[vnet] IPv4 DNS Server

[nklaassen]

What would you like Teleport to do?

Serve VNet DNS requests on an IPv4 address.

What problem does this solve?

VNet currently only serves DNS requests on a Unique Local IPv6 Unicast Address. Some network policies block all IPv6 traffic, even to local addresses. To address this, VNet could make the local DNS server available on an IPv4 address.

If a workaround exists, please include it.

Allow IPv6 traffic to VNet's current DNS nameserver.

Notes:
The reason VNet currently serves DNS on an IPv6 address only is twofold:

• VNet always has a unique IPv6 address space and can unconditionally serve DNS over IPv6. IPv4 ranges are configurable and VNet might not even have a usable IPv4 range until the user logs in to a Teleport cluster.
• On MacOS the process serving DNS requests is different from the process configuring the DNS nameservers, it's slightly more complicated to coordinate between the processes to select a common (set of) addresses on which to serve DNS.

Why not serve DNS on localhost/127.0.0.1?

• DNS must be served on port 53, there's no way to configure a different port. We currently avoid using 127.0.0.1 to avoid potential conflict with other local nameservers using that port, and select an address from VNet's unique IPv6 range instead.