Add domain-fronting-ip option

Allow specifying an explicit IP address for the domain fronting host
instead of relying on DNS resolution. Useful when DNS resolution of
the fronting hostname is blocked.

The hostname from the secret is still used for SNI in TLS handshake.

mtglib/proxy.go

@@ -27,6 +27,7 @@ type Proxy struct {
 	allowFallbackOnUnknownDC bool
 	tolerateTimeSkewness     time.Duration
 	domainFrontingPort       int
+	domainFrontingIP         string
 	workerPool               *ants.PoolWithFunc
 	telegram                 *dc.Telegram
 
@@ -40,8 +41,14 @@ type Proxy struct {
 }
 
 // DomainFrontingAddress returns a host:port pair for a fronting domain.
+// If DomainFrontingIP is set, it is used instead of resolving the hostname.
 func (p *Proxy) DomainFrontingAddress() string {
-	return net.JoinHostPort(p.secret.Host, strconv.Itoa(p.domainFrontingPort))
+	host := p.secret.Host
+	if p.domainFrontingIP != "" {
+		host = p.domainFrontingIP
+	}
+
+	return net.JoinHostPort(host, strconv.Itoa(p.domainFrontingPort))
 }
 
 // ServeConn serves a connection. We do not check IP blocklist and concurrency
@@ -317,6 +324,7 @@ func NewProxy(opts ProxyOpts) (*Proxy, error) {
 		eventStream:              opts.EventStream,
 		logger:                   opts.getLogger("proxy"),
 		domainFrontingPort:       opts.getDomainFrontingPort(),
+		domainFrontingIP:         opts.DomainFrontingIP,
 		tolerateTimeSkewness:     opts.getTolerateTimeSkewness(),
 		allowFallbackOnUnknownDC: opts.AllowFallbackOnUnknownDC,
 		telegram:                 tg,