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ZFS Remote Mirrors for Home Use

Update: Now with pre-built ZFS Raspberry Pi image! Jump to the Appendix for more information. In summary: Flash the image, change the keys, send your snapshots.

ECC Memory Note: The requirement for ECC memory with ZFS is a little contentious, it's not needed for this use but see the second Appendix for more information.

Why pay a nebulous cloud provider to store copies of our boring, but nice to keep data? Old photographs, home videos, college papers, MP3s from Napster; we typically stick them somewhere and hope the storage doesn't rot.

But we can do better. Magnetic storage is cheap; and our data is valuable. We don't need live synchronisation, cloud scaling, SLAs, NSAs, terms of service, lock-ins, buy-outs, up-sells, shut-downs, DoSs, fail whales, pay-us-or-we'll-deletes, or any of the noise that comes with using someone else's infrastructure. We'd just like a big drive that we can backup to, reliably, easily, and privately.

How about an automatic, remote, encrypted, verifiable, incremental backup of all your data, for about 100 currency units in outlay, less if you have existing hardware, and no upkeep costs? How?

You can have:

  • A block level copy of your ZFS pool, replicating all datasets and snapshots.
  • Automated incremental backups so that only changed data is transferred.
  • Complete at-rest encryption, with the key resident in memory only during backup operations.
  • Hardened communication security, so your data is safe in-flight.
  • A minimalistic, locked down, low-power remote server requiring almost no maintenance.
  • All wrapped up as a single command, powered entirely by tools in the FreeBSD base system.

You will need:

  • A FreeBSD 10.2 (or later) supported system, such as:
  • A Raspberry Pi, BeagleBoard or another FreeBSD ARM target, use a 4GB+ SD Card.
  • An old laptop, with around 512MB of memory, preferably something quiet.
  • A USB Hard Drive.
  • If using an SBC, a drive that comes with its own power supply is useful.
  • If using an old laptop, a drive powered directly by USB is probably better.
  • An Ethernet Internet connection in a location that is not your home.
  • Do not even consider Wi-Fi. Frustration abounds.
  • The ability to reach the system from the outside world. Potentially via:
  • Port forwarding (most likely).
  • IPv6.
  • Overlay routing (Tor, I2P, cjdns).

I'm specifying a laptop/SBC as it's a lot more palatable a device to ask someone to let you keep in their home. You can of course use a desktop if you have sufficient charm.

Please note, that ZFS is designed for serious, enterprise grade environments. Using it on older consumer hardware is doing an injustice to its many features, and the kernel will remind you of this on every boot. That said, it can be squeezed into this restricted use case, and you can have many of the benefits most important to a backup system. However, if you can supply more modern hardware, do. A system with ECC memory would be a wise investment also. Even if it is an older model.

If you must go with "previously loved", hardware, do yourself a favour and run a memory test first. Faulty DIMMs are a heartbreak one only tolerates once. MemTest86+ is a good choice, but their ISOs don't work on USB drives, only CDs. You could mess about with gpart to create a bootable partition manually but the good folks at Canonical added memtest to the boot-screen of some Ubuntu installers, which can be dandily dd'd to a memory stick. I can attest to 14.04.2 LTS Server having this. SBC users can try memtester, but as it runs on top of a full kernel it can't test the memory the kernel occupies.

Why not mirror the drives locally?


  • No remote system needed.
  • No network connection needed for backups, therefore much faster.
  • ZFS can auto-recover any corruption without missing a beat.


  • Devices are vulnerable to localised physical damage, e.g. fire, dogs, children.
  • Filesystems are susceptible to user stupidity as writes are synchronised live (snapshots help).

Why not some another way?

Some alternatives include rsync, bup, obnam, git-annex, something else.


  • Easier to setup.
  • OS agnostic.
  • Potentially local only decryption.


  • No ZFS features, such as:
  • Free snapshots.
  • Data integrity verification.
  • Block compression.
  • Deltas based on actual modified blocks, not file modification timestamps.

These solutions focus on backing up individual files, which I believe is a less useful system than a full mirror. They are certainly easier to setup though.

It's worth noting that as we send the decryption key to the remote device, our method is somewhat less secure than a method that never allows the remote system to view plaintext data. If someone has physical access then it's possible for the key to leak through side-channel analysis or system subversion.

The Solaris version of ZFS allows per-dataset encryption, which allows the pool to synchronise without exposing the plaintext data. This feature hasn't yet made it to FreeBSD, but for data that needs extra protection, we can approximate it (details further on).

Threat Model

Since nothing is really secure, just appropriately difficult, it's good to define what threats we'd like to defend against. This way we can spot when we've gone too far, or not far enough.

  • Data Loss - Made difficult by the existence of the remote backup and regular synchronisation.
  • Data Leaks - Made difficult by the at-rest encryption on the remote disk.
  • Server Breach (Digital) - Made difficult by judicious use of SSH keys.
  • Server Breach (Physical) - Made difficult by the front door lock of the home.

This list doesn't cover any of the security of our main system of course, only the remote one. Presumably you have some sort of disk encryption in use already.

The physical risk is the hardest to defend against. An attacker with physical access could potentially modify the system to capture the decryption key. Passwords are required for local logins, so the system would have to be shut down to modify it, unless they can exploit a faulty USB stack or similar alternative entry.

To defend against physical threats, you could encrypt the OS drive of the system and require a USB key to boot it, that way if it was ever powered down it could not be surreptitiously modified and rebooted. However, any time it crashes, or suffers a power cut, you'd have to boot it manually, or request your kind host to do so. I feel that this level of defence is more trouble than it's worth. See the section Extra Encryption for an easier alternative.

Secret Material

Any good crypto system is defined by the secrets it requires. For this setup, we need two:

  • The password for the master SSH signing key trusted by the backup system.
  • The encryption key for the hard disk.

Other secrets like the passwords for the root and the user account are less important. They will also not really be used beyond the initial setup stage, so I recommend choosing one single strong password and using it for all three cases. You can generate one like this:

hugh@local$ echo $(strings /dev/random | sed -E '/^.{6}$/!d;/[[:space:]]|[[:punct:]]/d' | head -n 2 | tr -d '\n')


That reads any strings that emerge from the noise of the PRNG, discards any not six characters long, and any containing whitespace or punctuation. After two such strings have been emitted it puts them together and echo ensures there's a newline at the end. We could just look for a single twelve character string matching our needs but that takes a while, those monkeys can only type so fast.

For future reference, all the commands I show you are written to run on a FreeBSD 10 system. GNU utilities often take different arguments so you might be able to translate with careful reference to the man pages. Also, I'll write the names of system components in italics. E.g. ZFS is the product, zfs is the command to control it.

Server Setup

OS Installation

Step one, is of course, to install FreeBSD. If this sounds daunting to you, take a look at the excellent Handbook. There's different ways to do this based on what platform you're using but here's a run down of some answers to the questions the installer will ask you on an i386/amd64 target. Do not connect your USB drive yet.

