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

example.config.toml

@@ -56,6 +56,19 @@ domain-fronting-port = 443
 # 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

@@ -184,6 +184,8 @@ func runProxy(conf *config.Config, version string) error {
 		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

@@ -71,6 +71,7 @@ func (s *SimpleRun) Run(cli *CLI, version string) error { // nolint: cyclop
 	}
 
 	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

@@ -10,6 +10,7 @@ import (
 
 type Config struct {
 	Debug                    TypeBool        `json:"debug"`
+	AllowFallbackOnUnknownDC TypeBool        `json:"allowFallbackOnUnknownDc"`
 	Secret                   mtglib.Secret   `json:"secret"`
 	BindTo                   TypeHostPort    `json:"bindTo"`
 	TCPBuffer                TypeBytes       `json:"tcpBuffer"`

internal/config/parse.go

@@ -10,6 +10,7 @@ import (
 
 type tomlConfig 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"`

mtglib/proxy.go

@@ -23,6 +23,7 @@ type Proxy struct {
 	ctxCancel       context.CancelFunc
 	streamWaitGroup sync.WaitGroup
 
+	allowFallbackOnUnknownDC bool
 	tolerateTimeSkewness     time.Duration
 	bufferSize               int
 	domainFrontingPort       int
@@ -209,7 +210,7 @@ func (p *Proxy) doObfuscated2Handshake(ctx *streamContext) error {
 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)
 
@@ -296,6 +297,7 @@ func NewProxy(opts ProxyOpts) (*Proxy, error) {
 		domainFrontingPort:       opts.getDomainFrontingPort(),
 		tolerateTimeSkewness:     opts.getTolerateTimeSkewness(),
 		bufferSize:               opts.getBufferSize(),
+		allowFallbackOnUnknownDC: opts.AllowFallbackOnUnknownDC,
 		telegram:                 tg,
 	}
 

mtglib/proxy_opts.go

@@ -90,6 +90,16 @@ type ProxyOpts struct {
 	// 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.
 	//