In OpenWRT, all config files under /etc/config are generated dynamically. There are scripts in /etc/uci-defaults to construct this configuration.

The scripts in /etc/uci-defaults are executed on first boot and then removed.

Although the scripts are removed on first boot, we could still find them in the image. For example, if you’re using an imagebuilder to build the image, once you built, under imagebuilder root, build_dir/target-aarch64_cortex-a72_musl/root-bcm27xx/ are the rootfs of bcm27xx targets, and the sub-directory /etc/uci-defaults is what we are looking for.

10_migrate-shadow        14_migrate-dhcp-release  50-dnsmasq-migrate-resolv-conf-auto.sh
12_network-generate-ula  15_odhcpd                90-resize-filesystem-to-partition-size
13_fix-group-user        20_migrate-feeds         99-override-network-proto-lan-to-dhcp

The content of 12_network-generate-ula is

[ "$(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

If we want to change this default configuration, just add another uci-defaults script with larger number in the filename, i.e. 00_customize_network_setting.

Sometimes we want to use built-in scripts to save our time of writing the script using raw UCI commands. There are functions in build_dir/target-aarch64_cortex-a72_musl/root-bcm27xx/lib/functions/ to provide such functionality. For example, there is network.sh which defines some functions to make the scripting more easily.