How to solve Home Assistant '400 Bad Request' error with Synology Reverse Proxy

Some assumptions

You have Home Assistant running nicely on your internal network.
You have a Synology Diskstation running.
You have a (sub)domain pointing to your home network IP / Synology.
You have already created a Reverse Proxy rule for Home Assistant.

Edit Reverse Proxy rule for Home Assistant

Since you already made a Reverse Proxy rule for your Home Assistant server, it is reachable from outside your internal network, but you get that error message. Here’s how to solve it.

Step 1

Open Synology DSM and navigate to Control Panel -> Login Portal -> Advanced. There click on the Reverse Proxy button. You should see your Reverse Proxy rule for Home Assistant, select it and click Edit.

Step 2

In the modal window click on Custom Header. Now click the Create button and select WebSocket.

Adding WebSocket headers to the Reverse Proxy rule. — Adding WebSocket headers to the Reverse Proxy rule

Now you should have these two headers added to the configuration:

Upgrade: $http_upgrade
Connection: $connection_upgrade

Step 3

Since we now have the Reverse Proxy configuration set up, we need to tell Home Assistant that a trusted proxy is used. Read more on this in the Home Assistant documentation.

Add the following to your configuration.yaml file:

http:
  use_x_forwarded_for: true
  trusted_proxies:
    - xxx.xxx.xxx.xxx # IP of Synology
                yaml

Now restart Home Assistant, and all should be working!