USB WiFi adapters that show up as a CD-ROM

A “multi-state” USB WiFi adapter carries on-board storage with a Windows driver, so on first plug it enumerates as a CD-ROM or flash drive. Windows autoruns the installer; other OSes see only a disk and no network interface, so the adapter must be told to switch.

Most mainstream distributions — Ubuntu, Fedora, Debian, OpenSUSE, Arch — ship usb-modeswitch with the adapter in its data files and switch silently; hand-switch only when the package is missing, the data predates the adapter, or the device is a rebadged clone.

Finding out

lsusb
sudo apt install usb-modeswitch usb-modeswitch-data
sudo usb_modeswitch -K -W -v 0e8d -p 2870     # vendor:product from lsusb

-K sends the eject, -W removes the fake storage. If lsusb then shows a new product ID and ip link a wireless interface, the switch worked — make it permanent.

Making it permanent

Two files: a udev rule triggering the switch on plug, and a config file for the device. The rule goes in /lib/udev/rules.d/40-usb_modeswitch.rules, below the GOTO="modeswitch_rules_end" line near the top:

# COMFAST CF-WU782AC/WU785AC, TEROW ROW02FD
ATTR{idVendor}=="0e8d", ATTR{idProduct}=="2870", RUN+="usb_modeswitch '/%k'"

The config file is named after the original IDs, /usr/share/usb_modeswitch/0e8d:2870:

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TargetVendor=0x0e8d
TargetProductList="7612"
StandardEject=1

TargetVendor/TargetProductList are the IDs the device takes after switching — what the kernel binds a driver to; StandardEject=1 is the common case, and quirky devices need a MessageContent byte sequence instead.

Reboot, or replug the adapter, and it should come up as WiFi.

The 8811cu entry is the exception: some adapters switch with just the -K eject in the rule, no config file.

ATTR{idVendor}=="0bda", ATTR{idProduct}=="1a2b", RUN+="/usr/sbin/usb_modeswitch -K -v 0bda -p 1a2b"

Two rule forms: RUN+="usb_modeswitch '/%k'" reads the config file on every plug; an inline usb_modeswitch -K … hardcodes the command. Prefer the config file — a switched device that re-enumerates back to storage usually means a rule that never ran.

Turning it off

To stop it fighting over a device, edit /etc/usb_modeswitch.conf:

DisableSwitching=1

Then reboot. Disabling is global, useful when something else handles the modem or dongle.

References

• morrownr/USB-WiFi: How to Modeswitch
• usb_modeswitch