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Much of the Mender client's configuration resides in /etc/mender/mender.conf on the root filesystem. This file is JSON structured and defines various parameters for Mender's operation. Some of the most common settings are tunable using Yocto variables. The remaining parameters can only be changed by providing your own mender.conf.

On systems where it is desired for one or more of the configuration options to be customized and survive future updates, there is an optional "fallback" configuration file /var/lib/mender/mender.conf. Because the directory /var/lib/mender is backed by persistent storage, the fallback configuration file will not be overwritten by Mender updates.

The fallback configuration file has the same JSON format as the main configuration file. Any setting value that appears in the main configuration file /etc/mender/mender.conf will be used, whether or not the setting appears in the fallback file /var/lib/mender/mender.conf. Therefore a setting in the fallback file will only be used if it does not appear in the main file.

Providing mender.conf

It is easy to provide your own mender.conf file. First you will need to have your own layer. If you don't already have one see the Yocto Manual for how to create one.

Inside the layer, you will need to create a .bbappend file for the mender recipe. It should be placed in a location like this: meta-<mylayer>/recipes-mender/mender/mender_%.bbappend. The content of the file should be the following:

FILESEXTRAPATHS_prepend := "${THISDIR}/files:"
SRC_URI_append = " file://mender.conf"

If your layer already contains a mender_%.bbappend file, then add the above contents into your file, taking care to not redefine any variables you are already using.

Then, inside meta-<mylayer>/recipes-mender/mender/files/mender.conf, you can put whatever configuration options you need, as a JSON structure. The file you provide will be merged with other settings taken from various variables in the build, with variables taking precedence.

Here is an example of a mender.conf file:

{
  "ServerURL": "https://mymenderserver.net",
  "ServerCertificate": "/etc/site-conf/server.crt"
}

Note that many mandatory entries are missing, such as RootfsPartA and RootfsPartB. These are filled in by the Yocto build system automatically, using the merge method described in the paragraph above. So you do not need to provide all values, those that are not provided will take on default values.