Homelast changed: 2020-11-04

Cross-building time capsule binaries to backup NetBSD


  • this is a followup to a previous post about Backup and Restore on NetBSD
  • it aims at using dump and restore as rescue tools provided by unmodified NetBSD installation media
  • this can be used with an Apple Time Capsule with a few preparations
  • the time capsule needs ssh access and
  • it needs a rmt(8) binary


As mentioned in a previous post, I'm using my time capsule (also) as a backup system for my NetBSD/macppc server. For this, I'm usually mounting the remote file system via mount_afp, then using the dump and restore tools to dump backups onto it. This works so far, but when booting into the ram disk of an installation kernel, the mount_afp command (provided by pkgsrc) is missing. So I was thinking of cross-compiling the tools needed on the time capsule to provide an easy access from a NetBSD rescue system. The restore(8) tool has the builtin ability to access remote hosts using rcmd(3) and rmt(8). For this, ssh can be used as remote command execution facility; rmt is used for an efficient random access to the backup dumps.

Activating ssh on the time capsule

First of all, we want to be able to ssh into the time capsule. The sshd is already in place, as the time capsule actually is a stripped down NetBSD system. The ssh access is disabled by default, but can be reactivated using acp (airport control protocol?).

Install airpyrt-tools by Vince Cali:

git clone https://github.com/x56/airpyrt-tools.git
python airpyrt-tools/setup.py install --user

Then set the dbug property and reboot the time capsule:

python -m acp -t {hostname} -p {password} --setprop dbug 0x3000
python -m acp -t 1 {hostname} -p {password} --reboot

Now, we can ssh into it as root, using the admin password. Afterwards, you may put your public key into /root/.ssh/autorized_keys, this way allowing automatic backups to access the system. But you should probably not deactivate password authentication. If you need to restore your backup'd system, you probably won't get in to the backup unless the key is restored…

Building rmt for the time capsule

Next, as it shows, the rmt tool, used for remote media access by dump and restore, is missing on the time capsule's reduced NetBSD installation. So, one might ask, where do I get this from? And the answer is of course, build it yourself. Using the mostly automated cross building system of NetBSD, this is quite an easy task. On my time capsule, the system identifies as NetBSD 6.0 evbarm. The binaries found on the system are built as ELF 32-bit LSB executables, ARM, EABI4, statically linked. So I started building a cross compilation environment on my NetBSD 9.0 box, hoping a statically linked binary built in this environment would still work on the older time capsule. While this took quite a little bit of time on my trusty PPC machine, it worked straight out of the box and was probably more easy than to setup a virtualized build service on x86.

These commands did it:

cd /usr/src
./build.sh -U -O ~/arm-build -m evbarm -a earmv4 tools
./build.sh -U -u -O ~/arm-build -m evbarm -a earmv4 distribution

Now the rmt tool can be built as a statically linked program:

export LDSTATIC=-static
cd /usr/src/usr.sbin/rmt

Finally, we transfer the binary over to the time capsule, put it into /usr/sbin and create a symlink to /etc/rmt, where dump and restore expect to find it.

cd ~/arm-build/usr.sbin/rmt/
scp rmt time-capsule:/usr/sbin/rmt
ssh time-capsule
cd /usr/sbin; chmod 555 rmt; ln -s /usr/sbin/rmt /etc/rmt


Now, we can directly address the remote disks mounted on the time capsule via the NetBSD onboard backup tools dump and restore. To do this, we configure rcmd(3) to use ssh:

export RCMD_CMD=ssh

Then we use dump(8) or restore(8) with option -f and give a filename in the form of {hostname}:{path}, where {hostname} is our time capsule and {path} addresses the backup directory over there. Here it is something like /Volumes/dk2/ShareRoot/nbsd-backup.

Once again, this can also be achieved by piping the data via ssh to dd, which works flawlessly in most cases. But if you need random access to different parts of the backup, e.g. when doing an interactive restore, it doesn't work so well, especially with very large backup files.

For example, to interactively browse such a backup, now you can just use something like this:

env RCMD_CMD=ssh restore -if time-capsule:/Volumes/dk2/ShareRoot/nbsd-backup/dump.2

Mounting and accessing the backup devices on the time capsule

Stays the question how to mount or access the disks on the time capsule if they are spun down. At the moment, I've no idea howo to get this done. So for now I will stay at my approach using afp to remotely access the time capsule for automatic nightly backups, as described in Backup and Restore on NetBSD. To access the backups via restore/rmt, now I first have to access the drives via AFP or SMB from another computer in order to get them mounted.

But at least I can rest assured I will reach the backup via restore from an unmodified default NetBSD installation set, if I need to restore to a new disk.

Nevertheless, if someone has an idea how to spin up and mount the time capsule disks via command line, I would be grateful if you leave a comment on Reddit.

Author: Jörg Kollmann (Reddit: u/e17i)

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