It is currently easy to enter an invalid agent hostname in tl-setup and web admin. We could implement checks to indicate to the admin if the entered hostname is likely to be valid. Here are some ideas: * Check if the IP address is in a public or private range * Check if the hostname looks like a fqdn Once those things are in place, we could attempt more complex systems that are technically uncertain, or require external services: * A reverse DNS lookup of the machine's IP address, hopefully getting something that is more universally useful * Look up candidate host names in DNS and check if the result is in a public or private range * Do the above DNS lookup with an external DNS (e.g. Google's) to get an external perspective * Is the hostname using a valid TLD? * Request an external service to attempt a connection to our candidate address * Ask the user to run some command from a client to attempt a connection to our candidate address
