SWUpdate: software update for embedded system


This project is thought to help to update an embedded system from a storage media or from network. However, it should be mainly considered as a framework, where further protocols or installers (in SWUpdate they are called handlers) can be easily added to the application.

One use case is to update from an external local media, as USB-Pen or SD-Card. In this case, the update is done without any intervention by an operator: it is thought as “one-key-update”, and the software is started at reset simply pressing a key (or in any way that can be recognized by the target), making all checks automatically. At the end, the updating process reports only the status to the operator (successful or failed).

The output can be displayed on a LCD using the frame-buffer device or directed to a serial line (Linux console).

It is generally used in the single copy approach, running in an initrd (recipes are provided to generate with Yocto). However, it is possible to use it in a double-copy approach by use of Software collections.

If started for a remote update, SWUpdate starts an embedded Web-server and waits for requests. The operator must upload a suitable image, that SWUpdate checks and then install. All output is notified to the operator’s browser via AJAX notifications.


General Overview

  • Install on embedded Media (eMMC, SD, Raw NAND, NOR and SPI-NOR flashes)
  • check if an image is available. The image is built in a specified format (cpio) and it must contain a file describing the software that must be updated.
  • SWUpdate is thought to update UBI volumes (mainly for NAND, but not only) and images on devices. Passing a whole image can still be updated as a partition on the SD card, or a MTD partition.
  • new partition schema. This is bound with UBI volume. SWUpdate can recreate UBI volumes, resizing them and copying the new software.
  • support for compressed images, using the zlib and zstd library. tarball (tgz file) are supported.
  • support for partitioned USB-pen or unpartitioned (mainly used by Windows).
  • support for updating a single file inside a filesystem. The filesystem where to put the file must be described.
  • checksum for the single components of an image
  • use a structured language to describe the image. This is done using the libconfig library as default parser, that uses a JSON-like description.
  • use custom’s choice for the description of the image. It is possible to write an own parser using the Lua language. An example using a XML description in Lua is provided in the examples directory.
  • Support for setting / erasing U-Boot variables
  • Support for setting / erasing GRUB environment block variables
  • Support for setting / erasing EFI Boot Guard variables
  • Support for preinstall scripts. They run before updating the images
  • Support for postinstall scripts. They run after updating the images.
  • Network installer using an embedded Web-server (Mongoose Server was chosen, in the version under Lua license). A different Web-server can be used.
  • Multiple interfaces for getting software
    • local Storage: USB, SD, UART,..
  • OTA / Remote
    • integrated Web-Server
    • pulling from remote Server (HTTP, HTTPS, ..)
    • using a Backend. SWUpdate is open to talk with back end servers for rolling out software updates. Current version supports the hawkBit server, but other backend can be added.
  • Can be configured to check for compatibility between software and hardware revisions. The software image must contain an entry declaring on which HW revision the software is allowed to run. SWUpdate refuses to install if the compatibility is not verified.
  • support for image extraction. A manufacturer can require to have a single image that contains the software for more as one device. This simplifies the manufacturer’s management and reduces their administrative costs having a single software product. SWUpdate receives the software as stream without temporary storing, and extracts only the required components for the device to be installed.
  • allow custom handlers for installing FPGA firmware, micro-controller firmware via custom protocols.
  • Features are enabled / disabled using “make menuconfig”. (Kbuild is inherited from busybox project)
  • Images are authenticated and verified before installing
  • Power-Off safe

Single image delivery

The main concept is that the manufacturer delivers a single big image. All single images are packed together (cpio was chosen for its simplicity and because can be streamed) together with an additional file (sw-description), that contains meta information about each single image.

The format of sw-description can be customized: SWUpdate can be configured to use its internal parser (based on libconfig), or calling an external parser in Lua.


Changing the rules to accept images with an external parser, let to extend to new image types and how they are installed. In fact, the scope of the parser is to retrieve which single images must be installed and how. SWUpdate implements “handlers” to install a single image: there are handlers to install images into UBI volumes, or to a SD card, a CFI Flash, and so on. It is then easy to add an own handler if a very special installer is required.

