• Mikrotik is a very popular supplier of networking equipment.
• They have their own operating system called RouterOS which most probably is Linux based.
• RouterOS however is licensed and includes different levels of licensing for different functionalities.
• To unlock a needed feature not included with the current license then requires you to upgrade which obviously involves cost.
• However, the hardware itself is good and durable and it is easy to source all over the world.
• Some of their models are also able to run OpenWRT as an alternative to RouterOS.
• OpenWRT is OpenSource and there is no paid for licensing involved on OpenWRT.
• This page will just cover the flash from RouterOS to OpenWRT on the Mikrotik RB750Gr3.
• We will cover the configuration of the device once OpenWRT is running on it on a separate page.

• Linux based machine to communicate with the Mikrotik and supply the OpenWRT firmware over the network.
• Mikrotik RB750Gr3 reset to factory defaults
• 1xCat5 LAN cable.

• The flash process is fairly simple and straight forward and involves three steps
  1. Export the RouterOS License Key.
  2. Load a RAM based version of OpenWRT.
  3. Flash the permanent (Flash) version of OpenWRT using the RAM based version.

• It is recommended that you export the RouterOS license key in the event of returning to the original RouterOS, you might need to use it again to activate the RouterOS.
• RouterOS can be accessed in various ways which include
  • Web based UI called Webfig (default http://192.168.88.1) on the LAN side of the board
  • Using the WinBox application.
• Unfortunately the Webfig interface does not support the export of the license key. (Only upgrade)
• We have to use WinBox for that.

• WinBox is a windows based program but since its a simple program its just as easy to run it on Ubuntu using Wine.
• We assume you have a standard install of Ubuntu 20.04 (with some GUI)
• Install Wine

sudo apt-get install wine

• Download the latest version of Winbox from Mikrotik
• https://mikrotik.com/download
• Use Wine to run winbox64.exe

#cd to where the winbox64.exe program is
wine winbox64.exe

• Configure the LAN port of your Ubuntu machine to have an IP Address on the 192.168.88 subnet.

#Here our LAN interface is enp8s0 
#Use ip a to discover what it is caleld on your machine
sudo ip addr add 192.168.88.10/24 dev enp8s0

• Use the LAN cable to connect to any of the LAN ports on the Mikrotik (2-5) to your Ubuntu machine.
• Go to the Neighbors tab and see if the device is listed.
• Click on the listed device and click Connect to enter into its configuration.
• Select System → License
• Click on Export Key to export it.
• Store it on a safe place

Now that we have the license key taken care of we can go to the next step which is to load the RAM based OpenWRT onto the device

• In order for us to get the RAM based version of OpenWRT onto the Mikrotik two things are involved.
  1. There has to be a BOOTP environment set up to serve the RAM based image
  2. The Mikrotik has to be set in such a mode as to use the BOOTP environment to fetch the RAM based image using BOOTP

• On Ubuntu it is very simple and we just create a small shell script to take care of it

loader.sh

#!/bin/bash
USER=system
#HEADS UP THIS MUST MATCH YOUR MACHINE'S LAN INTERFACE
IFNAME=enp8s0
/sbin/ip addr replace 192.168.1.10/24 dev $IFNAME
/sbin/ip link set dev $IFNAME up
/usr/sbin/dnsmasq --user=$USER \
--no-daemon \
--listen-address 192.168.1.10 \
--bind-interfaces \
-p0 \
--dhcp-authoritative \
--dhcp-range=192.168.1.100,192.168.1.200 \
--bootp-dynamic \
--dhcp-boot=openwrt-19.07.6-ramips-mt7621-mikrotik_rb750gr3-initramfs-kernel.bin \
--log-dhcp \
--enable-tftp \
--tftp-root=$(pwd)

• As you can see we will serve the openwrt-19.07.6-ramips-mt7621-mikrotik_rb750gr3-initramfs-kernel.bin image to the Mikrotik.
• Download this image (This was the latest 19.07 based image at the time of this writing - PLEASE CHECK IF ITS STILL THE LATEST!)
• https://downloads.openwrt.org/releases/19.07.6/targets/ramips/mt7621/
• Make sure it is located in the same directory as the loader.sh script.
• Do the following to start up the BOOTP service

#CD to where the RAM image and loader.sh files are
chmod 755 ./loader.sh
sudo ./loader.sh
#WE get the following output
[sudo] password for system: 
dnsmasq: started, version 2.80 DNS disabled
dnsmasq: compile time options: IPv6 GNU-getopt DBus i18n IDN DHCP DHCPv6 no-Lua TFTP conntrack ipset auth nettlehash DNSSEC loop-detect inotify dumpfile
dnsmasq-dhcp: DHCP, IP range 192.168.1.100 -- 192.168.1.200, lease time 1h
dnsmasq-tftp: TFTP root is /home/system/Documents/mt_flash 

• Our BOOTP environment is now ready for action.
• Next we will make the Mikrotik use this environment to load the RAM image

• To force the Mikrotik to use the BOOTP environment you need to connect the Internet port to the LAN interface of your Linux machine. (Internet Port is Port 1 on the Mikrotik)
• Press the RESET button while powering up the board.
• Wait for the first beep and release the RESET button.
• This is what our dnsmasq script outputs during the process

dnsmasq-dhcp: 274610437 available DHCP range: 192.168.1.100 -- 192.168.1.200
dnsmasq-dhcp: 274610437 vendor class: MMipsBoot
dnsmasq-dhcp: 274610437 tags: bootp, enp8s0
dnsmasq-dhcp: 274610437 BOOTP(enp8s0) 192.168.1.144 48:8f:5a:e3:d4:9c 
dnsmasq-dhcp: 274610437 bootfile name: openwrt-19.07.6-ramips-mt7621-mikrotik_rb750gr3-initramfs-kernel.bin
dnsmasq-dhcp: 274610437 next server: 192.168.1.10
dnsmasq-dhcp: 274610437 sent size:  4 option:  1 netmask  255.255.255.0
dnsmasq-dhcp: 274610437 sent size:  4 option: 28 broadcast  192.168.1.255
dnsmasq-dhcp: 274610437 sent size:  4 option:  3 router  192.168.1.10
dnsmasq-tftp: sent /home/system/Documents/mt_flash/openwrt-19.07.6-ramips-mt7621-mikrotik_rb750gr3-initramfs-kernel.bin to 192.168.1.144

• The USR LED will blink while OpenWRT is busy booting itself.
• After a while it will stop and this should happen on the output of the dnsmasq script

....
dnsmasq-dhcp: 3516861450 available DHCP range: 192.168.1.100 -- 192.168.1.200
dnsmasq-dhcp: 3516861450 vendor class: udhcp 1.30.1
dnsmasq-dhcp: 3516861450 client provides name: OpenWrt
dnsmasq-dhcp: 3516861450 DHCPDISCOVER(enp8s0) 48:8f:5a:e3:d4:9e 
dnsmasq-dhcp: 3516861450 tags: enp8s0
dnsmasq-dhcp: 3516861450 DHCPOFFER(enp8s0) 192.168.1.146 48:8f:5a:e3:d4:9e 
dnsmasq-dhcp: 3516861450 requested options: 1:netmask, 3:router, 6:dns-server, 12:hostname, 
....

• This simply means that OpenWRT are now fully booted and actually looking for an IP Address itself!
• You can now (While the board is still powered up) move the cable to and of the LAN ports of the Mikrotik
• Terminate the loader.sh script
• Reconfigure the LAN interface on the Linux machine to have IP 192.168.1.10

 sudo ip addr add 192.168.1.10/24 dev enp8s0

• Connect to OpenWRT on 192.168.1.1