  • Choose install from the Install / Shell / Live CD dialogue.
  • Choose your desired keymap. (I use UK ISO-8859-1)
  • Name the machine (knox is a good name).
  • Deselect all optional system components (doc, games, ports, src)
  • Choose Auto (UFS) over the entire disk. Defaults are usually fine.
  • Set your strong root password.
  • Set up IPv4 with DHCP unless you know better.
  • I don't bother with IPv6 as Irish ISPs haven't heard of it.
  • Your clock is usually UTC, so say yes.
  • Choose your timezone.
  • Disable all services on boot. We'll configure them manually.
  • Do not add users now. We'll do it later.
  • Choose Exit from the final menu.
  • YES, you do want to enter a shell to make final modifications.

Congratulations. You are standing in a root shell west of a fresh system, with a boarded network interface. There is a small mailbox here, but we'll soon disable it.

Before we continue, a word about FreeBSD partitions. Old wisdom was to split out /usr, /var and sometimes other aspects of the directory hierarchy onto different partitions. This has a few benefits, the most obvious is that some branches, especially /var tend to grow with system logs and other detritus. If these were allowed to grow unchecked, they might consume the entire disk which brings out edge-case complications. Splitting them off mitigates this, but with the system's specialised usage such growth isn't likely to be a problem, and it's nothing a quick check can't solve.

There was another argument to be made for performance; different branches have different read/write profiles, and splitting them reduced fragmentation for the read-mostlies. This is still true, but not significant on flash media due to uniform seek times and not really worth the hassle on a dedicated system like this. More than a moment's system tuning is for high-performance servers, and if we were interested in that we wouldn't be using old hardware. Let's keep it simple.

Finally, swap space used to be best placed at the edge of the platter (the last sectors) as the most sectors pass per rotation there. This reason goes out the window with flash, but it's still a good idea to put the swap at the end, it makes it easier to grow the partitions if we ever migrate to a larger card.

Thankfully, the FreeBSD installer will choose all of the above tweaks by default if you use guided mode.


Now we need to discover what IP address the DHCP server leased to the system. Ignore lo0 as it's the loopback interface. Also note the interface name, bfe0 in this case. Linux users may be used to eth0 style naming, but in FreeBSD the interface is named after the driver it uses, a Broadcom one in this case.

# ifconfig

bfe0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500
	ether 00:15:c5:00:00:00
	inet netmask 0xffffff00 broadcast
	media: Ethernet autoselect (100baseTX <full-duplex>)
	status: active

With this in hand we can SSH into the machine; sshd will generate some host keys automatically at this point, but before we let that happen, let's shake that odd feeling we both have that there hasn't been enough entropy in this baby-faced system yet to yield secure random numbers.

# dd if=/dev/random of=/dev/null bs=1m count=512

Now we haven't really added entropy of course, but at least we're far enough down the PRNG stream to be hard to predict. Use a different count value if you like.

Start OpenSSH. Note that this is being started in a chroot environment, so when we come to ssh in, we'll find ourselves within that same environment.

# service sshd onestart

Before we connect, we should first setup our local SSH configuration.

hugh@local$ mkdir ~/.ssh
hugh@local$ chmod 700 ~/.ssh
hugh@local$ touch ~/.ssh/config
hugh@local$ chmod 600 ~/.ssh/config

Edit your local ~/.ssh/config and insert the following:

HashKnownHosts yes

Host knox
	User root
	HostName # We'll swap in the FQDN later

Now we can simply use:

hugh@local$ ssh knox

There'll be a prompt to accept the host key as it's the first time ssh has seen it. If you think you are currently the victim of a LAN scale MitM attack you can compare the displayed key to the one on the new system before accepting. You may also want to switch to a low-sodium diet.

# ssh-keygen -lf /etc/ssh/

Now that we're in, we can use the copy/paste features of our terminal to make the config file editing a little more palatable.


We'll be editing several config files so I hope you know your vi key-bindings. If not, there's ee, which feels like a tricycle after vi, but I digress. Replace /etc/rc.conf with the following. You'll have to supply your own network adapter name gleaned from ifconfig.

keymap="uk" # use 'us' for USA layouts. Full selection in /usr/share/vt/keymaps
ifconfig_bfe0="DHCP" # Use your adapter's name, though the equivalent of this line may already be present

ntpd_enable="YES" # keep the system regular

powerd_enable="YES" # keep power usage down

sendmail_enable="NO" # no need for sendmail

dumpdev="NO" # defaults to no on RELEASE but you may be on CURRENT
#syslogd_enable="NO" # I think syslog should stay on, but you may feel otherwise

Let's have ntpd synchronise the clock before we proceed:

root@knox# service ntpd start


cron is the system command scheduler. cron really likes to send email, but since we've disabled sendmail this email will all pile up in /var/spool. Add the following line to the top of /etc/crontab to muzzle it.



This is the file system tab, which used to be a listing of all the file-systems the kernel should mount at boot time, though it's not as singular these days with the advent of removable devices and ZFS. Here's the contents of mine, note that the fields are separated by tabs, which is why that zero looks so lonesome.

# Device        Mountpoint      FStype  Options Dump    Pass#
/dev/ada0p2     /               ufs     rw,noatime      1       1
/dev/ada0p3.eli none            swap    sw      0       0
tmpfs           /tmp            tmpfs   rw,mode=1777    0       0
tmpfs           /var/run        tmpfs   rw      0       0
tmpfs           /var/log        tmpfs   rw      0       0

Your partition names may be different. I've enabled encrypted swap by appending the .eli suffix onto the device name of the swap partition. This ensures that no potentially sensitive memory is ever written out to disk in the clear. I've also added three tmpfs mounts; these are memory disks and are therefore cleared on reboot. By turning the /var/log directory into a memory disk, we should be able to keep the main system drive spun down most of the time, reducing power and extending its life. Of course, this doesn't really matter on flash devices. The other two are really just used for storing tiny files that should be cleared between boots anyway, so it makes sense to hold them in memory. The noatime setting on the root partition tells the system not to bother updating timestamps when a file is read (but still when written), this should also help to keep the drives spun down.

Be aware the using encrypted swap will prevent you from gathering automated crash dumps. This isn't a problem if your system isn't crashing, but if it starts to, switch back to regular swap (remove the .eli extension) and set dumpdev="AUTO" in /etc/rc.conf. Then, after a panic, you can run kgdb against the kernel and the dump to isolate what function caused the issue. Hopefully you can forget this paragraph exists though.


TTY stands for 'teletypewriter', which is an early method for interacting with systems of the UNIX era. This file controls the allocation of virtual TTYs. Here's my copy, you'll note that it's a lot smaller than the system default. Tabs are in use again.

console none                            unknown off insecure
ttyv1   "/usr/libexec/getty Pc"         xterm   on  secure

This disables all but one of the virtual consoles, leaving ttyv1, not v0 as the enabled one. This helps prevent the terrible temptation to login to ttyv0 and curse as various system messages overprint your session. The insecure option for console forces single user mode to require a password, which won't stop a serious physical attacker, but may deter a too-curious teenager.


This controls the environment the kernel loads into. It's a mix of some power control options and ZFS tuning parameters that should improve performance and stability on low memory systems. ZFS is very fond of memory, and runs in the kernel address space, so by limiting it we avoid some nasty memory conditions. The values here are for a 512MB system, if you have more memory than that you might consider increasing them after researching what they do, but they'll work for now. You should probably omit the two hint lines on SBC systems, though they're likely harmless. Some deeper info on ZFS tuning is available here.

