NVRAM Backup from GUI

[tracker.ca]

Very early in my experience with Merlin's firmware, I had experienced a problem restoring from a backup, where part of the configuration stays as before the restore (let's say, you are restoring onto a brand new router, and the 2.4GHz SSID is loaded from the restore, but not the 5GHz SSID)

At the time I dismissed as a glitch but, since then, it forced me into a "ritual" where I always make 3 backups of the NVRAM from the GUI. Throughout the years it has happened again, but I am usually in a hurry and just restore from another backup and move on.

A couple of weeks ago it happened again, and I found that 2 of the 3 backup files wouldn't restore properly. Moreover, the backup that worked was the #2: The first and the last of the three didn't work. That is more than just a glitch.

Using WRTSettings, I found that, while the "good" backup looks fine, the two "bad" backups had some entries that combined a bunch of NVRAM variables together with an "\x00" separator (see attached image).

I don't know what to make out of this information, but it has happened in the past (as early as 374.40 and as late as 380.59) on multiple models (RT-N16, N66U, AC56U).

 

[ColinTaylor]

The x00 character is the normal end-of-field delimiter for all of the NVAM variables, so that in itself isn't an issue.

Either there is a bug in the way WRTSettings is detecting those fields, or there is something else that is the source of the problem.

What do you see immediately before the corrupted output?

 

[tracker.ca]

Hi Colin,
Saving the corrupted NVRAM Backup file is no different than the ones which work well (which are the majority, btw). You can't tell until you need to restore (or open it in WRTSettings and try to find the variables)
Perhaps the bug in WRTSettings is related to how it parses the file for display, but the corruption is already there and the router itself cannot read it regardless.
Unfortunately the NVRAM Backup is encrypted, salted so every backup looks binary different and I don't have the knowledge to examine it without the tool.

 

[john9527]

Check your NVRAM utilization on the Tools > Sysinfo page. You need about 5000 bytes of free space, or all sorts of strange things can start happening.

 

[tracker.ca]

Hi John,
thanks for your input. I don't have NVRAM usage numbers at the time of the backup, but considering that they were done before the router has gone into production, it was probably lower than the current usage (which is at 48K) due to user lists, etc...
I feel that the problem occurs at the time of the backup.