Clarify Project Policy on Default Writes to NVRAM

[Thibaut]

> Le 22 août 2025 à 17:31, Tom Li via openwrt-devel <openwrt-devel at lists.openwrt.org> a écrit :
> 
> For OpenWrt routers, writing to NVRAM by default can serve useful
> purposes in several contexts:
> 
> 1. Saving a /dev/urandom seed file after boot, to prevent early-
> boot entropy starvation in the next boot. This was first discussed
> in 2016 [0], and raised again in 2022. [1]
> 
> 2. Saving the last-known system date and time to /etc/dnsmasq.time
> for DNSSEC timestep validation, on every SIGTERM and reboot.
> Discussed in [2].

On this: « reboot » on most of these devices really means « hard power cycle ». So this seems pretty useless IMHO, since all devices which do clean reboots (e.g. VMs / PCs etc) will already either have RTC or fully writeable fs.

> 3. Saving the current IPv6 prefix on the WAN port (obtained from
> the ISP) to a file. If the router reboots due to power loss, and
> the ISP changes the IPv6 prefix in the next boot, the old prefix
> can be invalidated in a Router Advertisements to prevent IPv6
> outages in the LAN due to stale addresses. This is known as "Flash
> Renumbering Workaround Across Reboot", a recommended feature by
> RFC 9096 to work around broken ISPs. This need is raised in a recent
> odhcpd devolopment discussion [3], and is my motivation of sending
> this mail.

I think this is the most useful feature.

If implemented properly, it should result in a single write (per prefix assignment from the ISP, which, for sane ISPs, means only 1) which is no worse than the writes performed during firstboot: it shouldn’t be very controversial.

My 2c,
T.