Fix #284

Added DNSCrypt upstreams support
Added DNS Stamps support

dnsforward/upstream.go

@@ -13,6 +13,9 @@ import (
 	"sync"
 	"time"
 
+	"github.com/jedisct1/go-dnsstamps"
+
+	"github.com/ameshkov/dnscrypt"
 	"github.com/joomcode/errorx"
 	"github.com/miekg/dns"
 )
@@ -177,6 +180,51 @@ func (p *dnsOverHTTPS) Exchange(m *dns.Msg) (*dns.Msg, error) {
 	return &response, nil
 }
 
+//
+// DNSCrypt
+//
+type dnsCrypt struct {
+	boot       bootstrapper
+	client     *dnscrypt.Client     // DNSCrypt client properties
+	serverInfo *dnscrypt.ServerInfo // DNSCrypt server info
+
+	sync.RWMutex // protects DNSCrypt client
+}
+
+func (p *dnsCrypt) Address() string { return p.boot.address }
+
+func (p *dnsCrypt) Exchange(m *dns.Msg) (*dns.Msg, error) {
+
+	var client *dnscrypt.Client
+	var serverInfo *dnscrypt.ServerInfo
+
+	p.RLock()
+	client = p.client
+	serverInfo = p.serverInfo
+	p.RUnlock()
+
+	if client == nil || serverInfo == nil {
+		p.Lock()
+
+		// Using "udp" for DNSCrypt upstreams by default
+		client = &dnscrypt.Client{Proto: "udp", Timeout: defaultTimeout}
+		si, _, err := client.Dial(p.boot.address)
+
+		if err != nil {
+			p.Unlock()
+			return nil, errorx.Decorate(err, "Failed to fetch certificate info from %s", p.Address())
+		}
+
+		p.client = client
+		p.serverInfo = si
+		serverInfo = si
+		p.Unlock()
+	}
+
+	reply, _, err := client.Exchange(m, serverInfo)
+	return reply, err
+}
+
 func (s *Server) chooseUpstream() Upstream {
 	upstreams := s.Upstreams
 	if upstreams == nil {
@@ -200,6 +248,20 @@ func AddressToUpstream(address string, bootstrap string) (Upstream, error) {
 			return nil, errorx.Decorate(err, "Failed to parse %s", address)
 		}
 		switch url.Scheme {
+		case "sdns":
+			stamp, err := dnsstamps.NewServerStampFromString(address)
+			if err != nil {
+				return nil, errorx.Decorate(err, "Failed to parse %s", address)
+			}
+
+			switch stamp.Proto {
+			case dnsstamps.StampProtoTypeDNSCrypt:
+				return &dnsCrypt{boot: toBoot(url.String(), bootstrap)}, nil
+			case dnsstamps.StampProtoTypeDoH:
+				return AddressToUpstream(fmt.Sprintf("https://%s%s", stamp.ProviderName, stamp.Path), bootstrap)
+			}
+
+			return nil, fmt.Errorf("Unsupported protocol %v in %s", stamp.Proto, address)
 		case "dns":
 			if url.Port() == "" {
 				url.Host += ":53"