For example we can think at a project with a main processor and one or several micro-controllers. Let’s say for simplicity that the main processor communicates with the micro-controllers via UARTS using a proprietary protocol. The software on the micro-controllers can be updated using the proprietary protocol.

It is possible to extend SWUpdate writing a handler, that implements the part of the proprietary protocol to perform the upgrade on the micro-controller. The parser must recognize which image must be installed with the new handler, and SWUpdate will call the handler during the installation process.

Streaming feature

SWUpdate is thought to be able to stream the received image directly into the target, without any temporary copy. In fact, the single installer (handler) receive as input the file descriptor set at the beginning of the image that must be installed.

The feature can be set on image basis, that means that a user can decide which partial images should be streamed. If not streamed (see installed-directly flag), files are temporary extracted into the directory pointed to by the environment variable TMPDIR with /tmp as fall-back if TMPDIR is not set. Of course, by streaming it is not possible to make checks on the whole delivered software before installing. The temporary copy is done only when updated from network. When the image is stored on an external storage, there is no need of that copy.

Images fully streamed

In case of remote update, SWUpdate extracts relevant images from the stream and copies them into the directory pointed to by the environment variable TMPDIR (if unset, to /tmp) before calling the handlers. This guarantee that an update is initiated only if all parts are present and correct. However, on some systems with less resources, the amount of RAM to copy the images could be not enough, for example if the filesystem on an attached SD Card must be updated. In this case, it will help if the images are installed directly as stream by the corresponding handler, without temporary copies. Not all handlers support to stream directly into the target. Streaming with zero-copy is enabled by setting the flag “installed-directly” in the description of the single image.

Configuration and build


There are only a few libraries that are required to compile SWUpdate.

  • mtd-utils: internally, mtd-utils generates libmtd and libubi. They are commonly not exported and not installed, but they are linked by SWUpdate to reuse the same functions for upgrading MTD and UBI volumes.
  • openssl / wolfssl / mbedtls (optional) for cryptographic operations
  • p11-kit & wolfssl (optional) for PKCS#11 support
  • Lua: liblua and the development headers.
  • libz is always linked.
  • libconfig (optional) for the default parser
  • libarchive (optional) for archive handler
  • librsync (optional) for support to apply rdiff patches
  • libjson (optional) for JSON parser and hawkBit
  • libubootenv (optional) if support for U-Boot is enabled
  • libebgenv (optional) if support for EFI Boot Guard is enabled
  • libcurl used to communicate with network

New handlers can add some other libraries to the requirement list - check if you need all handlers in case you get build errors, and drop what you do not need.

Building with Yocto

See corresponding chapter how to build in Yocto.

Configuring SWUpdate

SWUpdate is configurable via “make menuconfig”. The small footprint is reached using the internal parser and disabling the web server. Any option has a small help describing its usage. In the default configuration, many options are already activated.

To configure the options:

make menuconfig


  • to cross-compile, set the CC and CXX variables before running make. It is also possible to set the cross-compiler prefix as option with make menuconfig.
  • generate the code

The result is the binary “swupdate”. A second binary “progress” is built, but it is not strictly required. It is an example how to build your own interface to SWUpdate to show a progress bar or whatever you want on your HMI. The example simply prints on the console the current status of the update.

In the Yocto buildsystem,:

bitbake swupdate

This will build the package

bitbake swupdate-image

This builds a rescue image. The result is a Ramdisk that can be loaded directly by the bootloader. To use SWUpdate in the double-copy mode, put the package swupdate into your rootfs. Check your image recipe, and simply add it to the list of the installed packages.

For example, if we want to add it to the standard “core-image-full-cmdline” image, we can add a recipes-extended/images/core-image-full-cmdline.bbappend

                        swupdate \
                        swupdate-www \

swupdate-www is the package with the website, that you can customize with your own logo, template ans style.

Building a debian package

SWUpdate is thought for Embedded Systems and building in an embedded distribution is the first use case. But apart the most used buildsystems for embedded as Yocto or Buildroot, in some cases a standard Linux distro is used. Not only, a distro package allows one to run SWUpdate on Linux PC for test purposes without having to fight with dependencies. Using the debhelper tools, it is possible to generate a debian package.

