Get a list of all domains that a website must contact in order to work properly

I am designing a piece of software for a customer who wants to use a firewall to limit web browsing access to a certain set of domains. Because of the way websites are built, it is not sufficient to simply whitelist these domains; I must also whitelist all the domains which these sites must contact in order to properly display content.

I'm relatively inexperienced in web development, and while I have been able to get some sites successfully whitelisted just by poking around the developer console in Chrome and blindly whitelisting every domain I can find in the source for a given site, I have no systematic way of figuring out exactly what needs to be whitelisted in order for a site to function.

Is there a way to conveniently get a list of all the domains that a site is using/could potentially use?

Some details about the project itself:

• We are working on this for Samsung devices running Android OS, though I'm not sure if that really matters.
• The firewall rules are enforced using Samsung's built in Knox API. I can feed that API either a domain or an IP. Domains will be resolved using DNS, so these are preferred to IPs which might change over time resulting in an outdated firewall policy.
• The customer has specifically requested that this be done using firewall rules, not through other means, such as a restricted browser.