Replacing non-Alpine Linux with Alpine remotely

* What: These instructions are for installing Alpine Linux on a hosted physical server from a hosting company like serverpronto, bluehost, etc.
* Why: Typically these companies run Debian, Fedora, or another Linux distribution. This document explains how to get Alpine Linux on a machine with nothing but ssh access.
== With VNC access ==
Download an alpine iso that boots from ram, e.g. alpine-virt
 wget dl-cdn.alpinelinux.org/alpine/v3.5/releases/x86_64/alpine-virt-3.5.2-x86_64.iso
Flash the image to the drive of your server, e.g. /dev/sda
 dd if=alpine-virt-3.5.2-x86_64.iso of=/dev/sda
Reboot, and login again from vnc with root. Alpine is now running from ram, and should have mounted /dev/sda on /media/sda, paste it into ram.
 mkdir /media/setup
 cp -a /media/sda/* /media/setup
Copy also the kernel modules which are located in /.modloop
 mkdir /lib/setup
 cp -a /.modloop/* /lib/setup
Unmount modloop and the media folder
 rc-service modloop stop
 umount /dev/sda
Move back the files in place
 mv /media/setup/* /media/sda/
 mv /lib/setup/* /.modloop/
Finally you can format the disk to install alpine
 setup-disk
Or just run the the whole setup
 setup-alpine
== Without VNC access ==
Prerequisites:
* SSH access to the remote box (needs to have SSH server running)
* Alpine Linux supported hardware
* Adventurous mind
* Nerves of steel
These instructions are based on a debian (physical) server, and all steps are performed while logged into the machine. So you should start with a machine that has ssh running.
== Create apk overlay suitable for hard disk based tmpfs boot ==
The first step is to create Alpine configuration file with basic configuration of the host. We need the new box to start networking and ssh in the beginning so we can reconnect to it after reboot.
Create basic layout for the overlay:
 mkdir overlay
 cd overlay
 mkdir -p etc/ssh etc/network etc/runlevels/{default,boot,sysinit,shutdown} root/.ssh etc/lbu
You can also use default Alpine configuration files. In this case you should use ssh key to authorize yourself (as root password is empty, and ssh has empty passwords disabled).
If you want to keep the existing host identity (e.g. SSH key), you can copy them over:
 cp -a /etc/{passwd,group,shadow,gshadow,hostname,resolv.conf,network/interfaces,ssh} etc/
 cp /etc/network/interfaces etc/network
Copy over your ssh authorized_keys and make sure its included in future:
 cp -a /root/.ssh/authorized_keys root/.ssh
 echo "/root/.ssh" > etc/lbu/include
Find out which shell is used for root:
 grep ^root /etc/passwd
If its /bin/ash, you are good. If not, edit etc/passwd and change it to /bin/ash.
 sed -i -e '/^root:/s:/bin/bash:/bin/ash:' etc/passwd
Create etc/network/interfaces with network configuration (unless it was previously copied over). It should look something like:
 auto lo
 iface lo inet loopback
 
 auto eth0
 iface eth0 inet static
 address ip.ad.dr.es
 netmask 255.255.255.0
 gateway gw.ad.dr.es
Since Alpine 3.13 must be:
 auto lo
 iface lo inet loopback
 
