Routing 100% of a device's network traffic through a single remote proxy server is an anti-pattern in modern restrictive networks. When every packet—including local banking apps, national delivery services, and domestic CDN video streams—is hairpinned through a foreign datacenter:

1. Domestic Services Break: Local banking and government portals block foreign datacenter ASNs or trigger endless CAPTCHA loops.

2. Latency Doubles: Local traffic travels thousands of kilometers out of the country and back unnecessarily.

3. DPI Traffic Volume Anomalies: Sustained high-volume streams to a single unrecognized foreign IP on a mobile carrier (MTS, Beeline, MegaFon, Tele2, Yota) attract heuristic throttling faster than selective routing.

In this guide, we examine how mobile carrier SNI/CIDR whitelists operate and how to architect resilient Split-Tunneling and Auto-Balanced Subscriptions across sing-box, Clash Meta (Mihomo), Happ, v2RayTun, and Hiddify.

1. How Mobile Carrier SNI & CIDR Whitelists Work

During elevated filtering windows on cellular networks, DPI policy shifts from blacklisting known banned domains to whitelisting approved TLS Server Name Indication (SNI) domains and domestic cloud CIDR ranges:

• Default-Drop on Unrecognized SNIs: Any TLS 1.3 ClientHello presenting an unknown foreign SNI on port :443—or any raw WireGuard/OpenVPN UDP handshake—is dropped immediately.

• VLESS + XTLS-Vision + Reality: By borrowing the authentic certificate chain and TLS 1.3 handshake parameters of a permitted destination domain (sni) and authenticating the client via a Curve25519 ShortID (sid) inside ClientHello, Reality makes the handshake cryptographically indistinguishable from a legitimate visit to the whitelisted origin.

• Hysteria2 over QUIC/UDP: On congested 4G/LTE links where UDP is still permitted, Hysteria2's Brutal congestion control recovers throughput across high packet-loss wireless hops where standard TCP Cubic collapses.

• Fake-TLS (ee-Prefix) MTProto for Telegram: For lightweight messenger connectivity without a full TUN interface, ee-prefixed MTProto secrets wrap Telegram frames inside a standard TLS 1.3 record carrying a hex-encoded SNI domain. (We maintain a live, auto-tested 1-click MTProto directory at vpnkeys.me/proxy/).

2. Production sing-box Split-Tunneling Configuration

To route domestic .ru / .su / .xn--p1ai domains and geoip-ru subnets directly through the local ISP (direct) while tunneling international traffic through verified VLESS Reality or Hysteria2 nodes, use a split-DNS and rule-set topology.

We publish an open reference configuration in our repository (rules/singbox-ru-direct.json):

{
  "dns": {
    "servers": [
      {
        "tag": "dns-remote",
        "address": "https://1.1.1.1/dns-query",
        "detour": "proxy"
      },
      {
        "tag": "dns-direct",
        "address": "77.88.8.8",
        "detour": "direct"
      }
    ],
    "rules": [
      {
        "domain_suffix": [".ru", ".su", ".xn--p1ai"],
        "server": "dns-direct"
      }
    ],
    "final": "dns-remote",
    "strategy": "ipv4_only"
  },
  "route": {
    "auto_detect_interface": true,
    "rules": [
      { "protocol": "dns", "action": "hijack-dns" },
      { "ip_is_private": true, "outbound": "direct" },
      {
        "domain_suffix": [
          ".ru", ".su", ".xn--p1ai",
          "gosuslugi.ru", "sberbank.ru", "tbank.ru",
          "yandex.ru", "vk.com", "ozon.ru", "wildberries.ru"
        ],
        "outbound": "direct"
      },
      {
        "rule_set": ["geoip-ru", "geosite-category-ru"],
        "outbound": "direct"
      }
    ],
    "final": "proxy"
  }
}

When debugging an encrypted proxy tunnel (vless://, trojan://, hysteria2://, vmess://, ss://, or tg://proxy), engineers typically face two problems:

