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diff --git a/mod/web/blog/content/log/deterministic-hostnames.md b/mod/web/blog/content/log/deterministic-hostnames.md deleted file mode 100644 index 43dc7d8..0000000 --- a/mod/web/blog/content/log/deterministic-hostnames.md +++ /dev/null @@ -1,80 +0,0 @@ ---- -title: "Deterministic and unique network hostnames" -date: "2024-11-10" ---- - -As part of building out a Kubernetes cluster, I wanted to build and distribute a -single OS image to create stateless worker nodes. Using network booting, and -some clever tricks to differentiate nodes, we can create a scaleable and -efficient farm of workers for a cluster that don't even need disks. - -The idea came from a plan to build a cluster using the -[compute blade](https://computeblade.com/), and a few Raspberry Pi SBCs I -already own. Running the cluster from an SD card is not recommended due to the -not-so-great reliability of the flash used by most manufacturers, so I wanted to -try PXE booting each Pi to save money rather than purchasing an SSD for each -one. The compute blades do support an NVMe disk, but I plan to use those for a -storage cluster later, so they need to remain empty. - -## Base image - -Alpine Linux has been my preferred server OS for a long time. It provides a very -lightweight base system, and bundles an excellent bootstrapping system, -[apkovl](https://wiki.alpinelinux.org/wiki/Alpine_local_backup), that allows the -user to save a set of customisations to an system as an overlay to a stock -Alpine live image. In other words, we can create our image once, save the -changes as an `apkovl.tar.gz` file, and apply the same changes to a base system -on boot. This file can even be provided as a -[kernel parameter](https://wiki.alpinelinux.org/wiki/PXE_boot#Guide_to_options) -and will be fetched from a remote webserver automatically! - -Since the image and configuration will be shipped to the node via the network, -an added benefit of using Alpine is its tiny space consumption. I'm not using -enough nodes for this to really matter, but it's a cool optimization regardless. - -## Differentiating the nodes - -One of the main goals of this project is that there should be no persistent -storage required outside the boot image itself. Since every node will download -and generate the same root file-system on startup, the first problem that arises -is how the nodes will identify themselves both on the network and the cluster, -given that it's not possible to name them ahead of time. In other words, any -given node has to generate a unique hostname that won't collide with other -workers, and that will be the same each time that node boots. - -Since these nodes will not have a predefined name, we have to rely on -characteristics of the hardware to differentiate each one. The hardware MAC -address is perfect for this, since it's unique to to each node and will not be -wiped away after the node reboots. On a system like Linux that exposes its -hardware through a _sysfs_, we can find a file containing the address at -`/sys/class/net/eth0/address`. I don't really like the idea of attaching the -literal MAC address of the node to its network hostname, since it's a security -risk, and a bit too verbose. Instead, we can transform it into something safer -using a `sha1sum`, which is already present on our Alpine base system: - -```console -sha1sum /sys/class/net/eth0/address | head -c 6 | awk '{print "worker-" $0}' -``` - -### Applying the new name - -Ideally, the node should apply its generated hostname before reaching out for an -address over DHCP or joining the cluster. We can make sure it happens before any -traffic is sent out by adding a `pre-up` command to the right interface in -`/etc/network/interfaces`: - -``` - -... - -auto eth0 -iface eth0 inet dhcp - pre-up sha1sum /sys/class/net/eth0/address | head -c 6 | awk '{print "worker-" $0}' > /etc/hostname - -... -``` - -The VM I tested with looks outputs `worker-e2fae8`. Pretty clean result, and if -you want to know the physical node that maps to each hostname, you can take note -of the MAC address beforehand and generate the same hash on another computer to -match them up. |