resolver: more liberal factory

resolver/factory.go

@@ -0,0 +1,52 @@
+package resolver
+
+import (
+	"errors"
+	"net"
+	"net/url"
+	"strings"
+
+	"github.com/ncruces/go-dns"
+)
+
+func FromURL(u string) (*net.Resolver, error) {
+	parsed, err := url.Parse(u)
+	if err != nil {
+		return nil, err
+	}
+	host := parsed.Hostname()
+	port := parsed.Port()
+	switch scheme := strings.ToLower(parsed.Scheme); scheme {
+	case "", "udp", "dns":
+		if port == "" {
+			port = "53"
+		}
+		return NewPlainResolver(net.JoinHostPort(host, port)), nil
+	case "tcp":
+		if port == "" {
+			port = "53"
+		}
+		return NewTCPResolver(net.JoinHostPort(host, port)), nil
+	case "http", "https", "doh":
+		if port == "" {
+			if scheme == "http" {
+				port = "80"
+			} else {
+				port = "443"
+			}
+		}
+		if scheme == "doh" {
+			parsed.Scheme = "https"
+			u = parsed.String()
+		}
+		return dns.NewDoHResolver(u, dns.DoHAddresses(net.JoinHostPort(host, port)))
+	case "tls", "dot":
+		if port == "" {
+			port = "853"
+		}
+		hp := net.JoinHostPort(host, port)
+		return dns.NewDoTResolver(hp, dns.DoTAddresses(hp))
+	default:
+		return nil, errors.New("not implemented")
+	}
+}