Add 'img/live/ubuntuPXEInitRamFS.md'

img/live/ubuntuPXEInitRamFS.md

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+#Generate initramfs for ubuntu that can fetch rootfs from http server
+## Steps
+* Boot in to live Ubuntu disk  
+* Open Terminal
+* Run `sudo -i`
+* Put insert content of below to files `/usr/share/initramfs-tools/modules.d/pxe` and `/usr/share/initramfs-tools/scripts/pxe`
+* Run `mkinitramfs -o /root/initrd.img`
+* Migrate `/root/initrd.img` to PXE server
+* Add new boot parameters to pxelinux config on pxe server
+
+### New Required boot parameters
+```
+rooturl=http://<serverIP>/path/to/file boot=pxe maxTryCount=10
+```
+
+### /usr/share/initramfs-tools/modules.d/pxe
+```
+overlayfs
+squashfs
+```
+
+### /usr/share/initramfs-tools/scripts/pxe
+Script from https://habr.com/en/post/513568/
+```
+#!/bin/bash
+mountroot()
+{
+        maxTryCount=5
+        squashfsFile="/dwn/rootfs.squashfs"
+        squashfsMountPoint="/mnt/ro"
+        tmpfsMountPoint="/mnt/rw"
+        overlayfsUppderDir="$tmpfsMountPoint/upper"
+        overlayfsWorkDir="$tmpfsMountPoint/work"
+        overlayfsDir="/mnt/overlayfs"
+        tryCount="1"
+
+        # run udevadm
+        wait_for_udev 10
+
+        # parce kernel cmdline args. rooturl needed
+        for x in $(cat /proc/cmdline); do
+                case $x in
+                rooturl=*)
+                        export rooturl=${x#rooturl=}
+                        ;;
+                maxTryCount=*)
+                        export maxTryCount=${x#maxTryCount=}
+                        ;;
+                initdwnsize=*)
+                        export initdwnsize=${x#initdwnsize=}
+                        ;;
+
+                esac
+        done
+
+        log_begin_msg "Loading modules" 
+        modprobe squashfs || panic "can't modprobe squashfs"
+        modprobe af_packet || panic "can't modprobe af_packet"
+        modprobe overlay || panic "can't modprobe overlayfs"
+        log_success_msg "modules loaded"
+
+	log_begin_msg "Generating RamDisk for download.."
+	mkdir /dwn
+	mount -t tmpfs -o rw,size=${initdwnsize} tmpfs /dwn
+	log_success_msg "Generating RamDisk for download [ OK ]"
+
+
+        log_begin_msg "Configure network"
+        configure_networking || panic "Can't configure network"
+        log_success_msg "Network configured"
+
+        log_begin_msg "Download rootfs"
+        while [ ! -f ${squashfsFile} ] && [ ${tryCount} -le ${maxTryCount} ]; do
+                wget ${rooturl} -O ${squashfsFile} || log_failure_msg "Can't download rootfs, count ${tryCount}"
+                tryCount=$(( ${tryCount} + 1 ))
+                sleep 0.5
+        done
+
+        if [ -f ${squashfsFile} ]
+        then
+                log_success_msg "Rootfs downloaded"
+        else
+                panic "Can't download rootfs"
+        fi
+
+        log_begin_msg "Mount rootfs"
+        mkdir -p $squashfsMountPoint
+        mount -t squashfs -o loop $squashfsFile $squashfsMountPoint || panic "Can't mount rootfs"
+        log_success_msg "Rootfs mounted"
+
+        log_begin_msg "Mount tmpfs"
+        mkdir -p $tmpfsMountPoint
+        mount -t tmpfs none $tmpfsMountPoint || panic "Tmpfs mount failed "
+        log_success_msg "Tmpfs mounted"
+
+        log_begin_msg "Mount overlayfs"
+        mkdir -p $overlayfsUppderDir $overlayfsWorkDir $overlayfsDir
+        mount -t overlay overlay -o lowerdir=$squashfsMountPoint,upperdir=$overlayfsUppderDir,workdir=$overlayfsWorkDir $overlayfsDir \
+                || panic "Overlayfs mount failed"
+        log_success_msg "Overlayfs mounted"
+
+        log_begin_msg "Move tmpfs and squashfs to new root"
+        mkdir -p $overlayfsDir/$tmpfsMountPoint $overlayfsDir/$squashfsMountPoint
+        mount --move $squashfsMountPoint $overlayfsDir/$squashfsMountPoint  || panic "squashfs move failed"
+        mount --move $tmpfsMountPoint $overlayfsDir/$tmpfsMountPoint  || panic "tmpfs move failed"
+        log_success_msg "Tmpfs and squashfs moved"
+
+        log_begin_msg "Move overlayfs to new root"
+        mount --move $overlayfsDir ${rootmnt}  || panic ""
+}
+```