Note that this file isn't present by default.

autoboot_delay=3 # speed up booting
hint.p4tcc.0.disabled=1 # use only modern CPU throttling (x86 only)
hint.acpi_throttle.0.disabled=1 # does your SBC use ACPI? RPi doesn't.
vm.kmem_size="150M" # limit the kernel's memory usage
vfs.zfs.arc_max="32M" # limit the ZFS cache size
vfs.zfs.vdev.cache.size="16M" # limit per-device cache


We're back to SSH now, and this is where I think things get really interesting. We're going to be using SSH's certificate authority features to authorise different keys. This means we give the system one key to love, honour, and obey, and then anything we sign with that key will work even if the system hasn't seen it before. Naturally, the CA key itself then should be protected with a password as good as the root password as it will essentially be another way of getting root access. Personally I think you should re-use the existing root password here but you may feel otherwise.

hugh@local$ cd ~/.ssh
hugh@local$ ssh-keygen -t ed25519 -f knox-ca
hugh@local$ scp knox:/etc/ssh/knox-ca

Now, on knox, replace the contents of /etc/ssh/sshd_config with the following (the whole thing). This avoids crypto suspected to be NSA/FVEYS vulnerable. Be aware that this machine will now only be contactable by recent versions of OpenSSH, which if you're running a recent OS will be fine.

HostKey /etc/ssh/ssh_host_ed25519_key
TrustedUserCAKeys /etc/ssh/knox-ca
AllowUsers root
PasswordAuthentication no
PermitRootLogin without-password
UseDNS no
UsePAM no
ChallengeResponseAuthentication no


# service sshd restart

SSH Client

Now we'll generate a login and sign it with the CA key, and we'll do it properly by using ssh-agent. This allows us the security of having password protected keys without the hassle of entering the password every time. We unlock the key once, add it to the agent, and it's available until we logout. ssh-agent operates as a wrapper around a shell so firstly we have to work out what shell you're using.

hugh@local$ ssh-agent $(grep $(whoami) /etc/passwd | cut -d ':' -f 7)

You can avoid that junk if you already know what shell you're using, echo $0 or echo $SHELL can sometimes also contain the shell name, but not too reliably.

Now we're in a sub-shell of the ssh-agent process, time to generate the new ID.

hugh@local$ ssh-keygen -t ed25519 -f ~/.ssh/knox-shell

Sign the key. The -I flag is just a comment.

hugh@local$ ssh-keygen -s ~/.ssh/knox-ca -I knox-shell -n root ~/.ssh/knox-shell

Now we tell ssh to use this key when connecting to knox. We can add some fanciness while we're at it. Edit your ~/.ssh/config:

HashKnownHosts yes
ControlMaster auto
ControlPath /tmp/ssh_mux_%h_%p_%r
ControlPersist 30m

Host knox
	User root
	HostName # We'll swap in the FQDN later
	HostKeyAlgorithms ssh-ed25519 # Prevent downgrade attacks
	IdentityFile ~/.ssh/knox-shell

The Control settings allow us to reuse connections which greatly speeds things up. Now that the key has its bona-fides (the file), we should unlock it and use it to login.

hugh@local$ ssh-add ~/.ssh/knox-shell
hugh@local$ ssh knox

If you get dropped into your shell without being asked for a password then all is well. For fun, let's log out and in again to see how snappy the persisted connection makes things:

root@knox# exit
hugh@local$ ssh knox
root@knox# exit



FreeBSD provides binary updates for Tier 1 architectures, i.e. i386 and amd64. You can check your architecture with uname -p. If you're not using one of those, (the RPi is ARMv6) you'll have to find alternative ways of keeping the system up to date (flashing a new image or doing a local buildworld). The system is pretty locked down though, so having it always have the latest software isn't really necessary. You can probably ignore this section for now.

Tier 1 users should execute the following, they may take a few minutes to run. fetch will present a long list of files that will be updated, you can simply press 'q' to exit this.

root@knox# freebsd-update fetch install

The pkg system

pkg, too is tier 1 only, so look to the venerable ports collection for package installations. I've heard tell of some third party pkg repositories. Though it usually better to build your own through ports, either locally or on another system. If you don't feel you need any packages, then you can ignore this.

The blessed ones can run the following; you'll be prompted to install pkg.

root@knox# pkg update

Install any other packages you like at this point.

At this point we can reboot the system.

root@knox# reboot

Log in again when it's back up and read through the boot messages to see all went well. '+G' tells less to start at the end of the file.

hugh@local$ ssh knox
root@knox# less +G /var/log/messages

You may, for instance, see:

Aug  2 00:53:21 knox root: /etc/rc: WARNING: failed to start powerd

Which tells us that for whatever reason, powerd isn't able to function on this machine. SBCs may show this, as will VMs. If you see it, remove the powerd line from /etc/rc.conf

Be aware, that if you want to login to the machine physically, instead of via SSH, you must switch to the second console with <Alt-F2>. <Alt-F1> will return you to the kernel messages screen.

Disk Setup


Time to plug in the USB drive. Let's find out what knox is calling it.

root@knox# camcontrol devlist

<FUJITSU MHY2160BH 0081000D>       at scbus0 target 0 lun 0 (ada0,pass0)
<TSSTcorp DVD+-RW TS-L632D DE03>   at scbus4 target 0 lun 0 (pass1,cd0)
<WD My Passport 0820 1007>         at scbus5 target 0 lun 0 (da0,pass2)
<WD SES Device 1007>               at scbus5 target 0 lun 1 (ses0,pass4)

The first entry, ada0 is knox's internal hard drive, the second entry is the laptop's disc drive. The 2TB USB drive sits on da0 with a monitoring interface we can disregard on ses0. Yours will likely be da0 too.

We should also work out what sector size the drive is using, though in all likelihood we're going to use 4KiB sectors anyway. Most large (2TiB+) drives will be using the 4KiB standard.

root@knox# camcontrol identify da0 | grep size

sector size logical 512, physical 4096, offset 0

Yup, it's a 4KiB drive. Now we'll generate the encryption key for the GELI full disk encryption (locally). The brackets in the below command are significant, they limit the duration of the umask change.

hugh@local$ (umask 177; touch ~/.ssh/knox-geli-key)
hugh@local$ dd if=/dev/random bs=1k count=1 > ~/.ssh/knox-geli-key

Strictly, we shouldn't store that in ~/.ssh, but it's as good a place as any. You'll have noticed that we're not using any password with this key, and since we can't back it up to the backup system (egg, chicken, etc.) we'll need to store it somewhere else. But while we might be happy to have it lying around unencrypted on our local system, where we can reasonably control physical access, we're better off encrypting it for storage on Dropbox or in an email to yourself or wherever makes sense as we don't know who might have access to those systems (presume everyone). You could also stick it on an old USB flash drive and put it in your sock drawer if you know what an NSL is.

