corepxe

corepxe (Very much WIP)

Pronounced "corpse-y", corepxe serves to automate the update strategy for CoreOS hosts that boot using a PXE server.

Description

CoreOS is a Linux distribution with the goal of being a read-only host system for Docker containers. The update strategy is in essence a systemd cronjob that polls the coreos omaha endpoint to see if there are any pending updates for your channel, downloads them in the background, and swaps the updated image into place on reboot. This update strategy is only used for CoreOS on-disk installations.

CoreOS also supports the ability to do network boot using PXE. This is superior (in my mind) to installing CoreOS to disk as it allows for easier management of host system versions in a central location, and you can just make an LV or a btrfs filesystem and let Docker go crazy.

Currently, updating PXE images is a manual process. This is where corepxe comes in. The update_engine requests to the coreos update endpoint is proxied through corepxe, which MITMs the response to extract the URL for the newest image.

When corepxe intercepts a request, it will check to see if there's an ongoing download. If not, then it'll parse the response to see if there is an update needed.

Snipped example of a response indicating an update is required:

  <updatecheck status="ok">
   <urls>
    <url codebase="https://commondatastorage.googleapis.com/update-storage.core-os.net/amd64-usr/410.0.0/"></url>
   </urls>
   <manifest version="410.0.0">
    <packages>
     <package hash="fCMDlzLpTyNnV8++4+kDoqeEuvA=" name="update.gz" size="111882133" required="false"></package>
    </packages>
    <actions>
     <action event="postinstall" ChromeOSVersion="" sha256="MclaAJ7f63k0cHtYs5Wv5dqGuveyXDfbYwDw7X5SaoA=" needsadmin="false" IsDelta="false" DisablePayloadBackoff="true"></action>
    </actions>
   </manifest>
  </updatecheck>

If an update is required, then the update is downloaded and placed in the appropriate directory to be served by tftp.

If there isn't an update required (<updatecheck status="noupdate"></updatecheck>) then it just passes along that response to the host that requested a check.

Since there isn't (as far as I know) any documentation about how update_engine works behind the scenes, here's a link to the source.

Notes

Example request:

<?xml version="1.0" encoding="UTF-8"?>
<request protocol="3.0" version="CoreOSUpdateEngine-0.1.0.0"
updaterversion="CoreOSUpdateEngine-0.1.0.0" installsource="scheduler"
ismachine="1">
<os version="Chateau" platform="CoreOS" sp="289.0.0"></os>
<app appid="{e96281a6-d1af-4bde-9a0a-97b76e56dc57}" oem="diskless"
version="289.0.0" track="stable" bootid="{fake-client-018}"
machineid="fake-machine-018" lang="en-US" hardware_class="" delta_okay="false"
>
<event eventtype="3" eventresult="2" previousversion=""></event>
</app>
</request>


Example response:

<?xml version="1.0" encoding="UTF-8"?>
<response protocol="3.0" server="update.core-os.net">
 <daystart elapsed_seconds="0"></daystart>
 <app appid="e96281a6-d1af-4bde-9a0a-97b76e56dc57" status="ok">
  <updatecheck status="ok">
   <urls>
    <url codebase="https://commondatastorage.googleapis.com/update-storage.core-os.net/amd64-usr/410.0.0/"></url>
   </urls>
   <manifest version="410.0.0">
    <packages>
     <package hash="fCMDlzLpTyNnV8++4+kDoqeEuvA=" name="update.gz" size="111882133" required="false"></package>
    </packages>
    <actions>
     <action event="postinstall" ChromeOSVersion="" sha256="MclaAJ7f63k0cHtYs5Wv5dqGuveyXDfbYwDw7X5SaoA=" needsadmin="false" IsDelta="false" DisablePayloadBackoff="true"></action>
    </actions>
   </manifest>
  </updatecheck>
 </app>
</response>

Glossary:

• app.appid: appid, essentially the id assigned to the release channel
• url.codebase: base url for the binary
• package.hash: base64 encoded SHA-1 hash of the binary
• package.name: filename of the binary. ''.join(url.codebase, package.name) for the download url