package dnsforward import ( "crypto/tls" "errors" "fmt" "net" "net/http" "runtime" "strings" "sync" "time" "github.com/AdguardTeam/AdGuardHome/dnsfilter" "github.com/AdguardTeam/AdGuardHome/querylog" "github.com/AdguardTeam/AdGuardHome/stats" "github.com/AdguardTeam/dnsproxy/proxy" "github.com/AdguardTeam/dnsproxy/upstream" "github.com/AdguardTeam/golibs/log" "github.com/joomcode/errorx" "github.com/miekg/dns" ) // DefaultTimeout is the default upstream timeout const DefaultTimeout = 10 * time.Second const ( safeBrowsingBlockHost = "standard-block.dns.adguard.com" parentalBlockHost = "family-block.dns.adguard.com" ) var defaultDNS = []string{ "https://dns.quad9.net/dns-query", } var defaultBootstrap = []string{"9.9.9.9", "149.112.112.112"} // Server is the main way to start a DNS server. // // Example: // s := dnsforward.Server{} // err := s.Start(nil) // will start a DNS server listening on default port 53, in a goroutine // err := s.Reconfigure(ServerConfig{UDPListenAddr: &net.UDPAddr{Port: 53535}}) // will reconfigure running DNS server to listen on UDP port 53535 // err := s.Stop() // will stop listening on port 53535 and cancel all goroutines // err := s.Start(nil) // will start listening again, on port 53535, in a goroutine // // The zero Server is empty and ready for use. type Server struct { dnsProxy *proxy.Proxy // DNS proxy instance dnsFilter *dnsfilter.Dnsfilter // DNS filter instance queryLog querylog.QueryLog // Query log instance stats stats.Stats access *accessCtx webRegistered bool sync.RWMutex conf ServerConfig } // NewServer creates a new instance of the dnsforward.Server // Note: this function must be called only once func NewServer(dnsFilter *dnsfilter.Dnsfilter, stats stats.Stats, queryLog querylog.QueryLog) *Server { s := &Server{} s.dnsFilter = dnsFilter s.stats = stats s.queryLog = queryLog if runtime.GOARCH == "mips" || runtime.GOARCH == "mipsle" { // Use plain DNS on MIPS, encryption is too slow defaultDNS = defaultBootstrap } return s } // Close - close object func (s *Server) Close() { s.Lock() s.dnsFilter = nil s.stats = nil s.queryLog = nil s.Unlock() } // WriteDiskConfig - write configuration func (s *Server) WriteDiskConfig(c *FilteringConfig) { s.Lock() *c = s.conf.FilteringConfig s.Unlock() } // FilteringConfig represents the DNS filtering configuration of AdGuard Home // The zero FilteringConfig is empty and ready for use. type FilteringConfig struct { // Filtering callback function FilterHandler func(clientAddr string, settings *dnsfilter.RequestFilteringSettings) `yaml:"-"` // This callback function returns the list of upstream servers for a client specified by IP address GetUpstreamsByClient func(clientAddr string) []string `yaml:"-"` ProtectionEnabled bool `yaml:"protection_enabled"` // whether or not use any of dnsfilter features BlockingMode string `yaml:"blocking_mode"` // mode how to answer filtered requests BlockedResponseTTL uint32 `yaml:"blocked_response_ttl"` // if 0, then default is used (3600) Ratelimit int `yaml:"ratelimit"` // max number of requests per second from a given IP (0 to disable) RatelimitWhitelist []string `yaml:"ratelimit_whitelist"` // a list of whitelisted client IP addresses RefuseAny bool `yaml:"refuse_any"` // if true, refuse ANY requests BootstrapDNS []string `yaml:"bootstrap_dns"` // a list of bootstrap DNS for DoH and DoT (plain DNS only) AllServers bool `yaml:"all_servers"` // if true, parallel queries to all configured upstream servers are enabled AllowedClients []string `yaml:"allowed_clients"` // IP addresses of whitelist clients DisallowedClients []string `yaml:"disallowed_clients"` // IP addresses of clients that should be blocked BlockedHosts []string `yaml:"blocked_hosts"` // hosts that should be blocked // IP (or domain name) which is used to respond to DNS requests blocked by parental control or safe-browsing ParentalBlockHost string `yaml:"parental_block_host"` SafeBrowsingBlockHost string `yaml:"safebrowsing_block_host"` CacheSize uint `yaml:"cache_size"` // DNS cache size (in bytes) UpstreamDNS []string `yaml:"upstream_dns"` } // TLSConfig is the TLS configuration for HTTPS, DNS-over-HTTPS, and DNS-over-TLS type TLSConfig struct { TLSListenAddr *net.TCPAddr `yaml:"-" json:"-"` CertificateChain string `yaml:"certificate_chain" json:"certificate_chain"` // PEM-encoded certificates chain PrivateKey string `yaml:"private_key" json:"private_key"` // PEM-encoded private key CertificatePath string `yaml:"certificate_path" json:"certificate_path"` // certificate file name PrivateKeyPath string `yaml:"private_key_path" json:"private_key_path"` // private key file name CertificateChainData []byte `yaml:"-" json:"-"` PrivateKeyData []byte `yaml:"-" json:"-"` } // ServerConfig represents server configuration. // The zero ServerConfig is empty and ready for use. type ServerConfig struct { UDPListenAddr *net.UDPAddr // UDP listen address TCPListenAddr *net.TCPAddr // TCP listen address Upstreams []upstream.Upstream // Configured upstreams DomainsReservedUpstreams map[string][]upstream.Upstream // Map of domains and lists of configured upstreams OnDNSRequest func(d *proxy.DNSContext) FilteringConfig TLSConfig // Called when the configuration is changed by HTTP request ConfigModified func() // Register an HTTP handler HTTPRegister func(string, string, func(http.ResponseWriter, *http.Request)) } // if any of ServerConfig values are zero, then default values from below are used var defaultValues = ServerConfig{ UDPListenAddr: &net.UDPAddr{Port: 53}, TCPListenAddr: &net.TCPAddr{Port: 53}, FilteringConfig: FilteringConfig{BlockedResponseTTL: 3600}, } // Start starts the DNS server func (s *Server) Start(config *ServerConfig) error { s.Lock() defer s.Unlock() return s.startInternal(config) } // startInternal starts without locking func (s *Server) startInternal(config *ServerConfig) error { err := s.prepare(config) if err != nil { return err } return s.dnsProxy.Start() } // Prepare the object func (s *Server) prepare(config *ServerConfig) error { if s.dnsProxy != nil { return errors.New("DNS server is already started") } if config != nil { s.conf = *config } if len(s.conf.UpstreamDNS) == 0 { s.conf.UpstreamDNS = defaultDNS } if len(s.conf.BootstrapDNS) == 0 { s.conf.BootstrapDNS = defaultBootstrap } upstreamConfig, err := proxy.ParseUpstreamsConfig(s.conf.UpstreamDNS, s.conf.BootstrapDNS, DefaultTimeout) if err != nil { return fmt.Errorf("DNS: proxy.ParseUpstreamsConfig: %s", err) } s.conf.Upstreams = upstreamConfig.Upstreams s.conf.DomainsReservedUpstreams = upstreamConfig.DomainReservedUpstreams if len(s.conf.ParentalBlockHost) == 0 { s.conf.ParentalBlockHost = parentalBlockHost } if len(s.conf.SafeBrowsingBlockHost) == 0 { s.conf.SafeBrowsingBlockHost = safeBrowsingBlockHost } if s.conf.UDPListenAddr == nil { s.conf.UDPListenAddr = defaultValues.UDPListenAddr } if s.conf.TCPListenAddr == nil { s.conf.TCPListenAddr = defaultValues.TCPListenAddr } proxyConfig := proxy.Config{ UDPListenAddr: s.conf.UDPListenAddr, TCPListenAddr: s.conf.TCPListenAddr, Ratelimit: s.conf.Ratelimit, RatelimitWhitelist: s.conf.RatelimitWhitelist, RefuseAny: s.conf.RefuseAny, CacheEnabled: true, CacheSizeBytes: int(s.conf.CacheSize), Upstreams: s.conf.Upstreams, DomainsReservedUpstreams: s.conf.DomainsReservedUpstreams, BeforeRequestHandler: s.beforeRequestHandler, RequestHandler: s.handleDNSRequest, AllServers: s.conf.AllServers, } s.access = &accessCtx{} err = s.access.Init(s.conf.AllowedClients, s.conf.DisallowedClients, s.conf.BlockedHosts) if err != nil { return err } if s.conf.TLSListenAddr != nil && len(s.conf.CertificateChainData) != 0 && len(s.conf.PrivateKeyData) != 0 { proxyConfig.TLSListenAddr = s.conf.TLSListenAddr keypair, err := tls.X509KeyPair(s.conf.CertificateChainData, s.conf.PrivateKeyData) if err != nil { return errorx.Decorate(err, "Failed to parse TLS keypair") } proxyConfig.TLSConfig = &tls.Config{ Certificates: []tls.Certificate{keypair}, MinVersion: tls.VersionTLS12, } } if len(proxyConfig.Upstreams) == 0 { log.Fatal("len(proxyConfig.Upstreams) == 0") } if !s.webRegistered && s.conf.HTTPRegister != nil { s.webRegistered = true s.registerHandlers() } // Initialize and start the DNS proxy s.dnsProxy = &proxy.Proxy{Config: proxyConfig} return nil } // Stop stops the DNS server func (s *Server) Stop() error { s.Lock() defer s.Unlock() return s.stopInternal() } // stopInternal stops without locking func (s *Server) stopInternal() error { if s.dnsProxy != nil { err := s.dnsProxy.Stop() s.dnsProxy = nil if err != nil { return errorx.Decorate(err, "could not stop the DNS server properly") } } return nil } // IsRunning returns true if the DNS server is running func (s *Server) IsRunning() bool { s.RLock() isRunning := true if s.dnsProxy == nil { isRunning = false } s.RUnlock() return isRunning } // Restart - restart server func (s *Server) Restart() error { s.Lock() defer s.Unlock() log.Print("Start reconfiguring the server") err := s.stopInternal() if err != nil { return errorx.Decorate(err, "could not reconfigure the server") } err = s.startInternal(nil) if err != nil { return errorx.Decorate(err, "could not reconfigure the server") } return nil } // Reconfigure applies the new configuration to the DNS server func (s *Server) Reconfigure(config *ServerConfig) error { s.Lock() defer s.Unlock() log.Print("Start reconfiguring the server") err := s.stopInternal() if err != nil { return errorx.Decorate(err, "could not reconfigure the server") } // On some Windows versions the UDP port we've just closed in proxy.Stop() doesn't get actually closed right away. if runtime.GOOS == "windows" { time.Sleep(1 * time.Second) } err = s.startInternal(config) if err != nil { return errorx.Decorate(err, "could not reconfigure the server") } return nil } // ServeHTTP is a HTTP handler method we use to provide DNS-over-HTTPS func (s *Server) ServeHTTP(w http.ResponseWriter, r *http.Request) { s.RLock() s.dnsProxy.ServeHTTP(w, r) s.RUnlock() } func (s *Server) beforeRequestHandler(p *proxy.Proxy, d *proxy.DNSContext) (bool, error) { ip, _, _ := net.SplitHostPort(d.Addr.String()) if s.access.IsBlockedIP(ip) { log.Tracef("Client IP %s is blocked by settings", ip) return false, nil } if len(d.Req.Question) == 1 { host := strings.TrimSuffix(d.Req.Question[0].Name, ".") if s.access.IsBlockedDomain(host) { log.Tracef("Domain %s is blocked by settings", host) return false, nil } } return true, nil } // handleDNSRequest filters the incoming DNS requests and writes them to the query log // nolint (gocyclo) func (s *Server) handleDNSRequest(p *proxy.Proxy, d *proxy.DNSContext) error { start := time.Now() if s.conf.OnDNSRequest != nil { s.conf.OnDNSRequest(d) } // disable Mozilla DoH if (d.Req.Question[0].Qtype == dns.TypeA || d.Req.Question[0].Qtype == dns.TypeAAAA) && d.Req.Question[0].Name == "use-application-dns.net." { d.Res = s.genNXDomain(d.Req) return nil } // use dnsfilter before cache -- changed settings or filters would require cache invalidation otherwise s.RLock() // Synchronize access to s.dnsFilter so it