I was thinking if there's a way to use libpcap or another tool in turn to forge an ARP-request frame and make all the nodes within the LAN answer to the broadcast address ( so you could sniff it) or to the port where my NIC is connected to.
It doesn't have to be that complicated.
As long as your switch isn't using a feature similar to Dynamic ARP Inspection, the simple way to detect addresses is to use
arping... you can find this in most linux distributions (although CLI options sometimes vary depending on what
arping build you have).
[mpenning@home ~]$ sudo arping -c 2 -S 0.0.0.0 -D -i eth0 172.16.100.40
60 bytes from 00:ae:de:ad:be:ef (172.16.100.40): index=0 time=3.513 msec
60 bytes from 00:ae:de:ad:be:ef (172.16.100.40): index=1 time=2.778 msec
You can write a script to cycle through as many addresses as you like; usually it's a good idea to sniff with wireshark and see what subnets could be used in that vlan (i.e. by looking at the source address in ARP requests).
Dynamic ARP Inspection
If you are running Dynamic ARP Inspection (DAI), you can't detect addresses like this as long as DAI checks IP address validity... I tested this on a Cisco 2960 running this configuration:
ip arp inspection vlan 100
ip arp inspection validate src-mac dst-mac ip
When I used
arping as shown above, the switch noticed the invalid
0.0.0.0 source IP address in the ARP frame, and dropped it:
Jun 1 04:05:29.561 CDT: %SW_DAI-4-INVALID_ARP: 1 Invalid ARPs (Req) on Gi0/1,
vlan 100.([0024.1bde.add7/0.0.0.0/0000.0000.0000/172.16.100.40/04:05:28 CDT
Mon Jun 1 2015])