lede/package/base-files/files/etc/uci-defaults/12_network-generate-ula

[ "$(uci -q get network.globals.ula_prefix)" != "auto" ] && exit 0

r1=$(dd if=/dev/urandom bs=1 count=1 |hexdump -e '1/1 "%02x"')
r2=$(dd if=/dev/urandom bs=2 count=1 |hexdump -e '2/1 "%02x"')
r3=$(dd if=/dev/urandom bs=2 count=1 |hexdump -e '2/1 "%02x"')

uci -q batch <<-EOF >/dev/null
	set network.globals.ula_prefix=fd$r1:$r2:$r3::/48
	commit network
EOF

exit 0