hugh@local$ cd; tar -cf - .ssh | xz | openssl aes-128-cbc > key-backup.txz.aes

Make sure you use a good password (maybe the same as you use for your login) and stick that .aes file somewhere safe (it also contains your SSH identity). Should you ever need to decrypt that file:

hugh@local$ < key-backup.txz.aes openssl aes-128-cbc -d | tar -xf -
hugh@local$ ls .ssh

Let's get a quick measurement on the drive's normal speed before we activate the encryption.

root@knox# dd if=/dev/zero of=/dev/da0 bs=1m count=100
104857600 bytes transferred in 15.057181 secs (6963960 bytes/sec)

Send the key over to knox, this is only for the initial setup, it won't hold a copy of it. Also, since /tmp is now a memory drive, we don't need to worry about anything as serious as Guttmann erasure.

hugh@local$ scp ~/.ssh/knox-geli-key knox:/tmp

The following command creates an AES-XTS block device with a 128 bit key. Other ciphers/lengths are available but the defaults are pretty good. I feel we can skip geli's integrity options as ZFS is going to handle any cases of incidental corruption and malicious corruption isn't really in our threat model.

root@knox# geli init -s 4096 -PK /tmp/knox-geli-key /dev/da0
root@knox# geli attach -pk /tmp/knox-geli-key /dev/da0

The other geli option of note is the sector size. By forcing geli to use 4KiB sectors, and only writing to the geli overlay, we get the best performance from our drive. Though, given the low power nature of this system, we're unlikely to ever see the benefit due to slower links in the rest of the chain. Since geli encrypts per-sector, specifying a larger size also reduces it's workload versus the default 512 byte sectors.

Note: There is a bug in older ARM versions of geli that may prevent you from attaching a drive you've just initialised. See here for a workaround

Let's see how this encryption has affected our drive's speed:

root@knox# dd if=/dev/zero of=/dev/da0.eli bs=1m count=100
104857600 bytes transferred in 17.759175 secs (5904418 bytes/sec)

root@knox# echo "5904418 / 6963960" | bc -l


Fifteen percent drop? Not too bad. Again, this was never going to be a high-performance system.


Now that we have an encrypted substrate, we can hand it over to ZFS. The zpool command handles all things low-level in ZFS. I'm calling my pool wd.

root@knox# zpool create -O atime=off -O compression=lz4 wd da0.eli

Note that I specified the block device as da0.eli which is the overlay device exposed by geli. atime is access time, which logs when a file is accessed. We don't need this, and it hurts performance a little, so out it goes. lz4 compression is extremely fast, to the point of being almost computationally free, and will let our already large drive go even further. Individual ZFS datasets can override these options later on but they make good defaults. I also have these options set on my local pool, but if your local pool differs then they will be overwritten when we send the filesystem.

ZFS is all setup now (wasn't that easy? No partitioning or anything). Let's see what we have:

root@knox# zpool status

  pool: wd
 state: ONLINE
  scan: none requested
		wd          ONLINE       0     0     0
		  da0.eli   ONLINE       0     0     0

errors: No known data errors

Wonderful, now we tear it all down.

root@knox# zpool export wd
root@knox# rm -P /tmp/knox-geli-key

I securely erased the key anyway...couldn't help myself. Detach the geli and verify it's been removed:

root@knox# geli detach /dev/da0
root@knox# geli status

You should only see references to the encrypted swap, probably on ada0p3.eli. No da0.eli in sight. What does eli stand for anyway? I haven't been able to figure that out.


ZFS datasets allow you to specify different attributes on different sets of data; whether to use compression, access control lists, quotas, etc. I find that I need precisely none of those features, preferring to treat my backup storage as one large dataset with sane properties. There's nothing stopping you from creating different datasets and synchronising them to the backup system, I just don't see the point for personal backups. Know also that it makes previewing the differences between snapshot more complex, as zfs diff cannot automatically recurse into dependent snapshots, it has to be done per dataset. This isn't really a big deal though.


Drop the following script into root's home directory, call it I would have preferred to invoke these commands remotely but after a lot of experimenting, triggering the script was the only way I found that properly detached the encrypted drive in the event of connection failure. So rest assured, you're protected against that. The dd at the start is to spin up the drive.


dd if=/dev/da0 of=/dev/null bs=1m count=3 2>/dev/null
geli attach -dpk /tmp/k /dev/da0
zpool export -f wd 2>/dev/null
zpool import -Nf wd
zfs receive -Fu wd
zpool export -f wd

What's this you say? I promised that we wouldn't store the key on the backup server? Behold! The Named Pipe

Don't set any passwords on these two keys, we need them to be scriptable.

hugh@local$ ssh-keygen -t ed25519 -f ~/.ssh/knox-fifo
hugh@local$ ssh-keygen -t ed25519 -f ~/.ssh/knox-send

Though these two keys are not password protected, but they are going to be completely restricted in what they can do. This allows us to use them in an automatic way, without the fear of them being abused. Now bless them. This will ask for the CA password.

hugh@local$ ssh-keygen -s ~/.ssh/knox-ca -I knox-fifo -O clear -O force-command="mkfifo -m 600 /tmp/k; cat - > /tmp/k; rm -P /tmp/k" -n root ~/.ssh/
hugh@local$ ssh-keygen -s ~/.ssh/knox-ca -I knox-send -O clear -O force-command="/root/" -n root ~/.ssh/

Terrified? Don't be. We're signing the keys we just created and specifying that if they are presented to the remote server, the only thing they can do is execute the described command. In the first case we create a fifo on the /tmp memory disk that we write to from stdin. This will block until someone reads from it, and that someone is the script that we call next. Upon reading the fifo the key is transferred directly from our local system to the geli process and never touches the disk, or the RAM disk.

And before you complain, that's not a useless use of cat, it's required for tcsh.

Let's add some shortnames for those keys in ~/.ssh/config.

Host knox-fifo
	User root
	IdentityFile ~/.ssh/knox-fifo

Host knox-send
	User root
	IdentityFile ~/.ssh/knox-send

Final Approach

I trust you're quite excited at this point. Let's take a fresh snapshot of our local pool and send it. This will involve sending the entire dataset initially, which is likely a lot of data and is the reason we specified a local network address in ~/.ssh/config.

hugh@local$ snapname="$(date -u '+%FT%TZ')"
hugh@local$ zfs snapshot -r "wd@$snapname"

Snapshots will be given names of the form '2015-07-24T16:14:10Z (ISO 8601 format). The time stamp is from UTC so it will probably be a few hours off from your local time. If you're convinced you'll never change timezone you could change this, but it's hardly an inconvenience.

Let's make sure your local user has the requisite permissions to use zfs:

root@local$ zfs allow -u hugh send,diff,snapshot,mount,hold wd

Back to user level, and drum roll please...

hugh@local$ ssh knox-fifo < ~/.ssh/knox-geli-key &
hugh@local$ zfs send -Rev "wd@$snapname" | ssh knox-send

Be prepared to wait...On the Raspberry Pi I get about 2.5MB/s, which is several times faster than my home upload speed, so not really a problem. Here's the output from top while the system is receiving a snapshot over the local network, you can see the load is roughly split between geli and ssh, indicating that the RPi's processor isn't so snappy for cryptographic operations. Interestingly ZFS is using fairly little resources, and as you can see the system memory is pretty empty. Perhaps we needn't have been so concerned with saving every byte.