won't be suddenly uninitialized while in use. // This could happen after proxy server has been stopped, but its workers are not yet exited. // // A better approach is for proxy.Stop() to wait until all its workers exit, // but this would require the Upstream interface to have Close() function // (to prevent from hanging while waiting for unresponsive DNS server to respond). res, err := s.filterDNSRequest(d) s.RUnlock() if err != nil { return err } var origResp *dns.Msg if d.Res == nil { answer := []dns.RR{} originalQuestion := d.Req.Question[0] if res.Reason == dnsfilter.ReasonRewrite && len(res.CanonName) != 0 { answer = append(answer, s.genCNAMEAnswer(d.Req, res.CanonName)) // resolve canonical name, not the original host name d.Req.Question[0].Name = dns.Fqdn(res.CanonName) } if d.Addr != nil && s.conf.GetUpstreamsByClient != nil { clientIP, _, _ := net.SplitHostPort(d.Addr.String()) upstreams := s.conf.GetUpstreamsByClient(clientIP) for _, us := range upstreams { u, err := upstream.AddressToUpstream(us, upstream.Options{Timeout: 30 * time.Second}) if err != nil { log.Error("upstream.AddressToUpstream: %s: %s", us, err) continue } d.Upstreams = append(d.Upstreams, u) } } // request was not filtered so let it be processed further err = p.Resolve(d) if err != nil { return err } if res.Reason == dnsfilter.ReasonRewrite && len(res.CanonName) != 0 { d.Req.Question[0] = originalQuestion d.Res.Question[0] = originalQuestion if len(d.Res.Answer) != 0 { answer = append(answer, d.Res.Answer...) // host -> IP d.Res.Answer = answer } } else if res.Reason != dnsfilter.NotFilteredWhiteList { origResp2 := d.Res res, err = s.filterResponse(d) if err != nil { return err } if res != nil { origResp = origResp2 // matched by response } else { res = &dnsfilter.Result{} } } } if d.Res != nil { d.Res.Compress = true // some devices require DNS message compression } shouldLog := true msg := d.Req // don't log ANY request if refuseAny is enabled if len(msg.Question) >= 1 && msg.Question[0].Qtype == dns.TypeANY && s.conf.RefuseAny { shouldLog = false } elapsed := time.Since(start) s.RLock() // Synchronize access to s.queryLog and s.stats so they won't be suddenly uninitialized while in use. // This can happen after proxy server has been stopped, but its workers haven't yet exited. if shouldLog && s.queryLog != nil { p := querylog.AddParams{ Question: msg, Answer: d.Res, OrigAnswer: origResp, Result: res, Elapsed: elapsed, ClientIP: getIP(d.Addr), } if d.Upstream != nil { p.Upstream = d.Upstream.Address() } s.queryLog.Add(p) } s.updateStats(d, elapsed, *res) s.RUnlock() return nil } // Get IP address from net.Addr func getIP(addr net.Addr) net.IP { switch addr := addr.(type) { case *net.UDPAddr: return addr.IP case *net.TCPAddr: return addr.IP } return nil } func (s *Server) updateStats(d *proxy.DNSContext, elapsed time.Duration, res dnsfilter.Result) { if s.stats == nil { return } e := stats.Entry{} e.Domain = strings.ToLower(d.Req.Question[0].Name) e.Domain = e.Domain[:len(e.Domain)-1] // remove last "." switch addr := d.Addr.(type) { case *net.UDPAddr: e.Client = addr.IP case *net.TCPAddr: e.Client = addr.IP } e.Time = uint32(elapsed / 1000) switch res.Reason { case dnsfilter.NotFilteredNotFound: fallthrough case dnsfilter.NotFilteredWhiteList: fallthrough case dnsfilter.NotFilteredError: e.Result = stats.RNotFiltered case dnsfilter.FilteredSafeBrowsing: e.Result = stats.RSafeBrowsing case dnsfilter.FilteredParental: e.Result = stats.RParental case dnsfilter.FilteredSafeSearch: e.Result = stats.RSafeSearch case dnsfilter.FilteredBlackList: fallthrough case dnsfilter.FilteredInvalid: fallthrough case dnsfilter.FilteredBlockedService: e.Result = stats.RFiltered } s.stats.Update(e) } // filterDNSRequest applies the dnsFilter and sets d.Res if the request was filtered func (s *Server) filterDNSRequest(d *proxy.DNSContext) (*dnsfilter.Result, error) { if !s.conf.ProtectionEnabled || s.dnsFilter == nil { return &dnsfilter.Result{}, nil } clientAddr := "" if d.Addr != nil { clientAddr, _, _ = net.SplitHostPort(d.Addr.String()) } setts := s.dnsFilter.GetConfig() setts.FilteringEnabled = true if s.conf.FilterHandler != nil { s.conf.FilterHandler(clientAddr, &setts) } req := d.Req host := strings.TrimSuffix(req.Question[0].Name, ".") res, err := s.dnsFilter.CheckHost(host, d.Req.Question[0].Qtype, &setts) if err != nil { // Return immediately if there's an error return nil, errorx.Decorate(err, "dnsfilter failed to check host '%s'", host) } else if res.IsFiltered { // log.Tracef("Host %s is filtered, reason - '%s', matched rule: '%s'", host, res.Reason, res.Rule) d.Res = s.genDNSFilterMessage(d, &res) } else if res.Reason == dnsfilter.ReasonRewrite && len(res.IPList) != 0 { resp := dns.Msg{} resp.SetReply(req) name := host if len(res.CanonName) != 0 { resp.Answer = append(resp.Answer, s.genCNAMEAnswer(req, res.CanonName)) name = res.CanonName } for _, ip := range res.IPList { if req.Question[0].Qtype == dns.TypeA { a := s.genAAnswer(req, ip) a.Hdr.Name = dns.Fqdn(name) resp.Answer = append(resp.Answer, a) } else if req.Question[0].Qtype == dns.TypeAAAA { a := s.genAAAAAnswer(req, ip) a.Hdr.Name = dns.Fqdn(name) resp.Answer = append(resp.Answer, a) } } d.Res = &resp } return &res, err } // If response contains CNAME, A or AAAA records, we apply filtering to each canonical host name or IP address. // If this is a match, we set a new response in d.Res and return. func (s *Server) filterResponse(d *proxy.DNSContext) (*dnsfilter.Result, error) { for _, a := range d.Res.Answer { host := "" switch v := a.(type) { case *dns.CNAME: log.Debug("DNSFwd: Checking CNAME %s for %s", v.Target, v.Hdr.Name) host = strings.TrimSuffix(v.Target, ".") case *dns.A: host = v.A.String() log.Debug("DNSFwd: Checking record A (%s) for %s", host, v.Hdr.Name) case *dns.AAAA: host = v.AAAA.String() log.Debug("DNSFwd: Checking record AAAA (%s) for %s", host, v.Hdr.Name) default: continue } s.RLock() // Synchronize access to s.dnsFilter so it won't be suddenly uninitialized while in use. // This could happen after proxy server has been stopped, but its workers are not yet exited. if !s.conf.ProtectionEnabled || s.dnsFilter == nil { s.RUnlock() continue } setts := dnsfilter.RequestFilteringSettings{} setts.FilteringEnabled = true res, err := s.dnsFilter.CheckHost(host, d.Req.Question[0].Qtype, &setts) s.RUnlock() if err != nil { return nil, err } else if res.IsFiltered { d.Res = s.genDNSFilterMessage(d, &res) log.Debug("DNSFwd: Matched %s by response: %s", d.Req.Question[0].Name, host) return &res, nil } } return nil, nil } // genDNSFilterMessage generates a DNS message corresponding to the filtering result func (s *Server) genDNSFilterMessage(d *proxy.DNSContext, result *dnsfilter.Result) *dns.Msg { m := d.Req if m.Question[0].Qtype != dns.TypeA && m.Question[0].Qtype != dns.TypeAAAA { return s.genNXDomain(m) } switch result.Reason { case dnsfilter.FilteredSafeBrowsing: return s.genBlockedHost(m, s.conf.SafeBrowsingBlockHost, d) case dnsfilter.FilteredParental: return s.genBlockedHost(m, s.conf.ParentalBlockHost, d) default: if result.IP != nil { return s.genResponseWithIP(m, result.IP) } if s.conf.BlockingMode == "null_ip" { switch m.Question[0].Qtype { case dns.TypeA: return s.genARecord(m, []byte{0, 0, 0, 0}) case dns.TypeAAAA: return s.genAAAARecord(m, net.IPv6zero) } } return s.genNXDomain(m) } } func (s *Server) genServerFailure(request *dns.Msg) *dns.Msg { resp := dns.Msg{} resp.SetRcode(request, dns.RcodeServerFailure) resp.RecursionAvailable = true return &resp } func (s *Server) genARecord(request *dns.Msg, ip net.IP) *dns.Msg { resp := dns.Msg{} resp.SetReply(request) resp.Answer = append(resp.Answer, s.genAAnswer(request, ip)) return &resp } func (s *Server) genAAAARecord(request *dns.Msg, ip net.IP) *dns.Msg { resp := dns.Msg{} resp.SetReply(request) resp.Answer = append(resp.Answer, s.genAAAAAnswer(request, ip)) return &resp } func (s *Server) genAAnswer(req *dns.Msg, ip net.IP) *dns.A { answer := new(dns.A) answer.Hdr = dns.RR_Header{ Name: req.Question[0].Name, Rrtype: dns.TypeA, Ttl: s.conf.BlockedResponseTTL, Class: dns.ClassINET, } answer.A = ip return answer } func (s *Server) genAAAAAnswer(req *dns.Msg, ip net.IP) *dns.AAAA { answer := new(dns.AAAA) answer.Hdr = dns.RR_Header{ Name: req.Question[0].Name, Rrtype: dns.TypeAAAA, Ttl: s.conf.BlockedResponseTTL, Class: dns.ClassINET, } answer.AAAA = ip return answer } // generate DNS response message with an IP address func (s *Server) genResponseWithIP(req *dns.Msg, ip net.IP) *dns.Msg { if req.Question[0].Qtype == dns.TypeA && ip.To4() != nil { return s.genARecord(req, ip.To4()) } else if req.Question[0].Qtype == dns.TypeAAAA && ip.To4() == nil { return s.genAAAARecord(req, ip) } // empty response resp := dns.Msg{} resp.SetReply(req) return &resp } func (s *Server) genBlockedHost(request *dns.Msg, newAddr string, d *proxy.DNSContext) *dns.Msg { ip := net.ParseIP(newAddr) if ip != nil { return s.genResponseWithIP(request, ip) } // look up the hostname, TODO: cache replReq := dns.Msg{} replReq.SetQuestion(dns.Fqdn(newAddr), request.Question[0].Qtype) replReq.RecursionDesired = true newContext := &proxy.DNSContext{ Proto: d.Proto, Addr: d.Addr, StartTime: time.Now(), Req: &replReq, } err := s.dnsProxy.Resolve(newContext) if err != nil { log.Printf("Couldn't look up replacement host '%s': %s", newAddr, err) return s.genServerFailure(request) } resp := dns.Msg{} resp.SetReply(request) resp.Authoritative, resp.RecursionAvailable = true, true if newContext.Res != nil { for _, answer := range newContext.Res.Answer { answer.Header().Name = request.Question[0].Name resp.Answer = append(resp.Answer, answer) } } return &resp } // Make a CNAME response func (s *Server) genCNAMEAnswer(req *dns.Msg, cname string) *dns.CNAME { answer := new(dns.CNAME) answer.Hdr = dns.RR_Header{ Name: req.Question[0].Name, Rrtype: dns.TypeCNAME, Ttl: s.conf.BlockedResponseTTL, Class: dns.ClassINET, } answer.Target = dns.Fqdn(cname) return answer } func (s *Server) genNXDomain(request *dns.Msg) *dns.Msg { resp := dns.Msg{} resp.SetRcode(request, dns.RcodeNameError) resp.RecursionAvailable = true resp.Ns = s.genSOA(request) return &resp } func (s *Server) genSOA(request *dns.Msg) []dns.RR { zone := "" if len(request.Question) > 0 { zone = request.Question[0].Name } soa := dns.SOA{ // values copied from verisign's nonexistent .com domain // their exact values are not important in our use case because they are used for domain transfers between primary/secondary DNS servers Refresh: 1800, Retry: 900, Expire: 604800, Minttl: 86400, // copied from AdGuard DNS Ns: "fake-for-negative-caching.adguard.com.", Serial: 100500, // rest is request-specific Hdr: dns.RR_Header{ Name: zone, Rrtype: dns.TypeSOA, Ttl: s.conf.BlockedResponseTTL, Class: dns.ClassINET, }, Mbox: "hostmaster.", // zone will be appended later if it's not empty or "." } if soa.Hdr.Ttl == 0 { soa.Hdr.Ttl = defaultValues.BlockedResponseTTL } if len(zone) > 0 && zone[0] != '.' { soa.Mbox += zone } return []dns.RR{&soa} }