I run my own email server, ownCloud server, this blog and git server on a VM running on a physical machine we rent from Hetzner. I depend on not losing data from these servers so I setup my Raspberry Pi as a backup server. Here are some thoughts on how I have tried to secure everything, perhaps it is interesting and I appreciate feedback.

Ideally the backup server shouldn't accept connections from the Internet at all, thereby reducing its surface area for attack. For this my Raspberry Pi securely protected from in-bound connections by my home router! Actually this isn't very secure at all as I don't trust the router or the other devices connected to it, although it is likely to see less random attacks then on the open Internet. As I run the server headless I need sshd running so I added some basic security; disallowed password and root SSH login and installed fail2ban.

The SSH keys that are used to access the server need to be stored on the Raspberry PI, this means someone could just take the SD card and copy the keys off it. To prevent this I encrypted the root partition and set it up so it will try and read the secret key from a USB stick during the boot. This way if I remove the USB stick after it has booted and the Raspberry Pi and SD card were stolen, the thief would still need to break the disk encryption to get the SSH keys. Of course this also means it won't restart after a power outage unless the USB stick is inserted.

It is tempting to allow the backup server root access on the target machine as it makes it easier to backup everything, but this is not ideal. If the backup server is somehow compromised then root access to the target machine is also given. To limit this risk I created an unprivileged user the backup server could connect to the as. I granted this user permissions to dump databases I wanted to backup and added secondary groups to the user for www-data and the private groups of the users home directories I wanted it to backup. This way the backup server has read access to all the data it needs much not much more.

I sync everything I want to backup from the server to a local encrypted disk connected to the Raspberry Pi. As the root partition of the Raspberry PI is itself encrypted it is reasonably safe to keep the key for the local disk on the root partition.

I don't really trust my local disk (disk failure, apartment disaster!), so I also transfer my backups to Amazon S3 who I trust not to lose data but not really for privacy. To protect my data privacy on Amazon I encrypt it with my GPG key. This is secure enough that it would cost more than I would bet anyone would pay to crack my backups.

Below is the script I run as a daily cron job. It backs up several databases and directories. It snapshots the backup directory every day to the local disk and every Sunday it pushes a PGP encrypted snapshot to Amazon S3. When it finishes, successfully or not, it emails me with the result and the logs. It also prevents multiple backups running simultaneously and cleans up old snapshots on the local disk. I don't bother removing old snapshot from S3 yet, but might consider it when I get an Amazon storage bill of more than $2.

#!/bin/bash
set -e

ssh_host=someone@somehost.com
ssh_port=22
email=someone@somehost
s3_bucket=some.bucket
log_path=/mnt/backup/logs
backup_path=/mnt/backup/current
snapshot_path=/mnt/backup/snapshots
snapshot_name=`date +"%Y%m%d_%H%M%S"`.tar.gz
snapshot_file=$snapshot_path/$snapshot_name
scriptname=$(basename $0)
pidfile="/tmp/${scriptname}"

function log {
    echo "[$(date)]: $*"
}

initialize_directories() {
    log "Initializing directories"
    mkdir -p $backup_path
    mkdir -p $snapshot_path
    mkdir -p $log_path
}

backup_databases() {
    cat databases | while read database; do
        log "Backing up database: $database"
        ssh -p $ssh_port $ssh_host "pg_dump --data-only $database" > $backup_path/$database.sql
    done
}

backup_files() {
    cat locations | while read remote_path local_path; do 
        log "Backing up $remote_path to $local_path"

        log "Ignoring files:"
        ssh -n -p $ssh_port $ssh_host "find $remote_path -type d -a ! -executable -prune -o ! -readable -a ! -type l" | 
            cut -c ${#remote_path}- > /tmp/unreadable
        cat /tmp/unreadable

        log "Start rsync"
        rsync --delete -rltzuv --exclude-from="/tmp/unreadable" -e "ssh -p $ssh_port" $ssh_host:$remote_path $backup_path/$local_path
    done
}

snapshot() {
    log "Taking snapshot $snapshot_name"
    tar -czf $snapshot_file $backup_path
}

remove_old_snapshots() {
        $snapshot_path -name "*.tar.gz" | sort -r | tail -n +10 | xargs rm
}

should_push_offsite() {
    # Returns successfully if the backups should be pushed offsite.
    # Currently it returns successfully on Sundays, but it would be better
    # if it checked how long since the last successful offsite push.
    day_of_week=$(date +%u)
    sunday=7
    if [ $day_of_week = $sunday ]; then
        return 0;
    else
        return 1;
    fi
}


push_offsite() {
    log "Encrypting snapshot"
    gpg --encrypt -o $snapshot_file.gpg --recipient $email $snapshot_file 
    log "Send encrypting snapshot"
    s3cmd put $snapshot_file.gpg s3://$s3_bucket/$snapshot_name.gpg
}

get_lock() {
    log "Attempting to aquire lock"
    exec 200>$pidfile
    if flock -n 200; then 
        pid=$$
        log "Lock aquired for process $pid"
        echo $pid 1>&200
        return 0;
    else 
        log "Failed to aquire lock"
        return 1;
    fi
}

do_backup() {
    get_lock
    initialize_directories
    backup_databases
    backup_files
    snapshot
    if should_push_offsite; then
        push_offsite
        remove_old_snapshots
    else
        log "Skipping offsite storage"
    fi
    log "Completed backup."
}

main() {
    do_backup | tee $log_path/$snapshot_name.log

    if [ ${PIPESTATUS[0]} -eq 0 ]; then
        log "Sending report to $email."
        gpg -ea -r $email -o $log_path/$snapshot_name.log.asc $log_path/$snapshot_name.log
        echo "Results attached" | mail -s "Buckup Succeeded" -a $snapshot_file.log.asc $email
    else
        log "Backup failed, sending report to $email."
        gpg -ea -r $email -o $log_path/$snapshot_name.log.asc $log_path/$snapshot_name.log
        echo "Results attached" | mail -s "Buckup Failed" -a $snapshot_file.log.asc $email
    fi
    log "Completed."
}

main

Now I think my backup system is relatively secure from attackers and unlikely to lose data, my main risk of data loss is probably me losing a key or un-plugging the Raspberry Pi.

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