Antminer S21 Firmware Recovery
How to recover and flash Antminer S21 firmware using SD card, web UI, and network methods. Covers stock and custom firmware.
Overview
Firmware recovery is needed when the Antminer S21 fails to boot, has a corrupted web interface, shows erratic behavior, or needs a fresh installation. The S21 supports multiple recovery methods: SD card flash (most reliable), web UI upgrade (when the miner is accessible), and network-based recovery.
This guide covers all recovery methods, including how to install custom firmware options like Braiins OS+, Vnish, and LuxOS.
When You Need Firmware Recovery
- Miner stuck in a boot loop (LEDs cycling repeatedly)
- Web dashboard is blank, partial, or inaccessible
- Mining software crashes repeatedly
- "NAND error" messages in kernel log
- After a failed firmware upgrade
- Miner was bricked by incorrect firmware
- Need to reset all settings to factory defaults
- Switching between stock and custom firmware
Required Tools
- MicroSD card — 2–16GB capacity (larger cards may not be recognized)
- SD card reader — USB adapter for your computer
- Computer — Windows, macOS, or Linux
- Ethernet cable — for network-based recovery and post-flash setup
- Firmware image — downloaded from Bitmain support or custom firmware vendor
- 7-Zip or equivalent — for extracting firmware archives
Method 1: SD Card Recovery (Recommended)
The SD card method is the most reliable recovery technique and works even when the miner is completely unresponsive.
Step 1: Download the Correct Firmware
Critical: Download firmware for the exact model (S21, not S21 Pro or S21 Hyd). Using incorrect firmware can permanently damage the control board's NAND flash.
- Visit Bitmain's official support page for S21 firmware downloads
- Select the firmware version appropriate for your hardware revision
- Download the archive file (typically
.tar.gzor.zip) - Verify the file checksum (MD5 or SHA-256) matches the published value
Firmware file naming convention:
Antminer-S21-release-{date}-{version}.tar.gzKeep the firmware file accessible — you will extract it in the next step.
Step 2: Prepare the SD Card
- Format the SD card:
- Insert the SD card into your computer
- Format as FAT32 (not exFAT or NTFS)
- Allocation unit size: Default or 4096 bytes
- On macOS: Use Disk Utility, select "MS-DOS (FAT)"
- On Linux:
mkfs.vfat -F 32 /dev/sdX1
Some SD cards larger than 32GB cannot be formatted as FAT32 using the Windows built-in tool. Use a third-party tool like Rufus or the SD Card Formatter from the SD Association.
-
Extract the firmware:
- Extract the downloaded archive
- You should see one or more files (e.g.,
uramdisk.image.gz,BOOT.bin,uImage, or a singleupgrade.bin) - Copy all extracted files to the root directory of the SD card (not inside a subfolder)
-
Safely eject the SD card from your computer
Step 3: Flash the Firmware
- Power off the miner completely — disconnect the AC cord from the PSU
- Insert the SD card into the microSD slot on the S21 control board
- Reconnect AC power to the PSU
The miner will detect the SD card and automatically begin the recovery process:
-
LED indicators during recovery:
- Red + Green alternating: Firmware is being written to NAND flash
- Green blinking fast: Writing in progress
- Green solid: Flash complete, miner is booting normally
-
Wait for the process to complete — typically 3–10 minutes
- Do NOT power off during this process
- Do NOT remove the SD card until the miner has fully rebooted
-
Remove the SD card after the miner boots successfully and the green LED is solid
If you leave the SD card inserted, some firmware versions will re-flash on every boot. Always remove the SD card after successful recovery.
Step 4: Post-Recovery Setup
After firmware recovery, all settings are reset to factory defaults:
- Network: The miner defaults to DHCP — it will obtain an IP address from your router
- Find the miner's IP:
- Check your router's DHCP lease table
- Use an IP scanner (
nmap -sn 192.168.1.0/24) - Use Bitmain's IP Reporter tool
- Access the web dashboard: Open
http://<miner-ip>in a web browser - Log in with default credentials (typically
root/ blank password) - Configure:
- Set your mining pool address and worker name
- Configure network settings (static IP if desired)
- Set fan mode and temperature limits
- Change the default password
# Verify firmware version via SSH
ssh root@<miner-ip>
cat /etc/versionMethod 2: Web UI Firmware Upgrade
If the miner is accessible via the web dashboard, you can upgrade firmware without an SD card.
