#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 ""
}
```