Added factory method for creating DNS upstreams

upstream/helpers.go

@@ -1,6 +1,84 @@
 package upstream
 
-import "github.com/miekg/dns"
+import (
+	"github.com/miekg/dns"
+	"golang.org/x/net/context"
+	"net"
+	"strings"
+)
+
+// Detects the upstream type from the specified url and creates a proper Upstream object
+func NewUpstream(url string, bootstrap string) (Upstream, error) {
+
+	proto := "udp"
+	prefix := ""
+
+	switch {
+	case strings.HasPrefix(url, "tcp://"):
+		proto = "tcp"
+		prefix = "tcp://"
+	case strings.HasPrefix(url, "tls://"):
+		proto = "tcp-tls"
+		prefix = "tls://"
+	case strings.HasPrefix(url, "https://"):
+		return NewHttpsUpstream(url, bootstrap)
+	}
+
+	hostname := strings.TrimPrefix(url, prefix)
+
+	host, port, err := net.SplitHostPort(hostname)
+	if err != nil {
+		// Set port depending on the protocol
+		switch proto {
+		case "udp":
+			port = "53"
+		case "tcp":
+			port = "53"
+		case "tcp-tls":
+			port = "853"
+		}
+
+		// Set host = hostname
+		host = hostname
+	}
+
+	// Try to resolve the host address (or check if it's an IP address)
+	bootstrapResolver := CreateResolver(bootstrap)
+	ips, err := bootstrapResolver.LookupIPAddr(context.Background(), host)
+
+	if err != nil || len(ips) == 0 {
+		return nil, err
+	}
+
+	addr := ips[0].String()
+	endpoint := net.JoinHostPort(addr, port)
+	tlsServerName := ""
+
+	if proto == "tcp-tls" && host != addr {
+		// Check if we need to specify TLS server name
+		tlsServerName = host
+	}
+
+	return NewDnsUpstream(endpoint, proto, tlsServerName)
+}
+
+func CreateResolver(bootstrap string) *net.Resolver {
+
+	bootstrapResolver := net.DefaultResolver
+
+	if bootstrap != "" {
+		bootstrapResolver = &net.Resolver{
+			PreferGo: true,
+			Dial: func(ctx context.Context, network, address string) (net.Conn, error) {
+				var d net.Dialer
+				conn, err := d.DialContext(ctx, network, bootstrap)
+				return conn, err
+			},
+		}
+	}
+
+	return bootstrapResolver
+}
 
 // Performs a simple health-check of the specified upstream
 func IsAlive(u Upstream) (bool, error) {