When it's done, your data will be safe, secure, and (soon to be) far away. Accessible to only someone with your SSH key and its password (or physical access) and readable only by someone with your geli key.

Incremental Backups

All that was a lot of work, but we can automate the rest with a simple script.

To take a snapshot at the current time:

hugh@local$ ~/ snapshot

To preview the changes between your latest snapshot and the latest one on the remote system:

hugh@local$ ~/ preview

To send those changes:

hugh@local$ ~/ backup

To snapshot and send without previewing:

hugh@local$ ~/ snapback

Do this once a day, week, whenever and your backups will always be fresh. Remember that ZFS snapshots are cheap (use 'zfs list -t snapshot' to admire them) so feel free to make many. You might even consider adding it to your crontab:

hugh@local$ crontab -e

59	17	*	*	*	~/ snapback

Note that the way we send the snapshots will remove any data on the remote pool that isn't on the local pool - so don't store anything there manually. Store it in the local pool and let it propagate automatically.

Also be aware, that there isn't currently support for resuming failed transfers, so they'll have to be restarted. With small, regular snapshots this shouldn't pose much of an issue, and it is an upcoming feature.

Here's the whole script, save it as ~/ on your local machine.

hugh@local$ cd
hugh@local$ fetch
hugh@local$ chmod 744

If you're still in the same shell that you ran the initial backup from, we can set the remote state file now.

hugh@local$ echo "wd@$snapname" > .knox-last-sent

Physically Placing Your Remote System

Now we can place the backup system in its permanent location. This is highly subjective but here are a few points to bear in mind:

  • Connect the system to the modem/gateway via Ethernet. Not Wi-Fi.
  • Try and place it somewhere out of the way, so it can gather dust in peace.
  • (If necessary, instruct the home-keeper not to dust it.)
  • Be respectful and use as few power sockets as possible. A simple doubler can help here.
  • Use an extension cable to minimise the visible wires. Out of sight, out of mind.
  • Expect the system to crash or loose power, set it up to recover without intervention.
  • Configure the BIOS to ignore all boot sources but the root when booting.
  • To power up again if it looses power.
  • To auto-power on at a certain time in case the above fails.
  • Give the system a DHCP MAC reservation on the gateway, or a static IP if not possible.
  • Edit /etc/rc.conf to set a static IP if you can't reserve an address through DHCP.
  • Set up port forwarding. Take port 22 if available, or map to port 22 and update your local ssh config file with Port details.
  • Set up a Dynamic DNS name if the modem has a dynamic external IP.

If it's not possible to use port forwarding, there are some alternatives for connecting to the system:

  • Tor hidden service and SSH via torsocks.
  • IPv6.
  • SSH wizardry involving autossh, proxy commands, and reverse tunnels. In this case your local system, or some mutually accessible proxy system must have a static IP.

A Tor hidden service is probably the most reliable fall back, but it will be slow and it's not really in the spirit of Tor to use it in this way. One especially nice feature is that you can leave your remote system deep within a large network (e.g. your employer or University (get permission)) and as long as Tor can get out, you can get in. No need for a direct connection to the modem or DHCP reservations. Personally, I like to have a hidden service running even if I have port-forwarding working, as a fall back measure. e.g. if the modem were replaced or factory-reset, I could tor in and with SSH forwarding connect to the modem's WebUI and set up forwarding again.

Once you're worked out your method, adjust your config file on your local machine to use the new Internet accessible name.

Host knox
	User root
	HostName # FQDN for Internet routing
	#HostName # Local address for initial backup
	#HostName http://idnxcnkne4qt76tg.onion/ # Tor hidden service

# Remember to adjust the knox-fifo and knox-send entries also.
# Or make an entry in /etc/hosts, but that's just another thing to manage.

Care & Feeding


From time to time connect into the remote system and check the system logs and messages for anything suspicious. Also consider updating any installed packages and keeping up to date with errata patches. pkg and freebsd-update make this easy (on Tier 1 platforms).

root@knox# less +G /var/log/messages
root@knox# pkg upgrade
root@knox# freebsd-update fetch install

You will need to reboot if freebsd-update makes any changes.

It's also sound practice to occasionally exercise the disks, both your local and the remote one with a scrub operation. The instructs ZFS to re-read every block on the disk and ensure that they checksum correctly. Any errors can be found will be logged and they probably signal that you should replace the disk.

hugh@local$ ssh knox-fifo < ~/.ssh/knox-geli-key &
hugh@local$ ssh knox-shell
root@knox$ geli attach -dpk /tmp/k
root@knox$ zpool scrub wd
root@knox$ sleep 60; zpool status
# A considerable time later...
root@knox# zpool status # is it done?
root@knox# zpool detach
# geli detaches automatically here
root@knox# rm -P /tmp/k

zpool status will give you some information about the speed of the operation and an estimated time to completion. Be aware that your drive is unlocked in this state.

Extra Encryption

If the thought of the decryption key for some sensitive data being automatically sent to a system outside of your immediate physical control concerns you, but you still want all the advantages of ZFS, you might consider adding an encrypted volume.

This is a ZFS powered virtual storage device that we can layer GELI encryption on top of, using completely different key material but still stored in the ZFS pool such that it can be snapshotted, have only its changes transferred on backup and have the benefit of strong data integrity.

root@local# mkdir /wd/vol; chown hugh /wd/vol
root@local# zfs create -s -V 100G wd/vol
root@local# geli init -s 4096 /dev/zvol/wd/vol
root@local# geli attach -d /dev/zvol/wd/vol
root@local# newfs -Ujn /dev/zvol/wd/vol.eli
root@local# mount /dev/zvol/wd/vol.eli /wd/vol

I suppose you could use ZFS instead of UFS on the new volume, if you have a totem, but it's probably more trouble than it's worth.

/wd/vol is now available for secure storage. The -s flag to zfs create indicates a sparse allocation; the system won't reserve the full 100GiB and won't allocate any more data than you actually write to it. While 100GiB is the most it can hold, you can use ZFS properties to increase the volume size and then growfs if you ever hit that limit (geli may need to be informed of the resize too).

When you've finished using the encrypted drive, unmount it. Remember not to have any shells active in the directory or they'll hold it open. Since we attached it with the -d flag, geli will automatically detach it.

root@local# umount /wd/vol

To mount the volume for further use:

root@local# geli attach -d /dev/zvol/wd/vol
root@local# mount /dev/zvol/wd/vol.eli /wd/vol

You may wish to define some shell functions (using sudo, or scripts with setuid) to handle the repetitive attaching and detaching. The contents of vol will be included in any snapshots and will be sent to the remote system during zfs send. I recommend having the volume unmounted and detached before snapshotting though.

Disaster Recovery

One day, one of the following things will happen:

  • A motor or sensor in your USB drive will fail.
  • The GoldenEye will be fired.
  • A power surge will blow the regulator on the old laptop.
  • Someone will knock the remote drive onto the ground while cleaning.
  • A drive head will turn masochistic.
  • Cosmic rays will flip your bits.
  • The drive containing the OS will fail.

