Flash a lite version of Raspbian or similar onto a micro-SD card. The default user for Raspbian is pi, which can sudo the necessary privileged commands to install software, etc.

Ubuntu minimal seemed to be the recommeded lightweight OS for ODROID when I first got it. Flash the image onto a micro-SD card or eMMC chip, and that should give you a default root account. Being minimal, it's convenient to install a few more basic packages, starting with a language pack to silence a few warnings:

apt install language-pack-en

Also set up timezone and keyboard. These are interactive commands, so run them one at a time:

dpkg-reconfigure tzdata
dpkg-reconfigure keyboard-configuration

Make sure you have the latest of everything. You might have to repeat the following commands a few times to get to the latest versions:

apt update
apt upgrade
reboot
apt install linux-image-xu3
reboot

Make sure that uname -r reports 4.14.3-85+ or later to cope with a USB SSD being plugged in during boot.

Get rid of anything superfluous if you like:

apt autoremove

Create an ordinary account that can sudo, set its password, and disable password access on root. Some of these commands are also interactive:

useradd -s /bin/bash -G sudo -m -d /home/pi pi
passwd pi
passwd -l root

So that the instructions below remain uniform, the commands above call the new account pi, but you could name it how you like.

These are x86-based mini PCs, so install a regular 64-bit Ubuntu server or similar on them in the usual way. You usually get a choice of privileged account on these systems, but the instructions below will still assume you've chosen pi for this, which is what you get by default on Raspbian.

On each server, set the hostname the same, e.g., to media-centre. This should work on Raspbian and Ubuntu:

sudo hostnamectl set-hostname media-centre

On Raspbian, you could alternatively use an option in raspi-config:

sudo raspi-config

These instructions assume that all sites use the same local DNS domain, e.g., home. This is useful in that each server fulfils the same role at each site, so your device will find it under the same name (media-centre.home in this case) as you move from site to site.

If you want the host to have a different name (maybe you're re-tasking an existing machine), you could still create a DNS alias media-centre for it. Ideally, you'd configure your DNS server to do this, which is often integrated with DHCP, and both often integrated into your home gateway. If there seems to be no way to do this, but the DHCP server nevertheless honours hostname suggestions made by DHCP clients, you can trick it into giving the host's interface two addresses. See Two logical interfaces on one physical, on Ubuntu 18.04 without Netplan for details.

Get the basic stuff:

sudo apt-get install bash-completion ntp ntfs-3g exiv2 mediainfo inotify-tools pv bc minidlna fuse-convmvfs git subversion rsync imagemagick par gawk jmtpfs udisks2

sudo apt-get -y install libusb-1.0-0-dev
curl -o /tmp/libmtp-1.1.18.tar.gz 'https://deac-ams.dl.sourceforge.net/project/libmtp/libmtp/1.1.18/libmtp-1.1.18.tar.gz'
cd /tmp
tar xzf libmtp-1.1.18.tar.gz
cd libmtp-1.1.18
./configure
make
sudo make install
sudo ldconfig

A speech synthesizer is useful for automatic media import:

sudo apt-get install espeak-ng mbrola-en1 mbrola-us{1,2,3}

Don't worry if you can't get some of the mbrola packages on Raspbian. It will still work, but I find the extra voices a little easier to follow.

If you have a machine which seems to be out-of-sync a lot when it boots up, enable synchronization with NTP at boot:

sudo timedatectl set-ntp true

To check whether this setting is enabled, look for NTP enabled in the status:

$ timedatectl status
      Local time: Mon 2018-01-01 10:27:48 GMT
  Universal time: Mon 2018-01-01 10:27:48 UTC
        RTC time: n/a
       Time zone: Europe/London (GMT, +0000)
     NTP enabled: yes
NTP synchronized: yes
 RTC in local TZ: no
      DST active: no
 Last DST change: DST ended at
                  Sun 2017-10-29 01:59:59 BST
                  Sun 2017-10-29 01:00:00 GMT
 Next DST change: DST begins (the clock jumps one hour forward) at
                  Sun 2018-03-25 00:59:59 GMT
                  Sun 2018-03-25 02:00:00 BST

