This post is also available on https://reinhart1010.github.io/posts/2021/02/14/apparently-yes-you-can-install-openjdk-java-jre-and-yacy-on-openwrt.html

IMPORTANT NOTE: This guide is meant for OpenWrt-installed devices with really large memory and storage space, which in this case a Raspberry Pi 3 (Model B) with 1GB of RAM and 32GB of storage (loaded on a MicroSD Card). Most routers do not support those gigantic space, so they might not be capable on running YaCy at all.

YaCy (https://yacy.net) is a distributed search engine which can be installed on any devices running Java Runtime Environment (JRE) 8. YaCy itself can also be configured to be a private search engine - search your internal files and websites on the intranet.

It would be interesting to have your own SBC (such as a Raspberry Pi and Orange Pi) to run YaCy on OpenWrt, so you'll have your own private network and search engine at the same time, without needing to install YaCy on another device!

During the last few days I am trying to make this concept to work as expected. And as expected, it is possible can run JRE and YaCy on OpenWrt, too!

First thing first: Expand the root filesystem using Extroot!

The default OpenWrt image for Raspberry Pi and similar devices only uses 100MB as their main rootfs partition, which is used to load all the programs and configuration files used for OpenWrt. In this case I have a total of 32GB of storage, and the best way to "expand" the storage is by using Extroot, which is explained on https://openwrt.org/docs/guide-user/additional-software/extroot_configuration :

1. Create a new ext4 partition on the MicroSD card's unused space.
2. Perform extroot to use the new partition for expansion, for Devices ≥ 8 MiB flash. The new partition may likely to reside on /dev/mmcblk0p3 (or others) instead of /dev/sda, so please check it first using the block info command.
3. Restart device to apply changes.

Installing Java JRE on OpenWrt

Installing the JRE on OpenWrt is unlike other major Linux distributions. OpenJDK is not available on OpenWrt default repositories, so you can't simply install it using the opkg package manager.

Many Linux distributions including Debian, Ubuntu, and Red Hat commonly ship JRE packaged by OpenJDK, an open-source implementation of Java Standard Edition (Java SE) which was built and depends on glibc (GNU libc). OpenWrt, on the other hand, uses musl-libc instead since it's smaller than glibc so they can fit into routers with limited storage and memory.

Fortunately, the folks at Alpine Linux provides a copy of OpenJDK JRE which depends on musl-libc instead of glibc on their package repository. I decided to modify the install script from this GitHub Gist to download the binaries for my Raspberry Pi 3 (OpenJDK JRE 8 using aarch64 instead of regular armv7, since the Pi itself is capable for 64-bit and JRE 8 is required for YaCy to run).

#!/bin/sh

set -o errexit
set -o nounset
set -o pipefail
set -x

REVISION=8.252.09-r0
URL=http://dl-cdn.alpinelinux.org/alpine/v3.10/community/aarch64/
PACKAGES="openjdk8 openjdk8-jre openjdk8-jre-lib openjdk8-jre-base"

old_pwd=$(pwd)
tmp_dir=$(mktemp -d -t openjdk8-XXXXXXXXXX)
trap "rm -rf $tmp_dir" EXIT

cd "${tmp_dir}"

for package in $PACKAGES; do
    curl -LO "${URL}/${package}-${REVISION}.apk"
done

for package in $PACKAGES; do
    tar xzf "${package}-${REVISION}.apk"
done


cd "${old_pwd}"

mv $tmp_dir/usr/lib/jvm/java-1.8-openjdk /overlay/opt/java-1.8-openjdk

The above script requires a special program named curl, which is not installed by default in OpenWrt. Additionally, OpenJDK also requires libstdcpp6, libjpeg (substituted by libjpeg-turbo), libnss, and liblcms. Note that liblcms is also not available on opkg, so I decided to install it from Alpine repository (Hint: Alpine's .apk is simply a .tar.gz file) to /usr/lib.

# Don't forget to run "opkg update" first!
opkg install curl libstdcpp6 libjpeg liblcms2.so.2 libnss

wget http://dl-cdn.alpinelinux.org/alpine/edge/community/aarch64/liblcms-1.19-r8.apk
tar xvzf liblcms-1.19-r8
mv usr/bin/liblcms* /usr/bin/

Once these dependencies are installed, you can run the previous script to install OpenJDK under /overlay/opt/java-1.8-openjdk.

Lastly, you'll need to update the $PATH environment variable to let OpenWrt to find the JRE binaries. Open the /etc/profile file, then modify

export PATH="/usr/sbin:/usr/bin:/sbin:/bin"

to

export PATH="/usr/sbin:/usr/bin:/sbin:/bin:/overlay/opt/java-1.8-openjdk/bin"

then save the file and apply the configuration using source /etc/profile.

Installing YaCy

Once you have installed JRE 8 on OpenWrt, you can simply run YaCy on it, too!

At the time of this writing (February 14th, 2021), however, there is a then-unfixed bug on the latest YaCy release (yacy_v1.924_20201214_10042) where users cannot log into the admin interface (see this thread), hence I'll need to compile YaCy from their main repository by myself.

I have decided to put YaCy under the /overlay/opt directory as well. The YaCy scripts require another package named getopt, which is also installable using opkg:

opkg install getopt

Once I have extracted the YaCy package and installed their dependencies, I can simply run it using the startYACY.sh script file, which in this case located on

/overlay/opt/yacy/startYACY.sh

Once it runs for the first time, you will need to set the admin pasword by running

/overlay/opt/yacy/bin/passwd.sh

And that's it! YaCy is now accessible on <your OpenWrt IP address>:8090. Keep in mind that you can still access LuCi the original way, as the YaCy server does not take the regular HTTP ports (80, 443) unless otherwise specified.