 auto eth0
 iface eth0 inet static
 address ip.ad.dr.es/cidr
 gateway gw.ad.dr.es
Make sure there is no whitespace at end of lines in interfaces file. Busybox ifup is very picky.
Make sure your etc/resolv.conf exists; if not create etc/resolv.conf with the nameserver configuration like:
 nameserver dns.ip.ad.dr
 options edns0 trust-ad single-request-reopen
Create the apk world (etc/apk/world) with essential packages:
 mkdir -p etc/apk
 echo "alpine-base iproute2 openssh" > etc/apk/world
Double check the IP configuration and ssh keys.
Finally, make the essential services start up automatically and create the overlay file:
 ln -s /etc/init.d/{hwclock,modules,sysctl,hostname,bootmisc,syslog} etc/runlevels/boot/
 ln -s /etc/init.d/{devfs,dmesg,mdev,hwdrivers} etc/runlevels/sysinit/
 ln -s /etc/init.d/{networking,sshd} etc/runlevels/default/
 ln -s /etc/init.d/{mount-ro,killprocs,savecache} etc/runlevels/shutdown/
 tar czf ../host.apkovl.tar.gz *
Verify the overlay with "tar tzf" to see that it contains everything in proper places, and ensure it is in the / directory
 tar tzvf host.apkovl.tar.gz
 cp host.apkovl.tar.gz /
== Install Alpine cd-rom image on hard disk ==
We need to copy over two sets of information: the boot kernel (kernel, initramdisk and boot configuration) and operating system boot data (overlay, apk packages and kernel modules). These can reside on same partition if they fit. However, /boot is usually small, so you might want to put the apks on separate partition. This guide assumes they are on sda1 (/boot) and sda2 (/) with both having ext3 filesystems. If you don't have ext3 on / or /boot, then you might be able to disable swap and reformat the swap partition as ext3 and use that.
Download an alpine iso and mount it; for example
 wget 
 mount alpine*.iso /cdrom -o loop
Copy the apkovl and the contents of cd-rom image to root of current installation:
 cp host.apkovl.tar.gz /
 cp -a /cdrom/* /
Next we make the Alpine Linux kernel bootable (requires syslinux) and use fdisk to mark the /boot partition as bootable (if not done already):
 dd if=/usr/share/syslinux/mbr.bin of=/dev/sda # on some older systems its /usr/lib/syslinux/mbr.bin
 fdisk /dev/sda
Make sure that /boot dir has a symlink pointing to itself. This is to handle the case when /boot is on separate partition.
 ln -sf . /boot/boot
Create /boot/extlinux.conf with contents like (check the filesystem type for alpine_dev; kernel and initrd are relative to partition root):
 timeout 20
 prompt 1
 default grsec
 label grsec
 kernel /boot/grsec
 append initrd=/boot/grsec.gz alpine_dev=sda2:ext3 modloop=grsec.cmg modules=loop,cramfs,sd-mod,usb-storage,ext3 quiet
From Alpine Linux 2.1 you can use the UUID of the partition that holds the apkovl and the apks directory, the current root. Use blkid to get the proper UUID. By using UUID we solve the problem when there are multiple disks and we don't knowing how kernel enumerates them after boot. Example extlinux.conf:
 timeout 20
 prompt 1
 default grsec
 label grsec
 	kernel /boot/grsec
 	append initrd=/boot/grsec.gz alpine_dev=UUID=44ea33e2-deb4-4a29-88e2-fc8a4ef80ab0:ext3 modloop=grsec.cmg modules=loop,cramfs,sd-mod,usb-storage,ext3 quiet
[Comment added by IceCodeNew]
I tried this instruction on Ubuntu 20.04, and I'm installing Alpine-virt 3.13.2.
I found there is no such file named `*/syslinux/mbr.bin`, but even if I skip the step which issuing `dd if=/usr/share/syslinux/mbr.bin of=/dev/sda`, the machine was able to boot alpine without problem.
Here is the extlinux.conf I use (since there are lots of files that had been renamed): 
 timeout 20
 prompt 1
 default grsec
 label grsec
 	kernel /boot/vmlinuz-virt
 	append initrd=/boot/initramfs-virt alpine_dev=UUID=db153994-91fe-426f-bd88-f751e07f97f2:ext4 modloop=/boot/modloop-virt modules=loop,cramfs,sd-mod,usb-storage,ext4 quiet
[End of comment]
Finally make the /boot partition bootable by extlinux.
 extlinux -i /boot
(Tested on 2.1.2 and higher) Make sure:
* grsec.cmg is on /
* [something].apkovl.tar.gz is on /
* grsec.gz is on /boot
* The apline_dev UUID value points to whatever is currently /
* the partition for /boot is flagged as bootable
* the output of extlinux was for the partition currently mounted at /boot
== Install Alpine on hard-disk ==
Reboot the box. And wait for it to come alive again. If it doesn't, tough luck. So dry practice with local box, with as identical hardware as possible.
Once back in the box, we are running in Alpine Linux from tmpfs. So just do a regular installation of Alpine from network via ssh connection.
Edit /etc/apk/repositories to contain your favorite Alpine mirror or just:
 https://dl-cdn.alpinelinux.org/alpine/v3.18/main/
Finally run:
 rc-service modloop stop
 apk update
 setup-disk
After hard disk setup is complete, copy ssh_authorized_keys to hard disk.
Reboot once more, and you have the server with native Alpine hard disk installation.
Category:Installation