It can also appear as:

systemd-timesyncd.service active: yes

If you find you have to build FFmpeg:

sudo apt-get install build-essential pkg-config nasm yasm cmake libfreetype6-dev libtool autoconf zlib1g-dev libvpx-dev libxvidcore-dev libswscale-dev libvorbis-dev libtheora-dev libmp3lame-dev libomxil-bellagio-dev libopus-dev libass-dev libx265-dev

sudo apt-get install libx264-dev

mkdir -p ~pi/works
cd ~pi/works
git clone --depth 1 https://code.videolan.org/videolan/x264.git
cd x264
./configure --enable-static --enable-pic
make -j 4
sudo make install

mkdir -p ~pi/works
cd ~pi/works
git clone --depth 1 git://git.videolan.org/x264
cd x264
./configure --host=arm-unknown-linux-gnueabi --enable-static --disable-opencl
make -j 4
sudo make install

mkdir -p ~pi/works
cd ~pi/works
git clone --depth 1 git://github.com/mstorsjo/fdk-aac.git
cd fdk-aac
autoreconf -fiv
./configure --disable-shared
make
sudo make install

mkdir -p ~pi/works
cd ~pi/works
git clone --depth=1 git://source.ffmpeg.org/ffmpeg.git
cd ffmpeg
./configure --enable-gpl --enable-libx264 --enable-libx265 --enable-nonfree --enable-libfdk-aac --enable-libmp3lame --enable-omx --enable-libvorbis --enable-libxvid --enable-libtheora --enable-libass --enable-libfreetype --enable-libopus --enable-libvpx --arch=armhf --target-os=linux --enable-omx-rpi --extra-libs="-lpthread -lm" --pkg-config-flags="--static"
make -j 4
sudo make install

sudo apt install build-essential pkg-config libtool autoconf libchromaprint-dev frei0r-plugins-dev gnutls-dev ladspa-sdk libaom-dev liblilv-dev libavc1394-dev libiec61883-dev libass-dev libbluray-dev libbs2b-dev libcaca-dev libcodec2-dev libdc1394-22-dev libdrm-dev flite1-dev libgme-dev libgsm1-dev libmp3lame-dev libopenjp2-7-dev libopenmpt-dev libopus-dev libpulse-dev librsvg2-dev librubberband-dev libshine-dev libsnappy-dev libsoxr-dev libssh-dev libspeex-dev libtheora-dev libtwolame-dev libvidstab-dev libvpx-dev libwavpack-dev libwebp-dev libx264-dev libx265-dev libxvidcore-dev libzmq3-dev libzvbi-dev libopenal-dev libjack-dev libcdio-paranoia-dev libsdl2-dev
mkdir -p ~pi/works
cd ~pi/works
git clone --depth=1 git://source.ffmpeg.org/ffmpeg.git
cd ffmpeg
./configure --extra-version='1+rpt1~deb10u1' --toolchain=hardened --libdir=/usr/lib/arm-linux-gnueabihf --incdir=/usr/include/arm-linux-gnueabihf --arch=arm --enable-gpl --disable-stripping --enable-avresample --disable-filter=resample --enable-avisynth --enable-gnutls --enable-ladspa --enable-libaom --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --enable-libcdio --enable-libcodec2 --enable-libflite --enable-libfontconfig --enable-libfreetype --enable-libfribidi --enable-libgme --enable-libgsm --enable-libjack --enable-libmp3lame--enable-libopenjpeg --enable-libopenmpt --enable-libopus --enable-libpulse --enable-librsvg --enable-librubberband --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libspeex --enable-libssh --enable-libtheora --enable-libtwolame --enable-libvidstab --enable-libvorbis --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx265 --enable-libxml2 --enable-libxvid --enable-libzmq --enable-libzvbi --enable-lv2 --enable-omx --enable-openal --enable-sdl2 --enable-omx-rpi --enable-mmal --enable-libdc1394 --enable-libdrm --enable-libiec61883 --enable-chromaprint --enable-frei0r --enable-libx264 --enable-shared --enable-nonfree --enable-libfdk-aac
make -j 4
sudo make install

Ensure your sites have either fixed IPs, DNS names, or dynamic DNS names if your IPs are not stable. You might have to register with a dynamic DNS provider; most support a small number of free hostnames. When you have the account credentials, add them to the Dynamic DNS settings of your routers to ensure they are updated whenever your sites' IP addresses change. Check to see which providers your router supports before you register. Some sites also provide a client you can run on Linux, so support doesn't depend on your router's capabilities, though I prefer to get my router to be the client, as it reduces the number of points of failure.