In all but the last two cases, we must consider the drive totally lost. It might happen to your local drive first, because it experiences more activity than the backup. It might happen to the remote drive first, because it lives in a room without central heating and is subject to wider temperature fluctuations. No matter how it happens, it is going to happen.

What shall we do when it does? Simple. Buy a new drive. Set it up as above, create the first backup and then the incrementals as you've always done. Recycle the old drive. There's no need to worry about wiping it clean because everything is encrypted. It is enough to destroy whatever copies of the encryption key you have lying around.

I deleted a file by accident, can I recover it from a snapshot? Naturally, you don't even need to access the remote system, snapshots are accessible through the hidden .zfs directory at the root of your pool. e.g. /wd/.zfs/snapshot/2015-07-24T23:35:18Z

What if the backup computer dies, but the drive is okay? Recycle the computer. Buy/liberate another one, install FreeBSD as above, then just connect the drive and carry on.

What about slow, creeping drive death, as opposed to total failure? ZFS has your back. Take a look at 'zpool status' every now and then on both machines (the remote will have to be attached of course). If you see any checksum errors, buy a new disk. Every so often, run 'zpool scrub' on both disks to have ZFS read and verify every sector, then check the status and do what you need to do. Life is too short for bad hard disks, and 2TiB is a lot of data to loose.

My local disk failed, can I swap in my backup? Probably. Use geli to attach it locally (with the key) and then use 'zpool import'. Then buy a new drive and go through the motions again.

My local disk failed, but I can't physically access the remote one, what do I do? You've got your SSH and GELI keys backed up somewhere, right? Use those to access the remote machine and pull down any files you need (mount the datasets with 'zfs mount -a'). You could try a full backup onto a new disk over the Internet, but you'll be waiting a while, and your friendly server co-location administrators might be getting calls from their ISP. A better approach would be to buy a new drive, have it delivered to wherever the remote system lives and have someone connect it. Set it up as a second pool (use geli), do a local send/receive and once it holds a full copy politely ask that it be posted to you. Note: Some systems can't supply enough USB power for two high drain device like hard drives. If you're using a USB powered drive on the machine, connect the second drive through a powered hub or use one that has its own power.

Further Reading

I would first recommend the handbook sections on zpool and zfs, followed by their respective man pages. They're long, but you've already come this far. geli too, is required reading.

Here are some videos discussing ZFS:

Thank you for your attention, may you never need anything this guide helps prevent against. Please send details of any mistakes or areas for improvement to obrien.hugh at the Google mail system.

Appendix - Raspberry Pi

With some work, it's possible to use a Raspberry Pi Model B, and likely any other supported SBC, as the remote system. I'll take you through how to prepare a system image for this, but for more trusting readers I'll provide a pre-made image file.

The basic process is:

  • Download the source for our chosen branch.
  • Download the crochet image building tool.
  • Apply customisations.
  • Build image.
  • Customise built image.
  • Boot RPi.

I'm working with FreeBSD 10.2, which is the production branch at the time of writing. There are some pre-made images provided by the foundation but they're of 10.2-RELEASE, 10.2-STABLE has advanced a little since then. And since we're going to all the bother of building our own images, it makes sense to get the best available code. Also, rather crucially, these images do not support ZFS.

The FreeBSD Release Engineering process is worth reading up on. In a nutshell, most development occurs in current, also called head, which changes all the time, breaking and being repaired as new code is added. Every now and then the release team cleave off a section from current and call it a stable branch, such as 10. Code from current which is considered to be good stuff gets brought down into the latest stable branch. At some point the team branches off the stable branch to make a feature branch, such as 10.0. This branch stops getting the new toys added to the stable branch and the team focus on making everything in it work well together. When they're satisfied they tag a certain state of the feature branch as a release. Then they go to all the work of building the images and documentation for this release, make a big announcement, and we get to download and play with, for example, FreeBSD-10.0-RELEASE.

Of course, not everything is always rosy with a release, sometime minor bug fixes or security patches come out afterwards, known as errata. These make it into the feature branch, but since the release has already been tagged and distributed, it doesn't change. In an ideal world, we'd always run the most recent code from a feature branch. However this would mean each user would have to track the branch themselves and rebuild as necessary. Since most people use the RELEASE images (as recommended), the team also put out binary patches for the main supported architectures to allow people on a RELEASE to change just the necessary files, without compiling anything, bringing them to an equivalent state as if they were running a system compiled from the latest feature branch. This is provided by freebsd-update, for supported platforms.

I mention all of this, to answer the seemingly simple question, of what source branch should we download and compile for our Raspberry Pi? The Pi is an ARMv6 board, and thus isn't provided with binary updates. So if we want errata fixes, we have to get them ourselves. Here is the current list of branches:

  • head - no, too unstable.
  • stable/10 - currently working towards 10.3, so not quite stable.
  • releng/10.2 - the latest (at the time of writing) feature branch with all known errata applied.
  • releng/10.1 - an older 10 feature branch.
  • releng/10.0 - as above.
  • stable/9 - 9 isn't getting much love right now, but it's still supported.
  • releng/9.3 - the last feature branch for 9, will still get errata fixes if necessary.
  • ...
  • stable/8 - 8 is no longer supported, but it's still there for all the world to see. Doesn't support the RPi.
  • ...
  • stable/2.2 - FreeBSD is indeed old.

So, since we're building our own image, and compiling all our own code, we want the latest errata fixes from the latest feature branch. That's releng/10.2. Let's get the code. Depending on what version of FreeBSD you're currently running, you may already have an earlier version of this code in /usr/src, but it's cleaner if we grab a fresh copy.

hugh@local$ mkdir ~/knox
hugh@local$ svnlite co knox/src

svn is the short command name for Subversion, the source code management system the FreeBSD project uses 'co is shorthand for the checkout subcommand, which has a special meaning within Subversion, but for our purposes just think of it as download). However, Subversion is a somewhat large package, so in the name of minimalism, FreeBSD ships svnlite instead, which is just fine for our needs. The last argument is the destination folder.

This checkout process will take some time. On my system the process will occasionally fail due to network issues, leaving the source directory in an inconsistent state. If this happens, I recommend you delete the whole directory (rm -rf ~/knox/src) and try again.

If it succeeds, you'll get a message similar to:

Checked out revision 295681

Now we'll get the crochet build tool, which delightfully does almost all the hard work of image building for us. The package is maintained on GitHub, which is a little unusual for FreeBSD. If you have git, or indeed git-lite installed on your system you can get the code with:

hugh@local$ git clone
hugh@local$ cd crochet

If you don't have git, GitHub provide zipped archives:

hugh@local$ fetch
hugh@local$ unzip
hugh@local$ mv crochet-master crochet
hugh@local$ cd crochet

Crochet operates around a central build script, called There's a sample file in this directory, which I recommend you take a look at, but for expediency, simply create a new file in this directory called with the following contents:

board_setup RaspberryPi
option ImageSize 3900mb

I'm using a 4GB card, and leaving about 10% of the space unused so the internal chip can handle bad sectors more easily. The formula for ImageSize is n x 1024 x 0.9, where n is the number of Gigabytes on your card.

