Upgrading OpenWRT

At the time when I bought my Linksys WiFi routers, OpenWRT was in the version 19.x and now we have version 21.x. I replaced DD-WRT to OpenWRT in the past and I would not expect the DD-WRT config can be port to OpenWRT. But for upgrading the versions of OpenWRT, however, it is natural to expect the config can be ported. Except it is not seamless in my case.

It has been reported that upgrading OpenWRT to 21.x is not trivial. The reason probably lies in the use of DSA that changed the philosophy in handling switch interface in the kernel. While in OpenWRT, there is a sysupgrade command that allows us to flash the firmware without retaining the config, i.e.,

sysupgrade -n <image>

But it won’t work for the 19.x to 21.x upgrade on any of my Linksys WRT devices. It gives out the reason as “Image version mismatch: image 1.1, device 1.0”. That is not an issue of the config retained or not. Simply the image’s version triggered the safety logic of sysupgrade.

However, it would work if we use the -F (force) option. But we need the “factory” image, i.e., the one with extension .img and for flashing OpenWRT from the factory firmware. For example, the WRT3200ACM device, we can first, scp the image to the device (e.g., put it in /tmp as it is a tmpfs large enough to host a few MB), and then, run the following command:

sysupgrade -F -n openwrt-21.02.2-mvebu-cortexa9-linksys_wrt3200acm-squashfs-factory.img

This will flash the firmware and reset the machine. And after that, the device will return to the default config, i.e., no WiFi turned on and LAN IP set to 192.168.1.1. (This is also the method to flash the factory firmware on OpenWRT)

In OpenWRT, the config files are mostly in /etc/config (/etc/ethers and /etc/hosts may the the only exceptions for user contents). Therefore those unrelated to network (e.g., dropbear, or anything except /etc/config/network) should be able to copy over if you got the backup.

How’s OpenWRT 21.x?

One noticable caveat: Some older iOS device are not supporting WPA3. Hence we have to fall back to use WPA2 even OpenWRT supports it.