Also configure your routers to port-forward SSH onto your RPis, etc. You don't have to use the default SSH port as the external, and I will use port 9191 in the configurations below. The internal port will always be 22, the default for SSH. Only TCP is used, not UDP.

Create a less privileged user library to own and manipulate bulk data:

sudo useradd -N -s /bin/bash -g minidlna -G audio -m -d /home/library library

Set up passwordless SSH access from this account to its counterparts on other hosts (example):

sudo -i -u library
ssh-keygen -t ecdsa

When prompted, use an empty passphrase.

Set up aliases to the corresponding accounts at the other sites:

# In ~library/.ssh/config of (say) site3
ServerAliveInterval 300

Host site1
User library
Hostname site1.mydyndns.domain
Port 9191

Host site2
User library
Hostname site2.mydyndns.domain
Port 9191

(The ServerAliveInterval setting might help prevent long-lived SSH connections through firewalls from terminating before synchronization has completed.)

Copy ~library/.ssh/id_ecdsa.pub from each RPi to the others, appending its contents to the file ~library/.ssh/authorized_keys. Use ssh-copy-id to do this:

ssh-copy-id site1
ssh-copy-id site2

Test that access is available between sites:

# e.g., from site3
ssh site1 echo yes

On the first instance of calling ssh or ssh-copy-id, you might be asked to accept the remote site's public host key. Once you've done this, you shouldn't be asked again. Try again, and if you get a yes without prompting, you're good to go. If not, there's still something wrong with your SSH set-up.

sudo tee /var/lib/polkit-1/localauthority/50-local.d/uk.ac.lancs.snapjib.library.pkla > /dev/null <<EOF
[Allow legacy Snapjib to access removable media for auto-upload]
Identity=unix-user:library
Action=org.freedesktop.udisks2.filesystem-mount-other-seat;org.freedesktop.udisks2.eject-media-other-seat
ResultAny=yes
ResultActive=yes
ResultInactive=yes
EOF

sudo tee /etc/sudoers.d/99-snapjib-library-legacy > /dev/null <<EOF
library ALL = (root) NOPASSWD: /usr/bin/lsusb
library ALL = (root) NOPASSWD: /usr/bin/jmtpfs
library ALL = (root) NOPASSWD: /usr/bin/fusermount
EOF

Create a location for bulk storage:

sudo mkdir -p /var/media{,-monitored} 

If you intend to keep the bulk data on a separate storage device, set it to be mounted as appropriate. For example:

# In /etc/fstab:
UUID=xxxxxxxxxxxxxxxx /var/media ntfs defaults,auto,nouser,idmap=none,uid=library,gid=minidlna,dmask=022,fmask=133 0 0

(I reformatted my SSDs as NTFS so they would support hard links.)

You can get the UUID with sudo blkid when the device is connected. You could alternatively give the device a volume label, and then reference it as /dev/disk/by-label/your-label.

Mount immediately (or reboot):

sudo mount /var/media

Set /var/media-monitored as a mirror of /var/media:

# In /etc/fstab:
convmvfs /var/media-monitored fuse auto,nouser,srcdir=/var/media,icharset=UTF-8,ocharset=UTF-8,allow_other 0 0

Mount immediately (or reboot):

sudo mount /var/media-monitored

Support more directory watches with inotifywait:

# In /etc/sysctl.d/local.conf (RPi)
# or in /etc/sysctl.d/60-snapjib.conf (ODROID)
fs.inotify.max_user_watches=524288

To apply at once without waiting to reboot:

sudo sysctl fs.inotify.max_user_watches=524288

…or perhaps a more general:

sudo service procps start

Merge these with the defaults:

# In /etc/minidlna.conf
media_dir=PV,/var/media/Exposed/Gallery
media_dir=A,/var/media/Exposed/Music
root_container=B
friendly_name=My Media Centre
inotify=yes

Of course, you can give it a different friendly name, but it's helpful if all servers use the same name, so they will appear as the same device whichever site you're at.

