Pull request 1965: 3701-fallback-dns

Updates #3701.

Squashed commit of the following:

commit 5801acd3a919a55be6cb1de3b5c8afb61d5136d8
Author: Stanislav Chzhen <s.chzhen@adguard.com>
Date:   Mon Aug 21 13:37:57 2023 +0300

    all: upd chlog

commit 5c40913f76131854d321950f80ae9d5b78e56d9d
Author: Stanislav Chzhen <s.chzhen@adguard.com>
Date:   Thu Aug 17 13:15:19 2023 +0300

    dnsforward: add fallback dns servers

internal/dnsforward/config.go

@@ -118,6 +118,9 @@ type FilteringConfig struct {
 	// resolvers (plain DNS only).
 	BootstrapDNS []string `yaml:"bootstrap_dns"`
 
+	// FallbackDNS is the list of fallback DNS servers.
+	FallbackDNS []string `yaml:"fallback_dns"`
+
 	// AllServers, if true, parallel queries to all configured upstream servers
 	// are enabled.
 	AllServers bool `yaml:"all_servers"`

internal/dnsforward/dnsforward.go

@@ -263,6 +263,7 @@ func (s *Server) WriteDiskConfig(c *FilteringConfig) {
 	*c = sc
 	c.RatelimitWhitelist = stringutil.CloneSlice(sc.RatelimitWhitelist)
 	c.BootstrapDNS = stringutil.CloneSlice(sc.BootstrapDNS)
+	c.FallbackDNS = stringutil.CloneSlice(sc.FallbackDNS)
 	c.AllowedClients = stringutil.CloneSlice(sc.AllowedClients)
 	c.DisallowedClients = stringutil.CloneSlice(sc.DisallowedClients)
 	c.BlockedHosts = stringutil.CloneSlice(sc.BlockedHosts)
@@ -584,6 +585,11 @@ func (s *Server) Prepare(conf *ServerConfig) (err error) {
 		return fmt.Errorf("setting up resolvers: %w", err)
 	}
 
+	err = s.setupFallbackDNS()
+	if err != nil {
+		return fmt.Errorf("setting up fallback dns servers: %w", err)
+	}
+
 	s.recDetector.clear()
 
 	s.setupAddrProc()
@@ -593,6 +599,28 @@ func (s *Server) Prepare(conf *ServerConfig) (err error) {
 	return nil
 }
 
+// setupFallbackDNS initializes the fallback DNS servers.
+func (s *Server) setupFallbackDNS() (err error) {
+	fallbacks := s.conf.FallbackDNS
+	if len(fallbacks) == 0 {
+		return nil
+	}
+
+	uc, err := proxy.ParseUpstreamsConfig(fallbacks, &upstream.Options{
+		// TODO(s.chzhen):  Investigate if other options are needed.
+		Timeout:    s.conf.UpstreamTimeout,
+		PreferIPv6: s.conf.BootstrapPreferIPv6,
+	})
+	if err != nil {
+		// Do not wrap the error because it's informative enough as is.
+		return err
+	}
+
+	s.dnsProxy.Fallbacks = uc
+
+	return nil
+}
+
 // setupAddrProc initializes the address processor.  For internal use only.
 func (s *Server) setupAddrProc() {
 	// TODO(a.garipov): This is a crutch for tests; remove.