There's really no way to intercept all traffic from an unmanaged switch unless you do something seriously not nice such as exhausting its forwarding table so that it goes back to broadcasting all traffic on all ports (which will introduce serious lag and other problems into your network). As TenTenths said, most managed switches have some way to set up a monitoring port that all traffic can be mirrored to. If you have administrative access to your switch, you might want to look around in its configurationto see if you can find this option. If you have multiple PCs on the LAN, you probably have some sort of switch somehow.
The easiest thing honestly seems to put some sort of security gateway between the switch and the router; maybe just a Linux box that has a transparent bridge and running Wireshark. You can definitely use Wireshark for this - you just need to put your data collector someplace where it can see all traffic, e.g.:
Code:
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