Captcha can be enabled for this web client / home server. This file explains how to do that.
The captcha mechanism used is Google's ReCaptcha. This requires API keys from Google.

Getting keys
------------
Requires a public/private key pair from:

https://developers.google.com/recaptcha/


Setting Private ReCaptcha Key
-----------------------------
The private key is a config option on the home server config. If it is not 
visible, you can generate it via --generate-config. Set the following value:

  recaptcha_private_key: YOUR_PRIVATE_KEY
  
In addition, you MUST enable captchas via:

  enable_registration_captcha: true

Setting Public ReCaptcha Key
----------------------------
The web client will look for the global variable webClientConfig for config 
options. You should put your ReCaptcha public key there like so:

webClientConfig = {
    useCaptcha: true,
    recaptcha_public_key: "YOUR_PUBLIC_KEY"
};

This should be put in webclient/config.js which is already .gitignored, rather 
than in the web client source files. You MUST set useCaptcha to true else a
ReCaptcha widget will not be generated.

Configuring IP used for auth
----------------------------
The ReCaptcha API requires that the IP address of the user who solved the
captcha is sent. If the client is connecting through a proxy or load balancer,
it may be required to use the X-Forwarded-For (XFF) header instead of the origin
IP address. This can be configured as an option on the home server like so:

  captcha_ip_origin_is_x_forwarded: true