The Image Builder (previously called the Image Generator) is a pre-compiled environment suitable for creating custom images without the need for compiling them from source.
It downloads pre-compiled packages and integrates them in a single flashable image.

Doing so is useful if:

• you want to fit more packages in a small flash size
• you want to follow development snapshots
• your device has 32MB or less RAM and opkg does not work properly
• you want to mass-flash dozens of devices and you need a specific firmware setup

Alternative guides to achieving the same goal: Quick Image Building Guide, Beginners guide to building your own firmware.

Consider also removing packages if you have a device with very little firmware space: Saving Firmware Space.

The Image Builder runs only in 64bit linux. You can however run a 64bit linux in VM (i.e. virtualbox) even from 32bit windows.
The Image Builder has some of the same prerequisites of the Build system Build system – Installation.

Example dependencies in the most common distros:

apt-get install build-essential libncurses5-dev zlib1g-dev gawk git gettext libssl-dev xsltproc wget unzip python

yum install git gawk gettext ncurses-devel zlib-devel openssl-devel libxslt wget
yum group install "Development Tools"

dnf install @c-development @development-tools @development-libs zlib-static which

You can download an archive that contains the Image Builder, it is usually located in the same download page where you find the firmware image for your device.
For example, this is the page where you can download all firmware images for ar71xx/generic devices
https://downloads.openwrt.org/snapshots/targets/ar71xx/generic/ .
and you will find a openwrt-imagebuilder-ar71xx-generic.Linux-x86_64.tar.xz archive with the image builder in it.

Also, it is always created by the build system because it is needed to create the image file. If the option “Build the OpenWrt Image Builder” is enabled, the image builder will be generated in the same folder you find firmware images (source/bin/targets/xxx) and you can use it to create more images from the packages you obtained during compilation.

The Image Generator you download from the OpenWrt pages is already configured to download any non-default packages from official repositories.
The package sources are configured in the repositories.conf file in the extracted directory. Sources are specified in opkg native config format. This can be either the official package repositories or custom generated repositories.

An example of the contents of the repositories.conf from the openwrt-imagebuilder-18.06.0-rc2-ramips-mt7621.Linux-x86_64.tar.xz :

## Place your custom repositories here, they must match the architecture and version.
# src/gz %n http://downloads.openwrt.org/releases/18.06.0-rc2
# src custom file:///usr/src/openwrt/bin/ramips/packages

## Remote package repositories
src/gz openwrt_core http://downloads.openwrt.org/releases/18.06.0-rc2/targets/ramips/mt7621/packages
src/gz openwrt_base http://downloads.openwrt.org/releases/18.06.0-rc2/packages/mipsel_24kc/base
src/gz openwrt_luci http://downloads.openwrt.org/releases/18.06.0-rc2/packages/mipsel_24kc/luci
src/gz openwrt_packages http://downloads.openwrt.org/releases/18.06.0-rc2/packages/mipsel_24kc/packages
src/gz openwrt_routing http://downloads.openwrt.org/releases/18.06.0-rc2/packages/mipsel_24kc/routing
src/gz openwrt_telephony http://downloads.openwrt.org/releases/18.06.0-rc2/packages/mipsel_24kc/telephony

## This is the local package repository, do not remove!
src imagebuilder file:packages

The repositories.conf in an imagebuilder you compile from source will lack the “Remote package repositories” links.

If you want to add a custom local repository, copy the

src custom file:///usr/src/openwrt/bin/ramips/packages

line and modify it to point to the local folder you have your packages and package lists in.
If you have custom repositories online, copy and modify the

src/gz reboot http://downloads.openwrt.org/snapshots

line instead.

make image command will create a default image for a default device with default (essential) packages. In most cases it's not what you wanted.

To change this not-so-useful default behavior you can use some variables passed as arguments:

• PROFILE - specifies the target image to build
• PACKAGES - a list of packages to embed into the image
• FILES - directory with custom files to include
• BIN_DIR - alternative output directory for the images
• EXTRA_IMAGE_NAME - Add this to the output image filename (sanitized)
• DISABLED_SERVICES - Which services in /etc/init.d/ should be disabled. Use the initscript name you find in /etc/init.d, so for example “dhcp” for dnsmasq.

(see also the makefile used, here )

Example syntax:

$ make image PROFILE=XXX PACKAGES="pkg1 pkg2 pkg3 -pkg4 -pkg5 -pkg6" FILES=files/

See the sections below for a more in-depth explanation. After the make command is finished, the generated images are stored in the bin/device-architecture directory, just like if you were compiling them.

here the output of make help:

Available Commands:
      help:   This help text
      info:   Show a list of available target profiles
      clean:  Remove images and temporary build files
      image:  Build an image (see below for more information).

Building images:
      By default 'make image' will create an image with the default
      target profile and package set. You can use the following parameters
      to change that:

      make image PROFILE="<profilename>" # override the default target profile
      make image PACKAGES="<pkg1> [<pkg2> [<pkg3> ...]]" # include extra packages
      make image FILES="<path>" # include extra files from <path>
      make image BIN_DIR="<path>" # alternative output directory for the images
      make image EXTRA_IMAGE_NAME="<string>" # Add this to the output image filename (sanitized)

The built image will be found under the subdirectory ./bin/targets/<target>/generic or look inside ./build_dir/ for a files *-squashfs-sysupgrade.bin and *-squashfs-factory.bin (e.g. /build_dir/target-mips_24kc_musl/linux-ar71xx_tiny/tmp/openwrt-18.06.2-ar71xx-tiny-tl-wr740n-v6-squashfs-factory.bin)

Syntax:

$ make image PROFILE=NAME_OF_PROFILE

Run make info to obtain a list of defined profiles. Example output from make info is listed below.

