Failed Repack — Huawei S7721u Recovery Image Verify

The keyword "repack" indicates that the device isn't using an official Huawei-signed image from the factory. Instead, someone (perhaps a developer or a third-party ISP) created a custom firmware image by:

Huawei’s bootloader uses a proprietary digital signature (RSA-2048 or HMAC) appended to the image header. When you repack a modified image without updating the signature, the verification algorithm fails.

The Huawei S7721U is a rugged, industrial-grade GPON/EPON Optical Network Terminal (ONT), often deployed in FTTH (Fiber to the Home) scenarios, MDUs (Multi-Dwelling Units), and outdoor cabinets. It is renowned for its stability—until it isn't.

One of the most terrifying errors an engineer can face on the serial console is: "Recovery image verify failed repack" huawei s7721u recovery image verify failed repack

This message signals a fatal checksum or signature mismatch during the bootloader’s integrity check. When this happens, the device refuses to boot the main firmware, drops into a recovery shell (often BusyBox), or enters an infinite reboot loop. Without intervention, the S7721U becomes a brick.

But there is hope. This article dissects the anatomy of this error, explains why it happens, and provides a step-by-step forensic guide to repack a valid recovery image and flash it via UART or TFTP.

If you'd like, I can start by writing the full feature article now — confirm to proceed. The keyword "repack" indicates that the device isn't

⚠️ Disclaimer
This guide is for educational purposes only. Modifying firmware may void warranties, brick the device, or violate terms of service. Proceed at your own risk.

First, unpack the main bin file:

binwalk -e S7721UV100R001C02SPC123.bin

Navigate to the extracted folder. Look for a file named recovery.img or a SquashFS filesystem with offset 0x200000. If you'd like, I can start by writing

Alternatively, from a working S7721U:

dd if=/dev/mtd2 of=/tmp/original_recovery.img bs=64k

Before flashing, ensure the file isn't corrupted.

You cannot simply "repair" a corrupted image by renaming it. You must source a clean copy.

If you extracted a signature earlier, re‑append it:

cat recovery_nosig.img signature.bin > recovery_new.img

If you have a shell but no network:

# Copy image to /tmp (via USB stick mounted as /mnt/usb)
cp /mnt/usb/repacked_recovery.img /tmp/
# Flash using mtd utility
mtd write /tmp/repacked_recovery.img recovery