Extract several packages to form a public API

framework/dns/override.go

@@ -0,0 +1,37 @@
+package dns
+
+import (
+	"context"
+	"net"
+	"time"
+)
+
+var (
+	overrideServ string
+)
+
+// override globally overrides the used DNS server address with one provided.
+// This function is meant only for testing. It should be called before any modules are
+// initialized to have full effect.
+//
+// The server argument is in form of "IP:PORT". It is expected that the server
+// will be available both using TCP and UDP on the same port.
+func override(server string) {
+	net.DefaultResolver.PreferGo = true
+	net.DefaultResolver.Dial = func(ctx context.Context, network, address string) (net.Conn, error) {
+		dialer := net.Dialer{
+			// This is localhost, it is either running or not. Fail quickly if
+			// we can't connect.
+			Timeout: 1 * time.Second,
+		}
+
+		switch network {
+		case "udp", "udp4", "udp6":
+			return dialer.DialContext(ctx, "udp4", server)
+		case "tcp", "tcp4", "tcp6":
+			return dialer.DialContext(ctx, "tcp4", server)
+		default:
+			panic("OverrideDNS.Dial: unknown network")
+		}
+	}
+}