🛠 Gadget USB : Ethernet sur le port OTG + relais Wi-Fi

Objectives

Use the micro-USB data port (OTG, BCM2835) in gadget mode: the Zero W enumerates as an Ethernet card on any PC while relaying Internet through its Wi-Fi — the "Linux USB stick", with no configuration required on the PC side.

Hardware

• 1 Raspberry Pi Zero W (legacy boot: FAT partition with config.txt, no EEPROM)
• 1 micro-USB data cable to the PC

Steps

1. Enable the controller: in /boot/config.txt add dtoverlay=dwc2, and modules=dwc2 on the dwc_otg.lpm_enable=0 console=... line in /boot/cmdline.txt.
2. Create the Ethernet gadget via configfs/libcomposite (modprobe libcomposite, usb0 in g_ether RNDIS/CDC).
3. Link the two network interfaces: enable routing sysctl -w net.ipv4.ip_forward=1 and masquerade behind wlan0: iptables -t nat -A POSTROUTING -o wlan0 -j MASQUERADE.
4. On the PC, the interface appears (usb0 / "Remote NDIS") and obtains an address via DHCP through dnsmasq on the board.

Verification

ip a shows usb0 UP; from the PC, ping to a public IP goes out through the Zero W's Wi-Fi.

Common Errors

• Plugging into the PWR port: power only, no USB signal.
• Confusing OTG device and host: the Zero W has no USB host port or hub — gadget mode is the only USB path.
• Forgetting NAT or ip_forward: the PC sees the link but cannot access the Internet.