1. Opaque Configuration Strings: A single vless:// or Base64 vmess:// URI packs transport layers, TLS fingerprint settings (fp), Curve25519 Reality public keys (pbk), short IDs (sid), XTLS flow modes (xtls-rprx-vision), and Server Name Indication (sni) parameters into one dense URL. Pasting sensitive URIs into server-side web decoders risks leaking private endpoints.
2. Silent Client-Side Leaks: Even when a tunnel connects, misconfigured TUN/system proxy stacks frequently leak the user's real residential ISP address via WebRTC STUN ICE candidates, fallback DNS resolvers, or split-horizon routing.

Here is how we built a 100% client-side diagnostic suite and cryptographic URI analyzer to solve both challenges without sending private keys to a backend server.

1. Zero-Knowledge Client-Side URI Parsing & Heuristic Scoring

In our interactive Key Checker & Decoder (freevpnkeys.com/tools/key-checker/ and vpnkeys.me/tools/key-checker/), all parsing and security evaluation runs strictly inside the browser's JavaScript runtime:

• Transport & TLS Parameter Extraction: Parses URL query parameters and Base64 payloads for VLESS, VMess, Trojan, Shadowsocks, Hysteria2, and MTProto (tg://proxy / https://t.me/proxy), including ee-prefixed Fake-TLS secrets (extracting the embedded hex domain name).
• Cryptographic Hygiene Checks:
  • Flags dangerous configurations such as allowInsecure=1 / insecure=true (which disables TLS certificate verification and enables trivial MITM interception on public networks).
  • Verifies whether VLESS over TCP :443 enables flow=xtls-rprx-vision alongside security=reality to eliminate inner-TLS length fingerprinting.
  • Checks uTLS client fingerprint settings (fp=chrome, fp=firefox, fp=safari) against modern browser JA3/JA4 profiles.
• Carrier SNI Whitelist Assessment: Evaluates the configured sni / host domain against known ISP whitelist behavior, warning operators when an SNI is prone to active probing or SNI blackholing.
• Instant QR & Deep-Link Generation: Generates client-side QR codes and properly escaped (%23 fragment) 1-click import links (happ://add/..., v2raytun://import/..., hiddify://import/...) so configurations can be transferred to mobile devices without touching a clipboard server.

For offline CLI workflows, we also maintain a zero-dependency Python 3 auditor in our open repository: scripts/audit_sub.py.

2. Browser-Based IP/ASN, DNS & WebRTC Leak Diagnostics

Once a configuration is active in a client, verifying that traffic actually exits through the intended autonomous system requires checking multiple layers simultaneously. On our Connection & DPI Test page (vpnkeys.me/test/), the browser executes a multi-stage verification:

1. Ingress vs. Egress GeoLite2 ASN Comparison:
   • Queries /api/v1/geo/me (backed by in-memory MaxMind GeoLite2 Country, City, and ASN databases) and compares the result against external trace endpoints (1.1.1.1/cdn-cgi/trace).
   • If the primary HTTP request shows a domestic ASN while external trace endpoints resolve to a foreign datacenter ASN, the browser immediately detects active Split-Tunneling.
2. WebRTC ICE Candidate Leak Detection:
   • Instantiates an ephemeral RTCPeerConnection against public STUN servers (stun:stun.l.google.com:19302) and parses srflx (server reflexive) candidates.
   • If the STUN candidate exposes a residential IP that differs from the HTTP egress IP, the user's UDP stack is bypassing the proxy tunnel.
3. Browser-Side 64KB DPI Stream Probe:
   • Initiates parallel timed fetch() streams (>64 KB payload) with an AbortController deadline to detect whether the user's current ISP path is actively throttling foreign CDNs via TSPU byte-counter limits.

