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The best way not to let randos on the internet access to your MCP server is to not put the MCP server on the internet in the first place. Every Fly Organization has a private network. In most cases, you will want only a private v6 address on applications that are not available on the internet. When using:
  • The Machine API, specify private_v6.
  • fly ips, specify allocate-v6 --private
  • fly launch, specify --flycast
With this in place you can use fly proxy to create a tunnel, or you can follow our blueprint to Jack into your private network with WireGuard. With fly mcp proxy, this support is built in. To use, simply specify a --url ending in .internal or .flycast.
  • .internal addresses can be used to target individual machines or regions, but can only be used to access machines that are started. Just remember that the protocol to use is http not https, and the port you want to use it the internal port. So an typical URL would look like http://mcp.internal:8080/.
  • .flycast addresses target an external port for your application, and supports fly routing headers. If your request is routed to a machine that is stopped or suspended, that machine will be started first. Again the protocol to use is http not https, so an typical URL would look like http://mcp.flycast/.
Flycast - Private Fly Proxy services provides more information on the use of Flycast. fly mcp wrap has a --private flag which will cause the proxy to respond with a 403 Forbidden response to all requests that do not come in via the private network. This may be useful when combined with containers and machines with multiple services, some of which are public but the MCP server is private.