The KERNCONF is the specification of how to build the kernel for the Raspberry Pi. There's an existing config file in ~/knox/src/sys/arm/conf/RPI-B that I've modified as by default it doesn't come with, or support ZFS. Here are the modifications:

  • Change 'ident' line from RPI-B to RPI-B-ZFS.
  • Add 'options KSTACK_PAGES=6' as required for ZFS (it needs extra memory).
  • Add 'opensolaris' and 'zfs' to the MODULES_EXTRA makeoptions, to trigger the build of ZFS.

I've also removed the following modules, as I don't feel them necessary for this use case and as the RPi is so memory constrained, every byte helps. Some of these options are explained in more detail in the Developer's Handbook.

  • INET6 - Support for IPv6, you may want to leave this in.
  • SCTP - Stream Control Transmission Protocol, like an optimised TCP, not much use here.
  • UFS_DIRHASH - A speed/memory trade-off in the wrong direction for us.
  • QUOTA - Quota supports not relevant as we're the only human user of this system.
  • NFSCL - Network File System, no need for this at all.
  • NFSLOCKD - As above.
  • NFS_ROOT - As above.
  • KTRACE - Kernel debugging, not needed unless you're developing on this system.
  • KBD_INSTALL_CDEV - This system won't have a keyboard (KBD) so not necessary.
  • BREAK_TO_DEBUGGER - For developers.
  • ALT_BREAK_TO_DEBUGGER - For developers.
  • KDB - Debugging.
  • DDB - Debugging.
  • INVARIANTS - Kernel self-tests, they'll slow us down on an already slow system.
  • gpioled - Driver to control PWM on the RPi LEDs, not for us.
  • makeoptions DEBUG=-g - Don't build debug symbols, again, we're not developing on this.

This is, of course, somewhat cumbersome to do manually, so you can grab the config file directly from here:

hugh@$local$ cd ~/knox/src/sys/arm/conf
hugh@$local$ fetch

Verify that I'm not sneaking anything in with the following command:

hugh@local$ diff --side-by-side --suppress-common-lines RPI-B RPI-B-ZFS

There's one tweak to make to crochet before we build. Since we're being so security conscious, it make sense to use encrypted swap, on the off chance that some of our data might get paged out of memory. There's a possibility that the key-material might even be swapped out, so if it's going to be written to the SD card, let's make sure it's not readable afterwards.

To make it easy to enable encrypted swap, we're going to direct crochet to create an extra partition in the image. Edit the file ~/knox/crochet/board/RaspberryPi/ and find the function raspberry_pi_partition_image ( ). Above the line disk_ufs_create add disk_ufs_create 3000m. Here's a patch showing the change.

If you're using a card size other than 4GB you should tweak that 3000 figure, it's specifying the size of the root partition on the image. The next call to disk_ufs_create will use up all remaining space in the image, which for the 4GB case is about 700MB, plenty for swap. Bear in mind that this explicit specification of partition size will conflict with the Growfs option that crochet normally uses, though we've excluded it from our config file.

Just one last change, there's a DEFINE statement in the opensolaris code that causes some build issues, thankfully it's not needed so we can simply delete it. Edit the file ~/knox/src/sys/cddl/compat/opensolaris/sys/cpuvar.h and delete the line #define>cpu_id>-cpuid, it's on line 50 of the file at the time of writing. Here's the patch.

With all this done, we can kick off the build. It needs to run as root as it will mount and unmount some virtual file-systems as it goes. We also need the Raspberry Pi version of uboot installed, which will be automatically placed into the image.

root@local# pkg install u-boot-rpi
root@local# cd /home/hugh/knox/crochet
root@local# nice -n 20 ./ -c

This will take some time. nice puts the task at a lower priority so as not to interfere with the rest of the system.

Once it's finished, you'll have a 4GB image that's ready to be put on the SD card. But not so fast, we can do most of our post-installation configuration changes on the image itself, so that it boots up fully ready. To do this, we first mount the image as a memory device.

root@local# mdconfig work/FreeBSD-armv6-10.2-RPI-B-ZFS-295681M.img # note the name this returns, it will probably be md0.
root@local# mount /dev/md0s2a /mnt

Create the /mnt/boot/loader.conf file:


This causes the ZFS module to load with the kernel, sets the kernel memory size at a hard 180MB, which should be large enough for ZFS's needs, and restricts the size of the ARC, leaving more of that 180MB for the meat of ZFS.

Now edit /mnt/etc/fstab.

/dev/mmcsd0s1   /boot/msdos     msdosfs ro      0       0
/dev/mmcsd0s2a  /       ufs     ro      1       1
/dev/mmcsd0s3.eli       none    swap    sw      0       0
tmpfs   /tmp    tmpfs   rw,mode=1777    0       0
tmpfs   /var    tmpfs   rw      0       0

First thing to note here, is that we're using read-only mounts. The RPi will not make any writes to the file-system unless we explicitly mount it in read-write mode. The benefit of this, is that the system can always be reset to a stable state by switching it off and on again, and also that if it were to lose power during normal operation, it wouldn't find itself in an inconsistent state on reboot.

We're also directing the system to use the third partition (the one we edited the crochet setup file to create) as a swap device, but the addition of '.eli' causes it to be automatically encrypted with a one-time key at boot. Lastly, we're going to use a memory backed file system for the system's working directories. This means they're cleared on every reboot, and won't end up filling up the disk if they grow too much. Since we've added a swap partition of about 700MB, the contents of these memory disks are easily swapped out (unlike the kernel), so we're not likely to hit memory issues. A good trade off I think.

Now here's /mnt/etc/rc.conf:



powerd_flags="-M 800 -m 300 -n hiadaptive -p 3000"



Transatlantic sorts may wish to change the keymap to 'us'. Details of these choices are presented further up in this document, the only difference being that I also disable cron here.

We'll only ever access this system remotely, so it makes no sense for it to have terminal emulators hanging around in the background, this will also make local attacks more difficult. Replace mnt/etc/ttys with an empty file:

root@local# echo > /mnt/etc/ttys

Here's /mnt/etc/ssh/sshd_config, this is detailed earlier in the document.