Apply changes immediately, or reboot:

sudo service minidlna restart

Get Binodeps installed:

mkdir -p ~pi/works
cd ~pi/works
svn co http://scc-forge.lancaster.ac.uk/svn-repos/misc/binodeps/branches/stable/ binodeps
cd binodeps
make
sudo make install

Get the source:

mkdir -p ~pi/works
cd ~pi/works
svn co http://scc-forge.lancaster.ac.uk/svn-repos/ss/snapjib/branches/stable/ snapjib
cd snapjib
cat > snapjib-env.mk <<EOF
CFLAGS += -O2 -g
CFLAGS += -std=gnu11
CPPFLAGS += -D_XOPEN_SOURCE=600
CPPFLAGS += -D_GNU_SOURCE=1
CPPFLAGS += -pedantic -Wall -W -Wno-unused-parameter
CPPFLAGS += -Wno-missing-field-initializers
CXXFLAGS += -O2 -g
CXXFLAGS += -std=gnu++11
CPPFLAGS += -D_FILE_OFFSET_BITS=64
EOF
make
sudo make install

You only need the _FILE_OFFSET_BITS setting for OSes without LFS on by default. I had to use it for Raspbian Buster.

Configure Snapjib:

# In ~library/.config/snapjib/config.sh
snapjib_home=/var/media-monitored
snapjib_raw=/var/media
prepdir=/var/media-monitored/Exposed/Gallery/unsorted

logdir=$HOME/.cache/snapjib

sites+=(site2)
sites+=(site3)

speaker=(espeak-ng -p 99 -s 120 -v mb/mb-us1 -d plughw:1,0)

Drop the speaker setting if you're not doing automatic import, or you don't have a speaker. Replace it with a different command if you have a different means of getting message to the user, like a small screen. The message will be appended as a single argument.

You can define more sophisticated remote configuration:

sites+=(parents)
sites+=(brother)

site_host[parents]=millwell.hopto.org
site_host[brother]=foobar.hopto.org

site_host sets the SSH host name (or configuration label). site_port and site_user set other SSH parameters. site_conf sets the remote configuration file, and defaults to .config/snapjib/config.sh. User-defined keys like parents and brother now only identify entries in these arrays, or index into a cache of timestamps of sites that Snapjib synchronized with.

$ snapjib-scan
mtp:04e8-XXXXXXXXXXXXXXXXXX          usb:001:009 Samsung Electronics Co., Ltd Galaxy (MTP)

#!/bin/bash

function recognize_device () {
    local devid="$1" ; shift
    declare -n intentvar="$1" ; shift
    declare -n namevar="$1" ; shift
    declare -n langvar="$1" ; shift

    case "$devid" in
        (mtp:04e8-XXXXXXXXXXXXXXXXXX)
            intentvar=scan
            namevar="John's phone"
            langvar=en
            return
            ;;
    esac

    return 1
}

function device_sources () {
    local devid="$1" ; shift
    declare -n dirsvar="$1" ; shift
    declare -n patsvar="$1" ; shift
    declare -n tagsvar="$1" ; shift

    case "$devid" in
        (mtp:04e8-XXXXXXXXXXXXXXXXXX)
            dirsvar+=(Card/DCIM/Camera)
            patsvar+=('[0-9]{8}_[0-9]{6}\.(mp4|jpg)$')
            tagsvar+=('')
            return
            ;;
    esac

    return 1
}

            dirsvar+=(Card/DCIM/Camera)
            patsvar+=('[0-9]{8}_[0-9]{6}\.(mp4|jpg)$')
            tagsvar+=('')

