Install Open WRT
This chapter describes an installation of Open WRT on Mikrotik RB750GR3 router.
Download Mikrotik Winbox https://mt.lv/winbox64 and Tiny PXE Server http://reboot.pro/files/file/303-tiny-pxe-server/.
Download current Open WRT images for RB750GR3 router from https://openwrt.org/toh/mikrotik/rb750gr3. The firmware uploaded using TFTP and run from RAM is: https://downloads.openwrt.org/releases/21.02.1/targets/ramips/mt7621/openwrt-21.02.1-ramips-mt7621-mikrotik_routerboard-750gr3-initramfs-kernel.bin The firmware uploaded using router LUCi web interface and run from Flash is: https://downloads.openwrt.org/releases/21.02.1/targets/ramips/mt7621/openwrt-21.02.1-ramips-mt7621-mikrotik_routerboard-750gr3-squashfs-sysupgrade.bin Both of them are needed.Current version is 21.02.1. It is recommended to use always the newest stable version.
This step is optional. Set PC to use DHCP, connect it to LAN socket on router and use winbox to backup Mikrotik OS license key.
Set PC IP to 192.168.1.10, connect it to WAN socket on router.
Disable any other network interfaces on the PC.
Power the router on.
The RFC951 option in Tiny PXE config file must be set to 1 (RFC951=1).
Run Tiny PXE and set it as below. Select initramfs file as a boot file.
Click Online.
Power the router off, press and hold reset (check where it is!, for RB750 it is at the back, close to power socket), power it on, wait for activity in Tiny PXE (TFTPd...), release reset, wait for DHCPdACK. Do not power the router off as it is running the Open WRT from RAM here.
Reconnect to LAN socket,
Browse 192.168.1.1, LUCI shall appear, login with no password and upgrade (System – Backup/Flash Firmware – Flash new firmware image) using squashfs file which is permanent flash image. After flashing wait until Luci opens again.
Done.
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