Steps for building a debian package

./debian/rules clean
./debian/rules build
fakeroot debian/rules binary

The result is a “deb” package stored in the parent directory.

Alternative way signing source package

You can use dpkg-buildpackage:

dpkg-buildpackage -us -uc
debsign -k <keyId>

Running SWUpdate

What is expected from a SWUpdate run

The whole update process can be seen as a set of pipelines. The incoming stream (the SWU file) is processed by each pipe and passed to the next step. First, the SWU is streamed from one of the interfaces : local (USB, filesystem), Webserver, suricatta (one of the backend), etc. The incoming SWU is forwarded to the installer to be examined and installed. A run of SWUpdate consists mainly of the following steps:

  • extracts sw-description from the stream and verifies it It parses sw-description creating a raw description in RAM about the activities that must be performed.
  • if Signed Images is activated, extracts sw-description.sig and validate sw-description.
  • check for hardware-software compatibility, if any, reading hardware revision from hardware and matching with the table in sw-description.
  • Parse sw-description to determine which artefacts in the incoming SWU are required. Not required artifacts are simply skipped. The meta file sw-description is declarative and executive. If “hooks” are defined in sw-description, they are executed at parse time. Hooks are executed before any artefact is evaluated and they are the best way to implement pre install functions. At the end of the parsing, SWUpdate builds an internal mapping for each artifact to recognize which handler should be called for each of them.
  • Reads the cpio archive and proofs the CPIO checksum of each single file (this can be disabled in the configuration) and computes the sha256 sum if foreseen or if Signed Images are activated. SWUpdate stops if the archive is not complete verified.
  • check that all components described in sw-description are really in the cpio archive.
  • run the partitions handlers, if required.
  • runs pre-install scripts, if any. Please note: if artifacts are streamed, a preinstall script cannot be executed because it depends on the order the artifact (including the script) are packed into the CPIO archive. In this case, please use the “embedded-script” feature in sw-description to execute functions before any installation takes place.
  • if an artifact is found in SWU and it is marked with “installed-directly”, proceed with installation, else extract it temporarily into TMPDIR.
  • iterates through all images and call the corresponding handler for installing on target.
  • iterates through all files and call the corresponding handler for installing on target.
  • iterates through all scripts and call the corresponding handler for post-install scripts
  • iterates through all bootenv and updates the bootloader environment.
  • reports the status to the operator through the notification interface (logging, traces) and through the progress interface.
  • if a postinstall command is foreseen (for example to reboot the device), call it.

The first step that fails, stops the entire procedure and an error is reported.

To start SWUpdate expecting the image from a file:

swupdate -i <filename>

To start with the embedded web server:

swupdate -w "<web server options>"

The main important parameters for the web server are “document-root” and “port”.

swupdate -w "--document-root ./www --port 8080"

The embedded web server is taken from the Mongoose project.

The list of available options (depending on activated features) is shown with:

swupdate -h

This uses as website the pages delivered with the code. Of course, they can be customized and replaced. The website uses AJAX to communicate with SWUpdate, and to show the progress of the update to the operator.

The default port of the Web-server is 8080. You can then connect to the target with:


If it works, the start page should be displayed as in next figure.


If a correct image is downloaded, SWUpdate starts to process the received image. All notifications are sent back to the browser. SWUpdate provides a mechanism to send to a receiver the progress of the installation. In fact, SWUpdate takes a list of objects that registers itself with the application and they will be informed any time the application calls the notify() function. This allows also for self-written handlers to inform the upper layers about error conditions or simply return the status. It is then simply to add own receivers to implement customized way to display the results: displaying on a LCD (if the target has one), or sending back to another device via network. An example of the notifications sent back to the browser is in the next figure:


