The AppSketch/MAAT signature system contains a systematically dated, patch-versioned catalog of FQDN-based detection signatures for dozens of individually named commercial VPN apps (e.g. V2VPN, V2Netvpn, Turbo VPN, LetsVPN, AdGuard VPN, Avira Phantom VPN, TouchVPN, FinchVPN, Opera VPN, Ultrasurf VPN, GoFly VPN), each modeled as an 'app' object in category 'networking' / subcategory 'tunnels' whose default deny_action drops the flow while sending both an ICMP-unreachable and a TCP RST to the client. At least 35 such distinct VPN-app signature objects appear in this batch alone, dated July-November 2024, with repeated 'Patch0N' revisions to the same app as its domains rotate.
"deny_action": {"method": "drop", "after_n_packets": 0, "send_icmp_unreachable": 1, "send_tcp_reset": 1}
Defense implications
- Assume any circumvention app's FQDN/domain patterns will be individually fingerprinted and version-tracked by the censor within weeks of a new build (observed patch cadence is roughly weekly-to-monthly per app); rotate domains proactively rather than reactively.
- Blocking here is enforced via RST + ICMP-unreachable rather than a silent drop, so clients can distinguish active blocking from ordinary network failure -- useful as a signal for automated censorship-event detection.
Related findings
A production TSG maat.conf shows the MAAT rule-matching engine running four parallel instances (STATIC, DYNAMIC, APP_SIGNATURE_MAAT, CAPTURE); the APP_SIGNATURE_MAAT instance is explicitly configured against app_sketch_tableinfo.conf / app_sketch_maat.json -- directly confirming MAAT is the execution engine underlying the AppSketch signature system -- and is tagged with an ACCEPT_TAGS datacenter value 'xjlhs', indicating this specific instance is scoped to a Xinjiang deployment.
Side-by-side internal performance comparison across Xinjiang (China Mobile carrier deployment, hostname cmcc-xj-server1-sapp-244), Fujian (domestic), and Ethiopia (E21) explicitly references the same 'app_sketch_maat' diagnostic log format at all three sites, confirming the identical AppSketch app-fingerprinting stack is used for domestic Xinjiang surveillance and for the exported Ethiopia deployment.
TSG's DPI signature engine is organized around named internal components confirmed by config paths and error logs: SAPP (installed at /home/mesasoft/sapp_run or /opt/tsg/sapp), MAAT (config at .../tsgconf/maat.conf, tied to a Redis-backed APP_SIG_SESSION_ATTRIBUTE_STRING table), and App-SketchDB (a versioned, centrally-maintained app-identification signature database pulled periodically by field deployments). One deployment's provincial gateway alone had 1,667 TCP/UDP first-packet payload signature entries active (396 at the IDC site), confirming payload-prefix matching as a live, large-scale detection method, including custom byte-prefix entries for DingTalk and WeChat.
An internal TSG operations/troubleshooting manual lays out TSG's full traffic pipeline (NIC -> mrzcpd/marsio capture driver -> sapp DPI engine -> firewall/proxy(KNI->TFE)/active-defense/WAN-NAT policy branches) and shows engineers using maat_redis_tool to pull the live Redis-synced blocking policy tables (TSG_SECURITY_COMPILE, TSG_OBJ_IP_ADDR, TSG_OBJ_APP_ID) and filter them by numeric policy/object ID to debug why a block rule isn't firing.
The internal 'MAAT网络流处理配置统一描述框架' engineering manual (v3.1.20, author 郑超, 2021) documents MAAT's Redis-synced rule-compilation framework and its RuleScan/Hyperscan-based pattern-matching engine (libmaatframe.so / librulescan), and records that RuleScan's fast-scan feature caused a production outage on 2019-03-19 and has been disabled ever since.
Exported AppSketch/MAAT signature-object JSON shows named per-VPN detection rules combining multiple independent vectors: CyberGhost's WireGuard variant is matched by a hex UDP-payload pattern (04000000*), destination port 1337, and ip.proto=17 together (signature cyberghostvpn_wireguard); Windscribe's OpenVPN control channel is matched by two offset-anchored hex byte patterns plus an exact 86-byte payload length (windscribevpn_openvpn_payload); and ExpressVPN is matched independently via a literal JA3 hash (ssl.analysis.ja3), FQDN strings (expressapisv2.com, www.expressvpn.works), and a large enumerated IP list (700+ individual ip.dst entries observed before the 200KB text-extraction cutoff).