crontab -e

PATH=/usr/local/bin:/usr/bin:/bin

## Detect file movement in the presentation directory,
*/1 * * * * snapjib-monitor -q

## Normalize files deposited in the ingest directory.
*/5 * * * * snapjib-ingester -w 30 -q

## Move files off plugged-in devices into the ingest directory.
*/10 * * * * snapjib-importer -w 60 -q

## At 1:30am, ensure all files have hard links in the hash table.
30 1 * * * snapjib-unify -q

## At 2am, fetch files from other sites.
0 2 * * * snapjib-sync -q

## At 5am, clean out files that have moved or been removed.
0 5 * * * snapjib-purge -q

Append the client user's public key (in ~/.ssh/id_ecdsa.pub, for example) to ~library/.ssh/authorized_keys on each RPi. Use SSHFS to access the directory /var/media-monitored/.

Good security in NFS is probably not simple, but if you don't mind anyone on your home network being able to see all your media, a read-only NFS export is convenient. Beware of using NFS from laptops, as some applications lock up when trying to save a file if an automounted NFS resource is unavailable due to not being at home, even if you're not trying to save to it.

Install the NFS server:

sudo apt-get install nfs-kernel-server

# In /etc/exports
/var/media/Exposed *(ro,sync,no_subtree_check,no_root_squash,insecure)

Apply this change (or reboot):

sudo exportfs -a

Windows machines should most easily have access through a Samba share:

sudo apt-get install samba

# In /etc/samba/smb.conf
[media]
   comment = My Media Centre
   path=/var/media-monitored
   browseable=Yes
   writeable=Yes
   public=no
   only guest=no
   create mask=0644
   directory mask=0755

Set a CIFS password for the library account:

sudo smbpasswd -a library

Reload the configuration (or reboot):

sudo service samba reload

Or maybe:

sudo service smbd reload

The media are now accessible under \\media-centre.home\media.

Use whatever software you have for accessing remote Windows shares to open \\media-centre.home\media. Use library as the username, and the password as specified by the smbpasswd command mentioned earlier.

sudo apt-get install sshfs
sudo mkdir -p /var/media

Get the host keys for each site (in /etc/ssh/ssh_host_ecdsa_key.pub, for example), and make them known to your client machine, perhaps by adding them to /etc/ssh/ssh_known_hosts, so it looks something like this:

# In /etc/ssh/ssh_known_hosts on a client machine

# Site 1 key
media-centre.home ecdsa-sha2-nistp256 AAAA....

# Site 2 key
media-centre.home ecdsa-sha2-nistp256 AAAA....

# Site 3 key
media-centre.home ecdsa-sha2-nistp256 AAAA....

So far, this seems to be the most robust way of getting the client to recognize the same hostname against multiple keys. Let me know if you have a better suggestion.

Lazily mount whichever device is local:

# In /etc/fstab
library@media-centre.home:/var/media-monitored /var/media fuse.sshfs defaults,allow_other,idmap=none,uid=USERNAME,gid=GROUPNAME,noauto,x-systemd.automount,_netdev,reconnect,CheckHostIp=no,IdentityFile=/home/USERNAME/.ssh/id_ecdsa 0 0

The lazy mount should prevent your client device from spending a long time booting up when no media centre is available, but let you access the media centre without doing anything more than opening up a folder (say).

As configured, you should find your media in /var/media/Exposed. Drop new photos and movies into /var/media/Incoming to start the ingest process, leaving the converted files in /var/media/Preparation.

On the client machine:

mkdir -p /var/media

Lazily mount:

# In /etc/fstab
media-centre.home:/var/media/Exposed /var/media nfs ro,noauto,nouser,x-systemd.automount,x-systemd.device-timeout=10,timeo=14,_netdev,soft 0 0

On Linux, djmount acts as a DLNA client, and presents the media as a read-only filesystem.

sudo mkdir -p /var/upnp
sudo apt-get install djmount

# In /etc/fstab
djmount /var/upnp fuse ro,auto,allow_other,exec,sloppy,_netdev 0 0

At any one of your home sites, you should be able to read your media in /var/upnp/My Media Centre.

