Installation instructions don't work for mikrotik/rb750gr3 for latest supported release

Hello,

I've been following the installation instructions here (from a Win11 PC using TinyPXE Server):

The "Firmware OpenWrt Install" field links to version 23.05.0.

I tried all different things (disabling Wi-Fi, running TinyPXE as admin, manually allowing it through the firewall, DHCP vs BOOTP, holding reset vs changing to "try ethernet once"...) but it simply will not netboot 23.05.0. It just boots RouterOS and you do not see the USR light blinking.

Eventually I went to https://openwrt.org/toh/hwdata/mikrotik/mikrotik_rb750gr3 and saw that the "Supported Since Rel" says 19.07.0 so I went and downloaded that, by manually guessing at URLs on the downloads server ending up here (thank goodness for directory indexes). And that worked first time.

I'm going to figure out what all the versions in between are and where it breaks, but can someone please update the wiki to say that the latest version does not actually work?

Hopefully if someone else has this problem I'll have saved them some time.

Also, while I'm here: I've never used OpenWRT before, this is my first attempt trying it in general (I specifically bought this model of router to try it out on). If I manage to get an earlier version installed, is it then safe to upgrade it to 23.05.0? I'm a little worried about just bricking the device since it won't netboot from this version, and I don't want to end up in a situation where I have to start trying to solder things to recover it, as I'd probably just screw that kind of thing up.

Edit: Looks like it really is only the very latest release that doesn't work. I've just tested various versions up to and including 22.03.5 (which is the last one listed in https://openwrt.org/toh/hwdata/mikrotik/mikrotik_rb750gr3?do=revisions prior to the latest one) and those all work just fine.

Thanks!

This should be checked first. As far as I remember, I was able to install version 22.03.3 directly. I will try to upload the 23.05 initramfs image when I have time.

There's never a guarantee that something won't go wrong. If that helps, my device is currently running version 23.05.0 with no issues.

Just make sure you don't save the current configuration during reflash (using the -n option) and you should be fine.

Thanks for the quick reply. Hopefully if this turns out to be an odd bug with the latest version it can be fixed in a bugfix release. I did notice this:

NOTE2 the RouterBOOT included in RouterOS 6.49.x has a ~7MiB limit for TFTP image size

but the 23.05.0 image is 5,624 KB and the 22.03.5 image (which works) is actually slightly bigger at 5,629 KB, so it doesn't seem to be this.

At the moment I'm fiddling about with the configuration options on the "live" version without flashing it so far, to see if OpenWRT will achieve what I need. If it does I may end up just installing 22.03.5 to flash and sticking with that one, rather than trying to upgrade :slight_smile:

The issue is with RouterBOOT not accepting the entrypoint address for the netboot image:

If you are building images, you could try using the lzma loader for the initram images: https://github.com/john-tho/openwrt/pull/8. I occasionally generate kernel that stalls when starting after decompress, but have not yet had a chance to debug.

Workaround is to use rc3, or 22.03 initramfs image, then sysupgrade to the 23.05.

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