HostKey /etc/ssh/ssh_host_ed25519_key
TrustedUserCAKeys /etc/ssh/knox-ca
AllowUsers root
PasswordAuthentication no
PermitRootLogin without-password
UseDNS no
UsePAM no
ChallengeResponseAuthentication no

Now some SSH tasks. Install the fingerprint of the signing key, and generate the host key for the device while we're at it.

hugh@local$ ssh-keygen -t ed25519 -f ~/.ssh/knox-ca
root@local# cp /usr/home/hugh/.ssh/ /mnt/etc/ssh/knox-ca
root@local# ssh-keygen -t ed25519 -f /mnt/etc/ssh/ssh_host_ed25519_key # press <enter> when prompted for a passphrase

Note the key fingerprint generated from the above. Lastly, we should set a nameserver and add some entropy.

root@local# echo "nameserver" > /etc/resolv.conf
root@local# dd if=/dev/random of=/mnt/entropy bs=4k count=1

Last thing is to put the zfs-receive script into the root user's home folder. Edit /mnt/root/


geli attach -dpk /tmp/k /dev/da0
zpool import wd
zfs receive -Fu wd
zpool export wd

Then set the permissions:

root@local# chmod 744 /mnt/root/

All done. Let's unmount and write the image. Insert the SD card into the building system and take a look at 'dmesg | tail' to see what device name it gets. Mine is mmcsd0.

root@local# umount /mnt
root@local# mdconfig -du 0 # where 0 is from the name it gave you, here md0
root@local# dd if=/home/hugh/knox/crochet/work/FreeBSD-armv6-10.2-RPI-B-ZFS-295681M.img of=/dev/mmcsd0 bs=1m
root@local# sync

You can check the progress of the dd operation by typing ctrl-t. When it's done, put the SD card in your RPi and boot it up. To connect, we'll need to find out what IP address it's been assigned. Sometimes home routers have a 'connected devices' page that shows the active DHCP leases, if not we can do a quick scan for open SSH ports on the local network. (You may need to install nmap).

hugh@local$ nmap -Pn -p ssh --open <your local network>/<your CIDR mask>  # probably

Once you've found the new addition, connect in using a key signed by the knox-ca key, as detailed in the main section. Then, go back to the start of this guide, filling in all the blanks.

One last thing, because ARMv6 isn't a Tier 1 supported architecture, there aren't any binary packages provided by the FreeBSD Foundation. Thankfully, FreeBSD is all about the source code, and the famous Ports tree makes it easy to compile your own packages for whatever architecture you have a compiler for. Unfortunately...the RPi is very slow at compiling packages. Being a patient man, I've compiled a few myself that I find useful to use on this system, but I stress that none of these are necessary for the ZFS backup features - the base system has everything needed for that. RPi packages are available here. If you do decide to build some ports, bear in mind that ports tree from portsnap is approximately 900MB in size, before you begin to compile anything. Poudriere is an alternative that makes cross-compilation (building on your local machine for the RPi) easier, but I found it as easy to just wait for the RPi.

To use these packages, grab the pkg-static binary from the folder and use that to install pkg properly.

root@knox# ./pkg-static add pkg-1.6.3.txz

I should also note, that much to my surprise, my simple 1A USB power supply is able to both power the RPi, and the 2TB USB powered drive I attached to it, no powered hub needed - though this may be putting some strain on the RPi's linear regulators. To my further surprise, the choice of USB cable that connects the power supply to the RPi is also significant, lower quality ones seem less able to carry higher amounts of current.

Congratulations on making it to the end, as a reward, here's a pre-made RPi image file containing almost all of the above modifications. You'll have to install your own CA key, but otherwise it should speed things up quite a bit. Why didn't I mention this earlier? Think how much more you now know! Though this is sized for 4GB cards, it will also work with larger ones, but the extra space won't accessible.

It's easiest to flash the image directly, then connect in and make the necessary changes, you should also verify the checksum matches the one shown below.

hugh@local$ prefix=""
hugh@local$ fetch $prefix/FreeBSD-armv6-10.2-RPI-B-ZFS-295681M.img.xz
hugh@local$ sha256 FreeBSD-armv6-10.2-RPI-B-ZFS-295681M.img.xz
SHA256 (FreeBSD-armv6-10.2-RPI-B-ZFS-295681M.img.xz) = 338490fb8985d0778a5de743b1e8f4b5cac9b30ba3b398e6ffa49937c8a56137
root@local# xzcat FreeBSD-armv6-10.2-RPI-B-ZFS-295681M.img.xz | dd of=/dev/mmcsd0 bs=1m
# transfer to RPi, give it about 30 seconds to boot.
hugh@local$ fetch $prefix/keys/knox-login 
hugh@local$ fetch $prefix/keys/
hugh@local$ chmod 600 knox-login
hugh@local$ ssh -i knox-login root@ # replace with your assigned IP

ED25519 key fingerprint will be 8f:a5:94:2e:b5:a2:97:93:ed:71:2c:67:de:a1:32:9c

Now that you're in, you should replace the ssh host key and the ssh certificate authority key with your own (or else I'll be able to login). These were explained above. You'll have to mount the root file-system as read-write beforehand though:

root@knox# mount -o rw /
root@knox# cd /etc/ssh/
root@knox# rm ssh_host_ed25519_key
root@knox# ssh-keygen -t ed25519 -f ssh_host_ed25519_key # don't use any password
# replace /etc/ssh/knox-ca with the public key of your chosen CA key
root@knox# tzsetup # might as well set the timezone now
# maybe edit /etc/rc.conf as you desire, e.g. the keymap
root@knox# mkdir /wd # or whatever you're going to call your pool
root@knox# echo "nameserver" > /etc/resolv.conf # Select your nameserver
root@knox reboot

Now create and sign a login key as described in the main guide. When the system is back up and you attempt to reconnect, you'll get an SSH error about 'remote host identification changed', as indeed it has. Edit your ~/.ssh/known_hosts file to remove the offending entry (probably the last line in the file).

With that done, go back to the start of this guide and fill in any missing steps.

As a final note, it's common to overclock the Raspberry Pi using the /boot/msdos/config.txt file, simply add the following lines for a moderate speed increase: (More info here.)


However, any overclocking increases the potential for errors or system faults. Increasing the chip frequencies also increases the power draw which puts extra load on the power supply, which in my case is also powering the USB HDD. The benefit of the overclocking is about a 25% CPU boost, the drawback is considerable. I suggest you play it safe and ignore the temptation.

In the config.txt file there's also a line to adjust the memory split between the on-board GPU and the CPU. u-boot will default to 32MB, which I suggest you leave it alone. Lowering to 16M in my experience caused boot issues.

Appendix - ECC Memory

It's been pointed out that if ZFS detects a bad checksum while reading, it will always mark the disk as the problem, even if the real fault is a bad memory module. If ZFS then attempts to correct it, based on some online redundancy, that correction too may pass through the bad memory thus actually corrupting it.

A scrub operation, which is normally used to check the disks, might end up funnelling all the data through a bad memory module. However, there are a few factors that mitigate this potential disaster:

  • ZFS will take the pool offline if it detects too may errors, thus reducing the fallout.
  • ZFS will only attempt to auto-correct the data if it has some redundancy information such as provided by the copies parameter or by parity data. We use neither in this setup, though you may optionally enable copies on a per-dataset basis.

Here is a paper which analysed the result of many different types of memory corruption on a running ZFS system. The general conclusion is that while ZFS is not immune to memory issues, more often than not it will crash when it encounters them. Without performing a similar analysis for simpler file-systems, we cannot definitively say whether ZFS handles memory issues better or worse than its contemporaries.

As a general rule, use ECC memory if possible. Though I suspect that if your data were so critical as to require it, you would be using a more dedicated, specialised backup solution.

Here's some more debate on the matter:

In short, since this is a remote backup drive, we will always have two copies of our data. Should the backup system fail for any reason, memory or otherwise, we can easily get another one and recreate the backup. It is always better to have two independent copies of your data on systems without ECC memory than one copy on a system with it. And since non-ECC systems are easier to come by, I think it's the better move.


A detailed tutorial on setting up a remote ZFS mirror for home use.



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