Add configuration option allow-fallback-on-unknown-dc

example.config.toml

 # time range of this parameter.
 tolerate-time-skewness = "5s"
 
+# Telegram has a concept of DC. You can think about DC as a number of a cluster
+# with a certain purpose. Some clusters serve media, some - messages, some rule
+# channels and so on. But sometimes unknown DC number is requested by client.
+# It could be a bug or some global reconfiguration of the Telegram.
+#
+# By default, proxy rejects such requests. But it is also possible to fallback
+# this request to any DC. Telegram works in a way that any DC is able to serve
+# any request but sacrificing a latency.
+#
+# If this setting is disabled (default), mtg will reject a connection.
+# Otherwise, chose a new DC.
+allow-fallback-on-unknown-dc = false
+
 # network defines different network-related settings
 [network]
 # please be aware that mtg needs to do some external requests. For

internal/cli/run_proxy.go

 		BufferSize:         conf.TCPBuffer.Get(mtglib.DefaultBufferSize),
 		DomainFrontingPort: conf.DomainFrontingPort.Get(mtglib.DefaultDomainFrontingPort),
 		PreferIP:           conf.PreferIP.Get(mtglib.DefaultPreferIP),
+
+		AllowFallbackOnUnknownDC: conf.AllowFallbackOnUnknownDC.Get(false),
 	}
 
 	proxy, err := mtglib.NewProxy(opts)

internal/cli/simple_run.go

 	}
 
 	conf.Debug.Value = s.Debug
+	conf.AllowFallbackOnUnknownDC.Value = true
 	conf.Defense.AntiReplay.Enabled.Value = true
 
 	if err := conf.Validate(); err != nil {

internal/config/config.go

 )
 
 type Config struct {
-	Debug                TypeBool        `json:"debug"`
-	Secret               mtglib.Secret   `json:"secret"`
-	BindTo               TypeHostPort    `json:"bindTo"`
-	TCPBuffer            TypeBytes       `json:"tcpBuffer"`
-	PreferIP             TypePreferIP    `json:"preferIp"`
-	DomainFrontingPort   TypePort        `json:"domainFrontingPort"`
-	TolerateTimeSkewness TypeDuration    `json:"tolerateTimeSkewness"`
-	Concurrency          TypeConcurrency `json:"concurrency"`
-	Defense              struct {
+	Debug                    TypeBool        `json:"debug"`
+	AllowFallbackOnUnknownDC TypeBool        `json:"allowFallbackOnUnknownDc"`
+	Secret                   mtglib.Secret   `json:"secret"`
+	BindTo                   TypeHostPort    `json:"bindTo"`
+	TCPBuffer                TypeBytes       `json:"tcpBuffer"`
+	PreferIP                 TypePreferIP    `json:"preferIp"`
+	DomainFrontingPort       TypePort        `json:"domainFrontingPort"`
+	TolerateTimeSkewness     TypeDuration    `json:"tolerateTimeSkewness"`
+	Concurrency              TypeConcurrency `json:"concurrency"`
+	Defense                  struct {
 		AntiReplay struct {
 			Enabled   TypeBool      `json:"enabled"`
 			MaxSize   TypeBytes     `json:"maxSize"`

internal/config/parse.go

 )
 