Try the Open Tools

• Interactive Key Decoder & Security Scorer: freevpnkeys.com/tools/key-checker/ | vpnkeys.me/tools/key-checker/
• Live Browser Leak & DPI Stream Test: vpnkeys.me/test/
• CLI Subscription Auditor (audit_sub.py): github.com/TelegaVPN/free-vpn-keys/tree/main/scripts

Most public proxy aggregators rely on a deceptively simple health check: open a TCP socket—or complete a quick TLS ClientHello/ServerHello handshake—from a single cloud VPS in Frankfurt or Amsterdam. If the socket responds within 200 ms, the node is marked "online."

In modern deep packet inspection (DPI) environments—such as Russia's stateful TSPU middleboxes, Iran's MCI/Irancell filtering layers, or China's GFW—a passing TCP or TLS handshake no longer guarantees a working tunnel.

In this engineering deep dive, we break down how stateful byte-counter throttling works, why single-vantage checks fail, and how we engineered the dual-vantage verification pipeline and live telemetry feeds powering FreeVPNKeys.com and VPNKeys.me.

The Anatomy of a Stateful 16–64KB TCP Freeze

Modern inline DPI appliances avoid dropping suspicious connections on packet #1. Immediate TCP RST injection is noisy, easy to fingerprint, and triggers rapid client failover. Instead, middleboxes deploy delayed byte-window clamping:

1. TCP 3-Way Handshake (SYN / SYN-ACK / ACK): Passes with zero interference.

2. TLS 1.3 Handshake (ClientHello with SNI & ALPN): Passes cleanly. Even initial HTTP headers or small ping frames succeed.

3. The 16 KB – 64 KB Threshold: Once cumulative downstream payload bytes on the connection cross a stateful threshold (typically between 16,384 and 65,536 bytes) on a non-whitelisted ASN or fingerprinted flow, the middlebox silently drops subsequent packets without emitting a RST or FIN.

To the client OS, the socket remains ESTABLISHED, yet the TCP receive window stalls indefinitely until a read timeout occurs.

Engineering a Dual-Vantage Verification Engine in Go

To eliminate false positives ("zombie nodes" that pass datacenter pings but freeze on residential ISPs), our verification pipeline evaluates every candidate URI (vless://, trojan://, hysteria2://, ss://, vmess://) across two distinct network vantage points every 5 minutes:

1. Backbone Datacenter Probe (Europe):

   • Spins up an isolated Xray-core / sing-box outbound instance.

   • Verifies cryptographic handshake integrity (Curve25519 pbk/sid for VLESS Reality, QUIC ALPN negotiation for Hysteria2, AEAD cipher authenticity for Shadowsocks-2022).

   • Resolves the actual egress IP via in-memory MaxMind GeoLite2-City and GeoLite2-ASN MMDB readers (refreshed every 12 hours) and tests reachability against 7 target services (ChatGPT/OpenAI, Google Gemini, Claude, Netflix, YouTube, Spotify, Disney+).

2. Residential Domestic ISP Probe (TSPU Path):

   • Routes traffic from inside a domestic residential network directly through active TSPU hardware.

   • Streams a payload exceeding the 64 KB stateful inspection window over the established tunnel.

   • Measures real-world round-trip latency (RTT) and verifies whether the connection survives carrier-level SNI and CIDR subnet restrictions.

Nodes that pass both vantage checks and sustain throughput beyond the 64 KB window are tagged with for_russia (carrier whitelist ready) and promoted into the active rotation.

Reproducing the 64KB Freeze Test From Your Terminal

We published an open-source bash diagnostic utility in our repository (scripts/test_tspu_64k.sh) so network engineers can test any endpoint for byte-counter clamping using curl:

#!/usr/bin/env bash
# Test whether an endpoint stalls after 16KB-64KB of downstream transfer
TARGET_URL="${1:-https://speed.cloudflare.com/__down?bytes=131072}"

curl -sS -o /dev/null \
  --connect-timeout 5 \
  --max-time 12 \
  -w "HTTP: %{http_code} | Connect: %{time_connect}s | TTFB: %{time_starttransfer}s | Total: %{time_total}s | Downloaded: %{size_download} bytes\n" \
  "$TARGET_URL"