dev-docs: add rough etcd disaster recovery guide

dev-docs/howto/etcd-disaster-recovery.md

@@ -0,0 +1,139 @@
+# etcd disaster recovery
+When etcd loses quorum and (N-1)/2 control planes are lost, 
+etcd will not be able to perform any transactions anymore and will basically stall and cause timeouts.
+This makes the Constellation control planes unusable, resulting in a frozen cluster. The worker nodes will continue to function for a bit,
+but given that they cannot communicate to the control plane anymore, they will eventually also cease to function correctly.
+
+If the missing control plane nodes are still existent and their state disk still exists, you likely do not need this guide.
+If the missing are irrecoverably lost (e.g. scaled up and down the control plane instance set), follow through this guide to bring the cluster back up.
+
+## General concept
+1. Create snapshot of a state disk from a remaining control plane node.
+2. Download the snapshot and decrypt it locally
+3. Follow the [restoring a cluster](https://etcd.io/docs/v3.5/op-guide/recovery/#restoring-a-cluster) guide from etcd.
+4. Save the modified virtual disk and upload it back to the CSP.
+5. Modify the scale set (or remaining VM singularly, if you can) to use the new uploaded data disk.
+6. Reboot, wait a few minutes.
+7. Pray it workded ;)
+
+## How I did it once (Azure)
+
+1. If the VM has never been rebooted once after initialization, reboot it once to sync any LUKS passphrase changes to disk (not 100% sure if necessary to sync the change to the passphrase - would have to double-check that later with an experimental cluster)
+
+2. Create a snapshot from the disk using the CLI:
+```bash
+az snapshot create --resource-group dogfooding --name dogfooding-3 --source /subscriptions/0d202bbb-4fa7-4af8-8125-58c269a05435/resourceGroups/dogfooding/providers/Microsoft.Compute/disks/constell-f2332c74-coconstell-f2332c74-condisk2_dd460a6ae3124aa3a4c23be0ab39634e --location northeurope
+```
+
+3. Look up the snapshot online, export it as VHD
+Mount the disk: 
+```bash
+modprobe nbd && sudo qemu-nbd -c /dev/nbd0 /home/nils/Downloads/constellation-disk.vhd
+```
+
+4. Get the UUID of the disk: 
+```bash 
+sudo cryptsetup luksDump /dev/nbd0
+```
+
+5. Regenerate the passphrase to unlock the disk (the [code snippet below](#get-disk-decryption-key) might be useful for this)
+
+6. Decrypt the disk: 
+```bash
+sudo cryptsetup luksOpen /dev/nbd0 constellation-state --key-file passphrase
+```
+
+7. Mount the decrypted disk (I just did this via the Nautilus)
+
+8. Find the db file from etcd in `/var/lib/etcd/member/snap/db`
+
+9. Perform the etcd [Restoring a Cluster](https://etcd.io/docs/v3.5/op-guide/recovery/#restoring-a-cluster) step:
+
+```bash
+./etcdutl snapshot restore db --initial-cluster constell-f2332c74-control-plane000001=https://10.9.126.0:2380 --initial-advertise-peer-urls https://10.9.126.0:2380  --data-dir recovery --name constell-f2332c74-control-plane000001 --skip-hash-check=true
+```
+*(replace name & IP with the name & the private IP of the remaining control plane VM - this can be seen in the Azure portal)*
+
+10. Copy the contents of the newly created recovery directory to the mounted state disk and remove any remaining old files. 
+**Make sure the permissions are the same as before!**
+
+11. Unmount the partition:
+```bash 
+sudo umount /your/mount/path
+sudo luksClose constellation-state
+sudo qemu-nbd -d /dev/nbd0
+```
+
+12. Upload the modified VHD back to Azure (I just used Azure Storage Explorer for this).
+
+13. Patch the whole control-plane VMSS to remove LUN 0 from the VMs: 
+```bash
+az vmss disk detach --lun 0 --resource-group dogfooding --vmss-name constell-f2332c74-control-plane
+```
+
+14. Update the VM: 
+```bash
+az vmss update-instances -g dogfooding --name constell-f2332c74-control-plane --instance-ids 1
+```
+
+15. Attach the uploaded disk as LUN 0 (either via CLI or Azure Portal, I just used the Azure Portal).
+
+16. Start the VM and pray it works ;) It can take a few minutes before etcd becomes fully alive again.
+
+17. Patch the state disk definition back to the VMSS (no idea how, haven't done his yet) so newly created VMs in the VMSS have a clean state disk again.
+
+## Get disk decryption key
+```golang
+package main
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+	"encoding/json"
+	"fmt"
+	"io"
+	"os"
+
+	"golang.org/x/crypto/hkdf"
+)
+
+type MasterSecret struct {
+	Key  []byte `json:"key"`
+	Salt []byte `json:"salt"`
+}
+
+func main() {	
+	uuid := "4ae66293-57aa-4821-b99c-ebc598a6e5a8" // replace me
+
+	masterSecretRaw, err := os.ReadFile("constellation-mastersecret.json")
+	if err != nil {
+		panic(err)
+	}
+
+	var masterSecret MasterSecret
+	if err := json.Unmarshal(masterSecretRaw, &masterSecret); err != nil {
+		panic(err)
+	}
+
+	dek, err := DeriveKey(masterSecret.Key, masterSecret.Salt, []byte("key-"+uuid), 32)
+	if err != nil {
+		panic(err)
+	}
+
+	fmt.Println(hex.EncodeToString(dek))
+
+	if err := os.WriteFile("passphrase", dek, 0o644); err != nil {
+		panic(err)
+	}
+}
+
+// DeriveKey derives a key from a secret.
+func DeriveKey(secret, salt, info []byte, length uint) ([]byte, error) {
+	hkdfReader := hkdf.New(sha256.New, secret, salt, info)
+	key := make([]byte, length)
+	if _, err := io.ReadFull(hkdfReader, key); err != nil {
+		return nil, err
+	}
+	return key, nil
+}
+```