Snapjib works in conjunction with or makes use of several tools available on Raspbian Jessie Lite and Ubuntu (and surely other distributions), plus some you'll have to install yourself. Here are some of the media-related tools:

• MiniDLNA/ReadyMedia — This makes media arranged in a file system available to devices on the LAN, e.g., TVs and tablets, over uPnP /DLNA.
• ImageMagick — This is used to manipulate images, including auto-orientation of still images.
• FFmpeg — This converts videos from a camera's native format to something that should be universally accepted.
• exiv2 and mediainfo — These extract metadata from stills and videos, mainly dates, in order to choose a sensible name for ingested files.

Snapjib uses inotifywait to watch an ingest directory $incdir, and detect when the user has added files and directories to it. It looks for events MOVED_TO and CLOSE_WRITE, and uses the format %e X%w%fX so that comma-separated events are listed first, followed by the full path of the file pertaining to the event until the end of the line, so that spaces may appear in and around filenames. The output of inotifywait is passed through pv -q -B 1M as a buffer of 1 mebibyte, which is supposed to prevent events being dropped while Snapjib is busy with another event, although I still have some doubts about it.

When a new file is detected in $incdir, it undergoes a normalization process according to its suffix:

• Images that have an orientation other than 1 are passed through mogrify to give them the correct orientation, and saved in a hidden directory $convdir; other images are simply moved there.

• Videos are first passed through ffmpeg, which improves compression by making use of B frames, auto-orients, and converts to the most portable codecs I could determine (H.264 and LC-AAC). -movflags +faststart is used to place the MP4 navigational data at the start of the file. A bespoke C program is used to set the alternative groups and a few other odd bits in the MP4 file. The final output is deposited in $convdir. (The intermediate files are also placed in $convdir under hidden names.)

• Music files are not specially processed, just moved straight to $convdir.

inotifywait also watches $convdir for CLOSE_WRITE and MOVED_TO. To determine a creation time, stills are analysed with exiv2, and videos with mediainfo. Stills are also converted to RGB for hashing, and videos are hashed as they are. A new filename of the form YYYY-MM-DD_hh-mm-ss_HHHH is chosen by combining the date with the first few nibbles of the hash. The file is then moved from $convdir into $prepdir under the new name.

convmvfs is used to create /var/media-monitored, a duplicate view of the media directory /var/media. System operations such as synchronization are applied to directories in /var/media, while the user operates on /var/media-monitored. For example, $syncdir points to /var/media/Exposed, while $pubdir points to /var/media-monitored/Exposed, so they should have the same contents. However, inotifywait only monitors directories in /var/media-monitored, so it doesn't pick up events caused by system operations that operate on /var/media directly.

(I tried using mount --bind to create the duplicate, but it's no good because events on either the original or the duplicate can be picked up by watching either. Let me know if you have a better suggestion.)

Using the same inotifywait as for ingest, Snapjib also monitors $pubdir for changes made by the user. The events DELETE and MOVED_FROM on the file $pubdir/$subdir$leaf trigger the time-stamping of a file called $syncdir/$subdir.action-$leaf-delete, while MOVED_TO and CLOSE_WRITE trigger the time-stamping of a file called $syncdir/$subdir.action-$leaf-retain.

During the unification phase, which should complete before synchronization, non-hidden files in /var/media/Exposed are sought that are not hard links into /var/media/.hashes. Each is hashed, and a hard link to it is placed under that hash. Very old unlinked hash files are deleted.

During synchronization, the local site first askes the remote for a list of all its hash files, and their mtimes. Hash files with only one hard link are excluded. Those which the local site doesn't have are fetched and installed. Those with older timestamps than the remote are touched to make them match.

Information about remote retention and deletion markers is then fetched, and the corresponding local files are made to match. Hash codes of files in /var/media/Exposed at the remote site but not at the local are then requested, and if the local site has those hash files, they are hard-linked from the local /var/media/Exposed.

During purging, existing files whose deletion markers are younger than their retention markers are deleted, but only if they are hard-linked under some other name, usually a hash file. Markers for long-deleted files are also deleted.

Note that unification, synchronization and purging are all done on /var/media, not /var/media-monitored, so they neither create nor update retention/deletion markers.