 type tomlConfig struct {
-	Debug                bool   `toml:"debug" json:"debug,omitempty"`
-	Secret               string `toml:"secret" json:"secret"`
-	BindTo               string `toml:"bind-to" json:"bindTo"`
-	TCPBuffer            string `toml:"tcp-buffer" json:"tcpBuffer,omitempty"`
-	PreferIP             string `toml:"prefer-ip" json:"preferIp,omitempty"`
-	DomainFrontingPort   uint   `toml:"domain-fronting-port" json:"domainFrontingPort,omitempty"`
-	TolerateTimeSkewness string `toml:"tolerate-time-skewness" json:"tolerateTimeSkewness,omitempty"`
-	Concurrency          uint   `toml:"concurrency" json:"concurrency,omitempty"`
-	Defense              struct {
+	Debug                    bool   `toml:"debug" json:"debug,omitempty"`
+	AllowFallbackOnUnknownDC bool   `toml:"allow-fallback-on-unknown-dc" json:"allowFallbackOnUnknownDc,omitempty"`
+	Secret                   string `toml:"secret" json:"secret"`
+	BindTo                   string `toml:"bind-to" json:"bindTo"`
+	TCPBuffer                string `toml:"tcp-buffer" json:"tcpBuffer,omitempty"`
+	PreferIP                 string `toml:"prefer-ip" json:"preferIp,omitempty"`
+	DomainFrontingPort       uint   `toml:"domain-fronting-port" json:"domainFrontingPort,omitempty"`
+	TolerateTimeSkewness     string `toml:"tolerate-time-skewness" json:"tolerateTimeSkewness,omitempty"`
+	Concurrency              uint   `toml:"concurrency" json:"concurrency,omitempty"`
+	Defense                  struct {
 		AntiReplay struct {
 			Enabled   bool    `toml:"enabled" json:"enabled,omitempty"`
 			MaxSize   string  `toml:"max-size" json:"maxSize,omitempty"`

mtglib/proxy.go

 	ctxCancel       context.CancelFunc
 	streamWaitGroup sync.WaitGroup
 
-	tolerateTimeSkewness time.Duration
-	bufferSize           int
-	domainFrontingPort   int
-	workerPool           *ants.PoolWithFunc
-	telegram             *telegram.Telegram
+	allowFallbackOnUnknownDC bool
+	tolerateTimeSkewness     time.Duration
+	bufferSize               int
+	domainFrontingPort       int
+	workerPool               *ants.PoolWithFunc
+	telegram                 *telegram.Telegram
 
 	secret          Secret
 	network         Network
 func (p *Proxy) doTelegramCall(ctx *streamContext) error {
 	dc := ctx.dc
 
-	if !p.telegram.IsKnownDC(dc) {
+	if p.allowFallbackOnUnknownDC && !p.telegram.IsKnownDC(dc) {
 		dc = p.telegram.GetFallbackDC()
 		ctx.logger = ctx.logger.BindInt("fallback_dc", dc)
 
 
 	ctx, cancel := context.WithCancel(context.Background())
 	proxy := &Proxy{
-		ctx:                  ctx,
-		ctxCancel:            cancel,
-		secret:               opts.Secret,
-		network:              opts.Network,
-		antiReplayCache:      opts.AntiReplayCache,
-		ipBlocklist:          opts.IPBlocklist,
-		eventStream:          opts.EventStream,
-		logger:               opts.getLogger("proxy"),
-		domainFrontingPort:   opts.getDomainFrontingPort(),
-		tolerateTimeSkewness: opts.getTolerateTimeSkewness(),
-		bufferSize:           opts.getBufferSize(),
-		telegram:             tg,
+		ctx:                      ctx,
+		ctxCancel:                cancel,
+		secret:                   opts.Secret,
+		network:                  opts.Network,
+		antiReplayCache:          opts.AntiReplayCache,
+		ipBlocklist:              opts.IPBlocklist,
+		eventStream:              opts.EventStream,
+		logger:                   opts.getLogger("proxy"),
+		domainFrontingPort:       opts.getDomainFrontingPort(),
+		tolerateTimeSkewness:     opts.getTolerateTimeSkewness(),
+		bufferSize:               opts.getBufferSize(),
+		allowFallbackOnUnknownDC: opts.AllowFallbackOnUnknownDC,
+		telegram:                 tg,
 	}
 
 	pool, err := ants.NewPoolWithFunc(opts.getConcurrency(),

mtglib/proxy_opts.go

 	// This is an optional setting.
 	DomainFrontingPort uint
 
+	// AllowFallbackOnUnknownDC defines how proxy behaves if unknown DC was
+	// requested. If this setting is set to false, then such connection
+	// will be rejected. Otherwise, proxy will chose any DC.
+	//
+	// Telegram is designed in a way that any DC can serve any request,
+	// the problem is a latency.
+	//
+	// This is an optional setting.
+	AllowFallbackOnUnknownDC bool
+
 	// UseTestDCs defines if we have to connect to production or to staging
 	// DCs of Telegram.
 	//