Using C I am trying to discover the IP of a locally connected device without knowing any information about the device.
[ my box ]--eth0--------------[ unknown device ]
I can't seem to find a way to arp/broadcast to get the IP of the device. I understand that if I use multicast the device itself has to be a member of the group. When I arp or arp-scan they ask me to assign my eth0 interface an IP but I don't have one assigned, I can put one if I need to but I am unsure what it needs to be set to in order to discover anything.
Example: I have a Linksys wireless IP camera and I want to find the IP/Subnet. Without knowing its subnet/netmask I wouldn't know what to set my eth0 to.
I have written a C program to parse the arp table in Linux but I cannot seem to get the IP from the locally connected device in it. As mentioned above, I cannot use these tools without setting eth0 to something.
I think Your best chance to find something is by capturing all incoming network traffic, especially while unknown device is booting (using tcpdump, wireshark or something similar). Just filter out traffic, which is sent from my box.
Most likely You'll see DHCP requests and maybe ARP requests. If device is using DHCP, the easyest solution would be to run DHCP server on eth0 (so You can assign IP to this device).
If for some reason You need to program in C (and network capture gives expected results), libpcap may be usefull.
If evrything else fails, You can try to dicover unknown IP using fake ARP packets (so You don't need to change IP on eth0) and capturing any responses.