Patch for Busybox udhcpcd to add config options for restricting what types of clients can connect.
MAC filter, allows specifying a MAC address Mask with wildcards **, which will cause the DHCP server to ignore requests coming from any other devices. Wildcards must be in pairs, can't wildcard partial-bytes.
Client ID is DHCP Option #61, this filter allows specifying a string that Client ID must start with in order for the server to offer an address.
Example udhcpd.conf:
chaddr_filter a1b2c3******
client_id_filter my_device_