Software collections can be specified by passing –select command line option. Assuming sw-description file contains a collection named stable, with alt installation location, SWUpdate can be called like this:

swupdate --select stable,alt

Command line parameters

Parameter Type Description
-f <file> string SWUpdate configuration file to use. See examples/configuration/swupdate.cfg in the source code for details.
-b <string> string Available if CONFIG_UBIATTACH is set. It allows one to blacklist MTDs when SWUpdate searches for UBI volumes. Example: U-Boot and environment in MTD0-1: swupdate -b "0 1".
-e <sel> string sel is in the format <software>,<mode>. It allows one to find a subset of rules in the sw-description file. With it, multiple rules are allowed. One common usage is in case of the dual copy approach. Example: -e “stable, copy1” ==> install on copy1 -e “stable, copy2” ==> install on copy2
--excluded <sel>
string sel is in the format <software>,<mode>. It sets a blacklist of selections that cannot be used for an update. Selections can be activated not only with -e, but also via IPC. Multiple –excluded are allowed
Run usage with help.
-k <file> string Available if CONFIG_SIGNED is set. Filename with the public key.
-K <file> string Available on CONFIG_ENCRYPTED_IMAGES set. Filename with the symmetric key to be used for decryption.
–cert-purpose <purpose> string Available if CONFIG_SIGNED_IMAGES is set. Set expected certificate purpose.
–forced-signer-name <cn> string Available if CONFIG_SIGNED_IMAGES is set. Set expected common name of signer certificate.
–ca-path <file> string Available if CONFIG_SIGNED_IMAGES is set. Path to the Certificate Authority (PEM).
-l <level> int Set loglevel.
Send LOG output to syslog (local).
-i <file> string Run SWUpdate with a local .swu file.
Run SWUpdate in dry-run mode.
-N <version> string The minimum required version of software. This will be checked with the version of new software and forbids downgrading. Version consists of either 4 numbers (major.minor.rev.build with each field in the range 0..65535) or it is a semantic version.
-max-version <version>
string The maximum required version of software. This will be checked with the version of new software. Version consists of either 4 numbers (major.minor.rev.build with each field in the range 0..65535) or it is a semantic version.
-R <version> string The current installed version of software. This will be checked with the version of new software and forbids reinstalling.
-o <file> string Save the stream (SWU) to a file.
Activate verbose output.
Disable setting the bootloader transaction marker.
Disable setting the update state in the bootloader.
-w <parms> string Available if CONFIG_WEBSERVER is set. Start internal webserver and pass to it a command line string.
-d <parms> string Available if CONFIG_DOWNLOAD is set. Start internal downloader client and pass to it a command line string. See below the internal command line arguments for the downloader.
-u <parms> string Available if CONFIG_SURICATTA is set. Start internal suricatta client daemon and pass to it a command line string. See below the internal command line arguments for suricatta.
-H <board:rev> string Available on CONFIG_HW_COMPATIBILITY set. Set board name and hardware revision.
Check *.swu file. It ensures that files referenced in sw-description are present. Usage: swupdate -c -i <file>
-P <cmd> string Execute pre-update command.
-p <cmd> string Execute post-update command.

Downloader command line parameters

Example: swupdate -d "-u example.com"

Mandatory arguments are marked with ‘*’:

Parameter Type Description
-u <url> string * This is the URL where new software is pulled. URL is a link to a valid .swu image
-r <retries> integer Number of retries before a download is considered broken. With “-r 0”, SWUpdate will not stop until a valid software is loaded.
-t <timeout> integer Timeout for connection lost downloader or Webserver
-a <usr:pwd> string Send user and password for Basic Auth

Suricatta command line parameters

Example: swupdate -u "-t default -u localhost:8080 -i 1B7"

Note that different suricatta modules may have different parameters. The below listed options are for SWUpdate’s hawkBit support.

Mandatory arguments are marked with ‘*’:

