Added health-check method

upstream/helpers.go

@@ -0,0 +1,23 @@
+package upstream
+
+import "github.com/miekg/dns"
+
+// Performs a simple health-check of the specified upstream
+func IsAlive(u Upstream) (bool, error) {
+
+	// Using ipv4only.arpa. domain as it is a part of DNS64 RFC and it should exist everywhere
+	ping := new(dns.Msg)
+	ping.SetQuestion("ipv4only.arpa.", dns.TypeA)
+
+	resp, err := u.Exchange(nil, ping)
+
+	// If we got a header, we're alright, basically only care about I/O errors 'n stuff.
+	if err != nil && resp != nil {
+		// Silly check, something sane came back.
+		if resp.Response || resp.Opcode == dns.OpcodeQuery {
+			err = nil
+		}
+	}
+
+	return err == nil, err
+}