Available Profiles:

Default:
    Default Profile
    Packages: kmod-usb-core kmod-usb2 kmod-usb-ohci kmod-usb-ledtrig-usbport
ai-br100:
    Aigale Ai-BR100
    Packages: kmod-usb2 kmod-usb-ohci
rp-n53:
    Asus RP-N53
    Packages: 
rt-n14u:
    Asus RT-N14u
    Packages: 
whr-1166d:
    Buffalo WHR-1166D
    Packages: 
whr-300hp2:
    Buffalo WHR-300HP2
    Packages:
-and many many more-

After you find the appropriate profile pass it to the make image command:

For example, if we wanted to generate a default image for for Asus RT-N14u (from above).

$ make image PROFILE=rt-n14u

The PACKAGES variable allows to include and/or exclude packages in the firmware image. By default (empty PACKAGES variable) the Image Generator will create a minimal image with device-specific kernel and drivers, uci, ssh, switch, firewall, ppp and ipv6 support.

Syntax:

$ make image PACKAGES="pkg1 pkg2 pkg3 -pkg4 -pkg5 -pkg6"

The example above will include pkg1, pkg2, pkg3, and exclude pkg4, pkg5, pkg6, note the “-” before each excluded package.

You don't need to list all dependencies of the packages you need in this list, the Image Generator uses opkg to resolve automatically the package dependencies and install other required packages.

Tip: The list of currently installed packages on your device can be obtained with the command below:

echo $(opkg list_installed | awk '{ print $1 }')

The FILES variable allows custom configuration files to be included in images built with Image Generator. This is especially useful if you need to change the network configuration from default before flashing, or if you are preparing an image for mass-flashing many devices.

Syntax:

$ make image FILES=files/

Note: The files/ folder must be in the same folder where you issue the make command.

The following example shows:

1. Creating the directory for the configuration files
2. Using scp to transfer uci configuration files from a WL500GP router to the files/etc/config directory
3. Generating an image for WL500GP with custom packages and uci configuration files

mkdir -p files/etc/config
scp root@192.168.1.1:/etc/config/network files/etc/config/
scp root@192.168.1.1:/etc/config/wireless files/etc/config/
scp root@192.168.1.1:/etc/config/firewall files/etc/config/
make image PROFILE=wl500gp PACKAGES="nano openvpn -ppp -ppp-mod-pppoe" FILES=files/

To clean up temporary build files and generated images, use the make clean command.

1. Create file 'files_remove' with full filenames:

/lib/modules/3.10.49/ts_bm.ko
/lib/modules/3.10.49/nf_nat_ftp.ko
/lib/modules/3.10.49/nf_nat_irc.ko
/lib/modules/3.10.49/nf_nat_tftp.ko

2. Patch Makefile

 ifneq ($(USER_FILES),)
 	$(MAKE) copy_files
 endif
+
+ifneq ($(FILES_REMOVE),)
+	@echo
+	@echo Remove useless files
+
+	while read filename; do				\
+	    rm -rfv "$(TARGET_DIR)$$filename";	\
+	done < $(FILES_REMOVE);
+endif
+
 	$(MAKE) package_postinst
 	$(MAKE) build_image

3. Rebuild firmware

# make image \
    PROFILE=tlwr841 \
    PACKAGES="igmpproxy ip iptraf kmod-ipt-nathelper-extra openvpn-polarssl tcpdump-mini -firewall -ip6tables -kmod-ip6tables -kmod-ipv6 -odhcp6c -ppp -ppp-mod-pppoe" \
    FILES_REMOVE="files_remove"

It is possible to build the Image Generator and integrate in it all packages so it will be able to generate images without downloading packages:

In the graphical configuration, select “Build the OpenWrt Image Builder” to build the image builder, then select Global Build Settings → Select all packages by default, save and exit. Then build the image, including IGNORE_ERRORS=1 as there might be unmaintained packages that fail to compile.

Enabling IGNORE_ERRORS=1 should only be done once the kernel and required packages are known to compile successfully.

make IGNORE_ERRORS=1

Note: Don't call make defconfig or leave an old .config file in the path as Select all packages by default will only set the package selection to [m] for packages that are not already configured otherwise!(make defconfig will set most packages to [n], i.e. do not build.)

The image generation is tied to the profile names. If you add a new profile without also adding an appropriate macro to the image-generation Makefile, no suitable firmware file will get generated when using the custom profile. Make sure to remove the /tmp directory to get modified package selection from profiles to work.

The location of the profiles for the pre-compiled package for brcm47xx-for-Linux-i686 was target/linux/brcm47xx/profiles/

Remarkably, all that needs to be done to add a new profile, is to add a new file to the profiles directory. While this may have been the case in earlier releases, for 17.01, it appears that manual editing of .targetinfo is also required.

Here is what the profiles/100-Broadcom-b43.mk profile file looks like:

define Profile/Broadcom-b43
  NAME:=Broadcom BCM43xx WiFi (default)
  PACKAGES:=kmod-b43 kmod-b43legacy
endef

define Profile/Broadcom-b43/Description
	Package set compatible with hardware using Broadcom BCM43xx cards
endef
$(eval $(call Profile,Broadcom-b43))