Resolving Wi-fi Adapter Issues After a Linux Upgrade
Downgrading the kernel on Linux can help get your internet back.
Problem#
These days I do all of my work on Linux (Ubuntu). For wi-fi I use a cheap USB adapter. This last Friday the adapter stopped working. I had several remote meetings scheduled for the following Monday, so I needed a relatively quick solution.
Eventually I took the approach of downgrading my kernel. A summary of the relevant terminal commands is included below:
# check what kernel is used by the system
# list all installed kernels
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# find info on the kernel module for your wi-fi adapter
# reboot, hold SHIFT and change kernel from the Grub menu
# remove unneeded kernel to persist the change
Troubleshooting#
Initially, I went through a number of conventional troubleshooting steps that were unsuccessful:
- Plug the adapter into different USB ports
- Reboot the computer
- Run updates
At this point I contemplated buying a replacement adapter, but they are more expensive than I thought, so I returned to troubleshooting.
What was the root cause of this problem? I had recently upgraded to Ubuntu 22.04.2 LTS (Jammy Jellyfish) on two devices: a laptop and a PC. The upgrade caused no issues on the laptop but on PC it degraded the wi-fi; ultimately, the wi-fi stopped functioning altogether. As I only use an external wi-fi adapter on the PC, I figured there was a driver compatibility issue introduced during the upgrade on that device. So I tried another approach:
- Attempt reinstallation of drivers
The company that sold the adapter had a driver installation script on their website for my exact adapter. Every attempt to use this script failed, however, presumably because it was outdated.
I then opted to:
- Downgrade the Linux kernel
Downgrading#
First I identified the active kernel on the system:
This outputted 5.19. The presence of any other installed kernel (images) could also be checked with:
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This showed that an older kernel — 5.15 — was installed, in addition to 5.19. Note: the output was more verbose than stated above (i.e., 5.15.0-76-generic) but you get the idea.
The following command listed the kernel modules and their installation status:
One of the modules was for a wi-fi adapter: rtl8821CU/5.4.1, 5.15.0-76-generic, x86_64: installed. The code beginning 5.15 indicated that the module was built for the older kernel, which was not currently running on my system.
To downgrade the kernel the computer first needed to be rebooted:
Holding shift during the reboot opened the Grub menu. This is a minimalistic text-based interface that enables switching between different kernels installed on the system.
After selecting Advanced options for Ubuntu I saw that the two kernels were listed as expected — 5.19 (active) and 5.15 (desired).
The desired version could be selected using the arrow keys and activated with the return key.
This booted me back into the login screen and — much to my relief — the wi-fi connected automatically after login.
Removing a Kernel#
Rebooting again at this point would have reverted the system to the newer kernel (in this case 5.19), which would cause the same wi-fi connection issues to recur.
To prevent this from happening the newer kernel needed to be removed, at least for now.
This was achieved using the following command, with the full kernel name taken from the output of apt list --installed | grep linux-image:
After a fresh reboot, checking the active kernel again with uname -r confirmed that the 5.15 kernel was still being used.
"First Download X"#
If trying to troubleshoot this kind of problem yourself you might be frustrated to find that many of the available help guides require you to have internet access on your device. For example, you might be asked to run a command like sudo apt update or to git clone an online repo, neither of which will be possible with your faulty wi-fi.
In my case, the wi-fi router was located downstairs, the PC was too large to move and the ethernet cable was too short to connect. There were no alternative wi-fi adapters in the house and no obvious retailer nearby that might supply one. My only option was a wi-fi hotspot.
I was able to do this using an Android phone via bluetooth. Without going into the details — as they may vary by device — I activated bluetooth on my phone and then made it available as a hotspot through the Network and Internet settings. I already had Blueman Bluetooth manager installed on my PC, so running blueman-manager opened a GUI that included my phone as a selectable device. Once a connection was made wi-fi was now accessible on my PC through the phone (albeit with a crawlingly-slow internet speed).
Alternative Solutions#
The above solution was adequate for me, as I urgently needed a quick fix over the weekend so that I could fulfill work responsibilities by Monday morning.
If you have more time on your hands and do not wish to downgrade your kernel you might want to try to find an appropriate driver for your adapter.
If you search for drivers by their code (i.e., rtl8821CU) you might find GitHub repos with drivers and installation instructions. The supplier for your specific adapter might also provide drivers on their company website (though these might be outdated).
Unfortunately, a driver may not be currently available for your adapter/kernel until someone (or a future you) eventually uploads a patched version. In that case you might need to buy a new plug-and-play adapter after first double-checking that it is Linux-compatible.
If you do not have time for these explorations, if a driver is not readily available online or if you don't want to purchase a new adapter then downgrading your kernel might be the most immediately-viable option.