feat: preshared key, client port and DoH

Closes #2, Closes #3

internal/resolver/resolver.go

@@ -0,0 +1,55 @@
+package resolver
+
+import (
+	"context"
+	"crypto/tls"
+	"net"
+	"strings"
+)
+
+func New(addr string) *net.Resolver {
+	switch {
+	case strings.HasPrefix(addr, "tls://"):
+		return &net.Resolver{
+			PreferGo: true,
+			Dial: func(ctx context.Context, _, _ string) (net.Conn, error) {
+				d := tls.Dialer{}
+				address := addr[len("tls://"):]
+				return d.DialContext(ctx, "tcp", withDefaultPort(address, "853"))
+			},
+		}
+	case strings.HasPrefix(addr, "https://"):
+		return &net.Resolver{
+			PreferGo: true,
+			Dial: func(_ context.Context, _, _ string) (net.Conn, error) {
+				conn := &dohConn{addr: addr}
+				return conn, nil
+			},
+		}
+	case addr != "":
+		return &net.Resolver{
+			PreferGo: true,
+			Dial: func(ctx context.Context, _, _ string) (net.Conn, error) {
+				d := net.Dialer{}
+				address := addr
+				network := "udp"
+
+				if strings.HasPrefix(addr, "tcp://") || strings.HasPrefix(addr, "udp://") {
+					network = addr[:len("tcp")]
+					address = addr[len("tcp://"):]
+				}
+
+				return d.DialContext(ctx, network, withDefaultPort(address, "53"))
+			},
+		}
+	default:
+		return &net.Resolver{}
+	}
+}
+
+func withDefaultPort(addr, port string) string {
+	if _, _, err := net.SplitHostPort(addr); err == nil {
+		return addr
+	}
+	return net.JoinHostPort(addr, port)
+}