diff --git a/content/u/blog/how-to-build-an-openstack-alternative-step-3-automating-mac-addresses/contents.lr b/content/u/blog/how-to-build-an-openstack-alternative-step-3-automating-mac-addresses/contents.lr
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+title: How to build an OpenStack alternative: Step 3, Generating Mac Addresses
+---
+pub_date: 2020-01-13
+---
+author: ungleich virtualisation team
+---
+twitter_handle: ungleich
+---
+_hidden: no
+---
+_discoverable: yes
+---
+abstract:
+First step to automation
+---
+body:
+
+This time we describe how to begin automation in uncloud,
+our OpenStack alternative.
+
+The previous time we described
+[how to secure the network](../how-to-build-an-openstack-alternative-step-2-secure-the-network/).
+
+## Mac Addresses
+
+Mac addresses are identifiers for network cards. Like a street number
+is for a house. A virtual machine can have one or more network
+interfaces. Each of these interfaces need a unique number, the mac
+address.
+
+## Choosing the prefix
+
+We checked how [OpenNebula](https://opennebula.org/) is doing it.
+They basically use the **02:00** prefix and generate mac addresses after
+that. The
+[QEMU mac address
+prefix](https://gist.github.com/ashee/9241ab6281e6f4d1ef9b)
+seems to be 52:54:00.
+According to [Michael
+Stapelberg](https://michael.stapelberg.ch/posts/2012-11-21-kvm_best_practices_2012/)
+the **f6** prefix can be used due to the second lowest bit set to 1.
+
+Checking with [wikipedia](https://en.wikipedia.org/wiki/MAC_address),
+this bit indeed specifies whether or not a mac address is locally or
+universally administrated.
+
+For that reason we will use **42** as our prefix, which is **1000010**
+in binary and thus has the second least significant bit set to 1.
+
+## Generating Mac addresses
+
+Mac addresses are basically just numbers and so we treat them as
+such. We need to store the index of which is the current mac address
+and for the next time we create a VM, use the next mac address.
+
+The python code to generate a new mac address looks as follows:
+
+```
+    @staticmethod
+    def int_to_mac(number):
+        b = number.to_bytes(6, byteorder="big")
+        return ':'.join(format(s, '02x') for s in b)
+
+    def getnext(self):
+        if self.last:
+            last_number = int(self.last[0], 16)
+
+            if last_number == int('0xffffff', 16):
+                raise Error("Exhausted all possible mac addresses - try to free some")
+
+            next_number = last_number + 1
+        else:
+            next_number = 0
+
+        next_number_string = "{:012x}".format(next_number)
+
+        next_mac_number = self.prefix + next_number
+        next_mac = self.int_to_mac(next_mac_number)
+```
+
+It basically says:
+
+* If there has been a mac address used add 1 to it and use the next
+  one
+* Produce an error, if the whole prefix has been used
+* If no mac address has been used, use the first one
+* Convert the number to the standard format (aa:bb:cc:dd:ee:ff)
+
+There is some additional logic on saving the MAC address to which we
+will come back in one of the next posts.
+
+## Using the MAC address generator
+
+From our prototype, we modify the original `vm.sh` script to include a
+call to the mac address generator as follows:
+
+```
+...
+mac=$(./mac-gen.py)
+...
+$qemu -name "uncloud-${uuid}" \
+      -machine pc,accel=${accel} \
+      -m ${memory} \
+      -smp ${cores} \
+      -uuid ${uuid} \
+      -drive file=alpine-virt-3.11.2-x86_64.iso,media=cdrom \
+      -netdev tap,id=netmain,script=./ifup.sh,downscript=./ifdown.sh \
+      -device virtio-net-pci,netdev=netmain,id=net0,mac=${mac}
+```
+
+In good UNIX manner, the generator simply outputs the MAC address on
+stdout to be used by any software.
+
+## Status
+
+While being a small and very specific tool, the mac address generator
+is actually a key element of creating a virtualisation
+framework. Without it, it is practically impossible to spawn a dynamic
+amount of virtual machines.
+
+With our prototype, a secured network and the mac address generator we
+are not far from being able to migrate some test VMs to uncloud.!
diff --git a/content/u/blog/uncloud-next b/content/u/blog/uncloud-next
index 68b7819..4e07b4f 100644
--- a/content/u/blog/uncloud-next
+++ b/content/u/blog/uncloud-next
@@ -1,8 +1,6 @@
-- how to secure the network
-- rate limiting
+- metadata!
 - automating
 - trigger / postgres / etcd
-- metadata!
 - building vm images!
 - generate tap names matching vm??? problem, because inteerfaces are
   limited in length!
@@ -63,3 +61,5 @@ table bridge filter {
 can generate new full chain, change only the jump
 
 --------------------------------------------------------------------------------
+Much later:
+- rate limiting