- Log in to the web dashboard at
http://<miner-ip> - Navigate to System → Firmware Upgrade (or Upgrade in some firmware versions)
- Click Choose File and select the firmware
.tar.gzfile (do NOT extract it) - Click Upgrade
- Wait for the upload and flash process (5–15 minutes)
- The miner will reboot automatically
Do not navigate away from the page or close the browser during the upgrade. Do not power off the miner. A failed web upgrade may require SD card recovery.
Method 3: Network Recovery (SSH)
For advanced users who have SSH access:
# SSH into the miner
ssh root@<miner-ip>
# Upload firmware via SCP
scp Antminer-S21-firmware.tar.gz root@<miner-ip>:/tmp/
# On the miner, run the upgrade
cd /tmp
tar xzf Antminer-S21-firmware.tar.gz
./runme.sh # or the appropriate upgrade script
# The miner will reboot after flashingCustom Firmware Options
The S21 supports several third-party firmware options that offer features beyond the stock firmware:
Braiins OS+ is the most widely-used custom firmware:
- Auto-tuning: Automatically finds the most efficient operating point per chip
- Stratum V2 support: Improved mining protocol
- Cost: Free when mining on Braiins Pool, otherwise a pool fee applies
- Installation: BOS Toolkit (command-line) or web-based installer
- Supported models: S21, S21 Pro, T21
# Install Braiins OS+ via BOS Toolkit
pip install bos-toolbox
bos install <miner-ip> --model S21Vnish firmware focuses on overclocking and efficiency:
- Overclocking profiles: Up to 40% hashrate increase
- Low power mode: Up to 30% energy savings
- Per-chip optimization: Individual chip frequency tuning
- Cost: 2.8% dev fee on mining output
- Installation: SD card flash or web upgrade
Download the S21-specific Vnish firmware from vnish.net and flash via SD card or web UI.
LuxOS by Luxor offers fleet management features:
- LuxOS Manager: Centralized fleet dashboard
- Custom profiles: Power, hashrate, and efficiency presets
- API access: Full REST API for automation
- Cost: Varies by feature tier
- Installation: Web upload or SD card
Download from Luxor's website and follow their model-specific installation guide.
Firmware Downgrade
To downgrade from a newer firmware version to an older one:
- Download the target (older) firmware version
- Use the SD card method — web UI downgrades may be blocked by some firmware versions
- Some transitions (e.g., custom firmware → stock) may require an intermediate step or a specific "unlock" firmware
When switching from custom firmware back to stock, always use the SD card method. Custom firmware may modify the boot process in ways that make web-based downgrade unreliable.
Troubleshooting Recovery Issues
| Problem | Cause | Solution |
|---|---|---|
| SD card not detected | Card not FAT32, card >32GB, bad card slot | Reformat as FAT32, try smaller card, clean SD slot |
| Flash appears stuck | Large firmware file, slow SD card | Wait 15 minutes, use faster SD card (Class 10+) |
| Miner boots to old firmware | SD card files in wrong directory | Files must be in root, not a subfolder |
| "Invalid firmware" error | Wrong firmware for model | Verify firmware matches exact model (S21, not S21 Pro) |
| Boot loop after flash | Corrupted download | Re-download firmware, verify checksum |
| SD card flash succeeds but no network | Network settings reset | Miner is on DHCP, scan network for new device |
Troubleshooting FAQ
What size SD card should I use?
Use a 2–16GB microSD card. Cards larger than 32GB may not be recognized by the bootloader. Always format as FAT32.
Can I upgrade from stock to custom firmware via the web UI?
Some custom firmware (like Braiins OS+) has dedicated installation tools that work over the network. Others require SD card installation. Check the firmware vendor's documentation.
Will firmware recovery erase my mining pool settings?
Yes. All settings including pool configuration, network settings, and passwords are reset to factory defaults.
My S21 shows a different hashrate after firmware update. Is this normal?
Some firmware versions have different auto-tuning parameters. After a firmware change, allow 30–60 minutes for the auto-tuning to stabilize. Custom firmware like Vnish allows manual hashrate adjustment.
How often should I update firmware?
Update when there are security patches, performance improvements, or bug fixes relevant to your setup. Do not update simply because a new version exists — stable, working firmware should be left alone unless there is a specific reason to change.
Related Guides
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Antminer S21 Thermal Maintenance Guide
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