@Override
  public CompletableFuture<String> retrieveExternalIPAddress() {
    return externalIpQueryFuture.thenApply(x -> x);
  }
Sends a UPnP request to the discovered IGD for the external ip address.

@return A CompletableFuture that can be used to query the result (or error).
nat/src/main/java/org/hyperledger/besu/nat/upnp/UpnpNatManager.java