Kea-dhcp4 should reject queries with dhcp-server-identifier (54) of 0.0.0.0
While working on LeaseQuery I discovered that the logic in Dhcp4Srv::acceptServerId() relies on CfgSubnet4::hasSubnetWithServerId() when looking for matches. This in turn relies on Network4::getServerId() which returns IPV4_ZERO_ADDRESS() if the server-id option is not set for that Network4. This means any subnet/network which does not have this option set will "match" and this is probably not appropriate.
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