Prepare 2 files within top level directory /volume2. The name is optional but must be a directory outside of the encrypted /volume1 and also outside of any operating system or Synology created directory so that these 2 files are kept intact when DSM get's updated, they must be treated as user files. In my setup Synology created /volume2 automatically when I created the fully encrypted /volume1.

I wrote 2 shell files which run in /bin/sh or /bin/bash. They regularly check existence of /volume1/homes, and if the script detects the homes directory is not accessible it restarts the SSH daemon with password login enabled. To further safeguard it against misuse my DSM first contacts an external (cloud) server and requests the 2-factor-login key for my user account, which I activated with 2FA served through google authenticator. Without the 6 digit login key served from my cloud server, which serves it only if the request originates from my own fixed IP NAS, password login is not enabled.

There are 3 files required to make this concept work, the first 2 which are enablepwd and queryotp reside on my NAS under /volume2/rescue/batch/, the third one synology.php serves the OTP and resides on my cloud server.

Enable password login with enablepwd:

#!/bin/sh
#
# This script checks availability of a fully encrypted volume1 and enables password authentication for SSH in case the volume is locked
# After restarting the SSH daemon it allows to login even though the homes directory is not accessible
# It requires script 'queryotp' to provide a valid google authenticator code without which it will do nothing
# Must be run as root, place it in the task scheduler with regular execution
# (c) Bernard Condrau
# 2026-08-31: initial
#
VOLUME="/volume1/homes"
SSHD_CONFIG="/etc/ssh/sshd_config"
PWD_AUTH_STR="PasswordAuthentication yes"
BASE_PATH=$( cd -- "$( dirname -- "${BASH_SOURCE[0]}" )" &> /dev/null && pwd )
QUERY_OTP=$BASE_PATH"/queryotp"
LOG_FILE=$BASE_PATH"/../logs/enablepwd.log"
#
# check whether password authentication is enabled
head -n 1 $SSHD_CONFIG | grep -q "$PWD_AUTH_STR" && ENABLED="yes" || ENABLED=""
echo "$(date +'%Y-%m-%d %H.%M'): enablepwd task run" | tee -a $LOG_FILE
#
# check whether homes exists (e.g. volume1 is decrypted)
if [ -d "$VOLUME" ]; then
	echo "homes exists."
	if [[ -n "$ENABLED" ]]; then
		# disable password authentication
		$QUERY_OTP
		if [ $? -eq 0 ]; then
			sed -i "1d" $SSHD_CONFIG
			synosystemctl restart sshd
			echo "$(date +'%Y-%m-%d %H.%M'): \"$PWD_AUTH_STR\" removed" | tee -a $LOG_FILE
			exit 0
		fi
	fi
else
	echo "homes does not exist."
	if [[ -z "$ENABLED" ]]; then
		# enable password authentication
		$QUERY_OTP
		if [ $? -eq 0 ]; then
			sed -i "1i $PWD_AUTH_STR" $SSHD_CONFIG
			synosystemctl restart sshd
			echo "$(date +'%Y-%m-%d %H.%M'): \"$PWD_AUTH_STR\" added" | tee -a $LOG_FILE
			exit 0
		fi
	fi
fi
exit 1

Query OTP with queryotp:

#!/bin/sh
#
# Query OTP
#  must be run as root
# (c) Bernard Condrau
# 2026-08-31: initial
#
OTPQ="https://your.cloud.tld/html/synology.php?share=merkur-otp&auth=your-safe-authentication-password-which-should-have-24-or-more-characters"
AUTH=0;
CODE="`wget -qO - $OTPQ`"
SYNO=$(sudo synootp username=bco code=$CODE)
if [[ $SYNO == 'auth ok' ]]; then
	echo 'authenticated'
	exit 0
else
	sleep 35
	CODE="`wget -qO - $OTPQ`"
	SYNO=$(sudo synootp username=bco code=$CODE)
	if [[ $SYNO == 'auth ok' ]]; then
		echo 'authenticated'
		exit 0
	fi
fi
echo 'not authenticated'
exit 1

Serving the OTP through wget to synology.php:

<?php
/*
 * php to deliver encryption key to remote NAS
 *
 * (c) 2026, Bernard Condrau
 *
 */

// Build an array of trusted IP addresses...
$trusted=array('nas.server1.ip.addr', 'nas.server2.ip.addr');

// check if request is made from valid ip address
if (in_array($_SERVER['REMOTE_ADDR'], $trusted) && isset($_SERVER['QUERY_STRING'])) {
	$password = array(
		'merkur-otp'	=> 	['your-safe-authentication-password-which-should-have-24-or-more-characters',
					shell_exec('/home/user/html/merkur-otp.sh') // generate OTP
					],
	);
	// get the share name from the request
	$share = $_GET['share'];
	if (isset($password[$share]) && ($password[$share][0] == $_GET['auth'])) {
		echo $password[$share][1]; // echo the password, this will be grabbed by wget
		exit(0);
	}
}
echo '';
?>

Compute the OTP in merkur-otp.sh:

#!/bin/bash
source /home/bco/html/pyotp-env/bin/activate
python3 /home/bco/html/pyotp-env/merkur-otp.py
deactivate
exit 0

Set up a task in Task Scheduler of your DSM.

Open Storage Manager in Synology DSM

Go to Storage and click on your encrypted volume

Click the Settings button (or the three dots/action menu next to the volume)

Locate the Recovery Key section

Click Regenerate

Enter your Vault Password when prompted to authorize the action

The new recovery key file (.rkey) will download automatically to your computer's default download folder