Parameter Type Description
-t <tenant> string * Set hawkBit tenant ID for this device.
-u <url> string * Host and port of the hawkBit instance, e.g., localhost:8080
-i <id> integer * The device ID to communicate to hawkBit.
-c <confirm> integer Confirm update status to server: 1=AGAIN, 2=SUCCESS, 3=FAILED
Do not abort on flawed server certificates.
-p <polldelay> integer Delay in seconds between two hawkBit poll operations (default: 45s).
-r <retry> integer Resume and retry interrupted downloads (default: 5 tries).
-w <retrywait> integer Time to wait prior to retry and resume a download (default: 5s).
-y <proxy> string Use proxy. Either give proxy URL, else {http,all}_proxy env is tried.
-k <targettoken> string Set target token.
-g <gatewaytoken> string Set gateway token.
-f <interface> string Set the network interface to connect to hawkBit.
Daemon enabled at startup (default).
Daemon disabled at startup.
Do not send authentication header when downloading SWU.
–cache-file string This allow to resume an update after a power cut. If the SWU is saved in a file, SWUpdate can reuse the file and download just the remaining part of the SWU.
-m <seconds> integer Delay in seconds between re-trying to send initial feedback specified with “-c” option. Default value is 10 seconds. If Suricatta is started with initial state of STATE_WAIT (“-c 6”), this value is ignored.
-s <seconds> integer Connection timeout to use in seconds. If user doesn’t set this option, default libcurl connection timeout value of 300 seconds is used. NOTE: it is not possible for Suricatta to respond to external program API requests during this period - adapt this value to your use case!

systemd Integration

SWUpdate has optional systemd support via the compile-time configuration switch CONFIG_SYSTEMD. If enabled, SWUpdate signals systemd about start-up completion and can make optional use of systemd’s socket-based activation feature.

A sample systemd service unit file /etc/systemd/system/swupdate.service may look like the following starting SWUpdate in suricatta daemon mode:

Description=SWUpdate daemon

ExecStart=/usr/bin/swupdate -u '-t default -u http://localhost -i 25'


Started via systemctl start swupdate.service, SWUpdate (re)creates its sockets on startup. For using socket-based activation, an accompanying systemd socket unit file /etc/systemd/system/swupdate.socket is required:

Description=SWUpdate socket listener



On swupdate.socket being started, systemd creates the socket files and hands them over to SWUpdate when it starts. So, for example, when talking to /tmp/swupdateprog, systemd starts swupdate.service and hands-over the socket files. The socket files are also handed over on a “regular” start of SWUpdate via systemctl start swupdate.service.

Note that the socket paths in the two ListenStream= directives have to match the socket paths CONFIG_SOCKET_CTRL_PATH and CONFIG_SOCKET_PROGRESS_PATH in SWUpdate’s configuration. Here, the default socket path configuration is depicted.

Changes in boot-loader code

The SWUpdate consists of kernel and a root filesystem (image) that must be started by the boot-loader. In case using U-Boot, the following mechanism can be implemented:

  • U-Boot checks if a sw update is required (check gpio, serial console, etc.).
  • the script “altbootcmd” sets the rules to start SWUpdate
  • in case SWUpdate is required, U-boot run the script “altbootcmd”

Is it safe to change U-Boot environment ? Well, it is, but U-Boot must be configured correctly. U-Boot supports two copies of the environment to be power-off safe during an environment update. The board’s configuration file must have defined CONFIG_ENV_OFFSET_REDUND or CONFIG_ENV_ADDR_REDUND. Check in U-Boot documentation for these constants and how to use them.

There are a further enhancement that can be optionally integrated into U-boot to make the system safer. The most important I will suggest is to add support for boot counter in U-boot (documentation is in U-Boot docs). This allows U-Boot to track for attempts to successfully run the application, and if the boot counter is greater as a limit, can start automatically SWUpdate to replace a corrupt software.

GRUB by default does not support double copies of environment as in case of U-Boot. This means that there is possibility that environment block get’s corrupted when power-off occurs during environment update. To minimize the risk, we are not modifying original environment block. Variables are written into temporary file and after successful operation rename instruction is called.

Building a single image

cpio is used as container for its simplicity. The resulting image is very simple to be built. The file describing the images (“sw-description”, but the name can be configured) must be the first file in the cpio archive.

To produce an image, a script like this can be used:

FILES="sw-description image1.ubifs  \
       image2.gz.u-boot uImage.bin myfile sdcard.img"
for i in $FILES;do
        echo $i;done | cpio -ov -H crc >  ${PRODUCT_NAME}_${CONTAINER_VER}.swu

The single images can be put in any order inside the cpio container, with the exception of sw-description, that must be the first one. To check your generated image you can run the following command:

swupdate -c -i my-software_1.0.swu

Support of compound image

The single image can be built automatically inside Yocto. meta-swupdate extends the classes with the swupdate class. A recipe should inherit it, and add your own sw-description file to generate the image.