create a new ed25519 key on the backup server, and copy the public Setup /root/.ssh/authorized_keys command=“/usr/bin/rrsync -ro /”,restrict,from=“192.0.2.2” ssh-ed25519 abc1234PublicKey

This will force the use of rrsync to the / directory as read-only, when logging in using the specified key, from 192.0.2.2 .

make sure to use the “–inplace” option for rsync when using the setup for btrfs, from the rsync man page “it can also help keep a copy-on-write filesystem snapshot from diverging the entire contents of a file that only has minor changes”

In order to take advantage of btrfs subvolumes with rsnapshot the following options in the rsnapshot configuration will need to be modified according to ¹⁾

cmd_cp      /root/bin/rsnapshot_plug_cp_btrfs
cmd_rm      /root/bin/rsnapshot_plug_rm_btrfs

snapshot_plug_cp_btrfs ²⁾

rsnapshot_plug_cp_btrfs

#!/bin/sh
# Arg 1: -al
# Arg 2: /testbtrfs/backups/hourly.0
# Arg 3: /testbtrfs/backups/hourly.1
btrfs subvolume snapshot $2 $3

rsnapshot_plug_rm_btrfs ³⁾

rsnapshot_plug_rm_btrfs

#!/bin/sh
 
# Arg 1: -rf
# Arg 2: /testbtrfs/backups/hourly.4/
 
# echo 1: $1  2: $@
 
# Try to delete the given path with btrfs subvolume delete first
# if this fails fall back to normal rm
if [  "$1" = "-rf"  -a  "$3" = ""  ]; then
        # "trying to delete with btrfs"
        btrfs subvolume delete $2
        error=$?
        if [ $error -eq 13 ]; then
                # EC 13 => The directory specified is no subvolume
                rm $@
        elif [ $error -ne 0 ]; then
                echo Error while deleting with btrfs $?
        fi
else
        rm $@
fi

https://web.archive.org/web/20140409045559/http://wwerther.de/2011/10/migrate-rsnapshot-based-backup-to-btrfs-snapshots/

create a user and limit it to a predefined sftp only chroot.

useradd  -m -s /bin/false gatewaybackup

setup chroot

mkdir -p /chrootbackup/gatewaybackup/backups
chown gatewaybackups /chrootbackup/gatewaybackup/backups

nightly script using sysupgrade ⁴⁾

#!/bin/sh
backupFile=/tmp/backup-${HOSTNAME}-$(date +%F).tar.gz
sysupgrade -b $backupFile
echo "put $backupFile backups/" |  sftp -i gatewaykey  -b - gatewaybackup@<backupserver>
rm -f "${backupFile}"

edit the /etc/sysupgrade.conf file on the openwrt machine with a list of files to backup

update cron to run it nightly /etc/crontabs/root

54 1 * * * /root/backup.sh

create ssh key to login to backupserver

dropbearkey -t ed25519 -f gatewaykey

copy the public key to the backup server /home/gatewaybackups/.ssh/authorized_keys

dropbearkey -y -f gatewaykey

https://wiki.archlinux.org/title/SFTP_chroot

https://passingcuriosity.com/2014/openssh-restrict-to-sftp-chroot/

https://web.archive.org/web/20140409045559/http://wwerther.de/2011/10/migrate-rsnapshot-based-backup-to-btrfs-snapshots/