Preserve dnsmasq DHCP leases on mounted storage

[gsanchietti]

Description

Persist dnsmasq DHCP leases across reboots when NethSecurity storage is configured on /mnt/data.

When storage is mounted, dnsmasq should use a persistent lease file under /mnt/data/dnsmasq/dhcp.leases.

When storage is not mounted, the system must keep using /tmp/dhcp.leases.

If storage is later unmounted or enters an error state, dnsmasq must switch back to /tmp/dhcp.leases before /mnt/data is removed.

The target directory must be created automatically before switching dnsmasq, otherwise the service will fail to start.

This preserves address continuity for clients across reboots, especially on systems with a small DHCP pool.

Alternative solutions

• Keep using /tmp/dhcp.leases and periodically copy it to persistent storage. Rejected because dnsmasq would still restart from a volatile lease file after reboot.
• Update every consumer to read the storage-backed lease file directly. Rejected because the change should stay inside ns-storage and keep existing /tmp/dhcp.leases consumers compatible.

References

• Requested flow: keep /tmp when storage is absent, switch to storage when mounted, revert to /tmp before unmount.
• Current config example: dhcp.ns_dnsmasq.leasefile='/tmp/dhcp.leases'

[gsanchietti]

Testing image: latest from nethsecurity-8.8 branch

To check dnsmasq config:

uci show dhcp.ns_dnsmasq.leasefile

Test case 1

• Install a clean machine without an extra storage
• Verify the dhcp lease file is saved inside /tmp

Test case 2

• After test case 1
• Add a storage
• Verify the dhcp lease file is saved inside /mnt/data

Test case 3

• After test case 2
• Remove the storage
• Verify the dhcp lease file is saved inside /tmp