binwalk -e S7721UV100R001C02SPC123.bin Navigate to the extracted folder. Look for a file named recovery.img or a SquashFS filesystem with offset 0x200000 .
One of the most terrifying errors an engineer can face on the serial console is:
mksquashfs recovery_root/ new_recovery.squashfs -comp lzo -b 131072 -noappend This is the trickiest part. You must preserve the original header, CRC, and signature placeholders. huawei s7721u recovery image verify failed repack
00000000 48 55 41 57 45 49 00 01 00 00 1c 20 00 00 00 10 |HUAWEI..... ....| The first 4 bytes ( HUAW or NAND ) define the image type. If you repack without preserving this, verification fails. 3.4 Mounting and Modifying the Recovery Image Use binwalk to find the SquashFS start offset:
This article is written for network engineers, embedded systems technicians, and advanced hobbyists dealing with Huawei enterprise ONT (Optical Network Terminal) devices. Introduction: The Bootloop Nightmare 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. binwalk -e S7721UV100R001C02SPC123
hexdump -C original_recovery.img | head -40 You should see something like:
# Step 1: Extract the first 262144 bytes (header + boot code) dd if=original_recovery.img of=header.bin bs=1 count=262144 cat header.bin new_recovery.squashfs > repacked_recovery.img Step 3: Padding to original size (e.g., 8MB) truncate -s 8M repacked_recovery.img You must preserve the original header, CRC, and
This error isn't a death sentence—it’s an invitation to understand deep embedded boot security. Happy repacking, and may your S7721U boot cleanly once more. Modifying firmware voids warranties and may violate local telecom regulations. This guide is for educational and authorized repair purposes only.