1, 0, 1, 0, 1, 0, 1 };
sock = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP);
addr.sin_addr.s_addr = htonl(-1);
addr.sin_port = htons(PORT);
addr.sin_family = AF_INET;
status = setsockopt(sock, SOL_SOCKET, SO_BROADCAST, &yes, sizeof(int));
status = setsockopt(sock, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof(int));
status = bind(sock, (struct sockaddr *) &addr, sinlen); // bind socket
status = getsockname(sock, (struct sockaddr *) &addr, &sinlen);
status = sendto(sock, broadcast, sizeof(broadcast), 0, (struct sockaddr *) &addr, sinlen);

fd_set fds;
struct timeval timeout;

timeout.tv_sec = sec;
timeout.tv_usec = usec;

FD_ZERO(&fds);
FD_SET(socket, &fds);

select(socket+1, &fds, 0, 0, &timeout);

status = recvfrom(sock, buffer, buflen, 0, (struct sockaddr *) &addr, &sinlen);
some_addr = inet_ntoa(addr.sin_addr);

I understand that it's sending a UDP broadcast packet to the broadcast address for the subnet, but what I don't get is what the f#$@ is the magic payload that got this to work and why doesn't it work with our new equipment ?

To replace this code, I'm looking to essentially implement a ping 192.168.1.0 -b command to ping the broadcast address using an ICMP packet. I've tried the code that was linked to at this question, but it doesn't work for broadcast addresses (presumably because it's not configured to do so). My question then is, how do I configure the code linked from the above question to sendto a broadcast address and receive results from each machine on the network ?

Before anybody points it out, the code above has been severely snipped for clarity and brevity, all error checking has been removed. — Alex Marshall, Oct 29 '13 at 23:12
Just to be sure, `ping -b` does work? If not, it is probably a problem with the network (equipment), not the code. Maybe some of the bridges block/ignore broadcast messages? — Kninnug, Oct 29 '13 at 23:17
Yes, ping -b does work, on all our different networks with different wireless bridges. — Alex Marshall, Oct 30 '13 at 13:37

score 0 · Answer 1 · answered Oct 30 '13 at 19:09

it might be a proprietary protocol used by acksys - they mention this port in their documentation (most other hits are related to the logitech squeezebox, but i don't think that's relevant).

[i realise i am not directly answering your question, but i think this means that the code above will only work with acksys hardware]

Finding all of the machines available on my local subnet

1 Answers1