From Wikipedia, about Default-free zone, it says:
In the context of Internet routing, the default-free zone (DFZ) refers to the collection of all Internet autonomous systems (AS) that do not require a default route to route a packet to any destination. Conceptually, DFZ routers have a “complete” BGP table, sometimes referred to as the Internet routing table, global routing table or global BGP table, but, realistically, the widespread use of route filtering and the rapid rate of change in Internet routing ensure that no router anywhere has an absolutely complete view of all routes, and any such routing table would, in any case, look different from the perspective of different routers, even if a stable view could be achieved.
I’m just wondering, if the DFZ receives a packet but has no matching route for this packet, what will do if it doesn’t have a default route?
It returns a network unknown message (code 6) to the src address.