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Bug 14: Ethernet Silent

Category: Boot/kernel Related docs: script-reference, overview

Symptom

The kernel launched successfully. The CPU was running—analysis showed the idle task and scheduler were active. The host expected to reach the device at 192.168.1.1, but there was no response to ARP, ICMP, or TCP probes.

Meanwhile, tcpdump on the host's Ethernet interface showed:

  • Meraki management frames with ethertype 0x0642 (Cisco's cloud management protocol)
  • DHCP discover packets from a Meraki client

These are signatures of the Cisco/Meraki userspace, not OpenWrt.

Root Cause

The OpenWrt initramfs kernel booted correctly, but during the preinit phase, the OpenWrt init scripts mounted the Meraki NAND overlay. The MR18's NAND flash contains the Cisco filesystem, and OpenWrt's preinit saw it as a valid overlay partition. Once mounted, the Cisco /etc, /bin, and other directories shadowed the initramfs contents.

The result: the OpenWrt kernel was running, but the userspace was Cisco's. The Meraki management daemon started, sending its cloud registration frames. OpenWrt's network configuration, uhttpd, telnetd, and other services never started.

The key evidence was the Meraki ethertype 0x0642 frames—these come from Cisco's proprietary management protocol and are never generated by OpenWrt.

Fix

Trigger OpenWrt failsafe mode. Failsafe mode skips the NAND overlay mount entirely, running purely from the initramfs with:

  • Static IP 192.168.1.1 on the Ethernet interface
  • telnetd listening on port 23
  • No overlay mounts
  • No normal init sequence

This provides a clean OpenWrt environment from which sysupgrade can be run to flash the NAND with OpenWrt, replacing the Cisco filesystem.

See Bug 15 for the overlay mechanics and Bug 16 through Bug 23 for the failsafe triggering challenges.

Lesson

When an initramfs kernel boots but the device behaves as if a different OS is running, check for overlay mounts. On devices with factory firmware on NAND, the OpenWrt preinit scripts may mount the factory filesystem as an overlay, replacing the initramfs userspace with the factory userspace.