Splitting and Desync Attacks

Relevant source files

• intra/core/ping.go
• intra/core/sigcond.go
• intra/core/sockopt.go
• intra/dialers/direct_split.go
• intra/dialers/example/main.go
• intra/dialers/retrier.go
• intra/dialers/split_and_desync.go
• intra/ipn/ground.go
• intra/netstack/icmpconn.go
• intra/protect/icmplistener.go
• intra/protect/protect.go
• intra/protect/xdial.go
• intra/rwconn.go

Purpose and Scope

This page documents the implementation of TCP/TLS splitting strategies and TCB desynchronization attacks used for censorship circumvention in Firestack. The system employs two primary mechanisms: splitter for direct TCP/TLS splitting and overwriteSplitter for TCB desynchronization with TTL manipulation. These techniques target the first data segment of TCP connections to bypass Deep Packet Inspection (DPI) systems.

Key Components:

• splitter — Direct TCP/TLS record splitting without content modification. intra/dialers/direct_split.go37-42
• overwriteSplitter — TCB desynchronization using fake payload and restricted TTL. intra/dialers/split_and_desync.go45-55
• UDP traceroute — Hop distance estimation for TTL calculation via tracert. intra/dialers/split_and_desync.go109-176
• ttlcache — Performance optimization for TTL storage using core.Sieve. intra/dialers/split_and_desync.go41
• SigCond — First-write coordination primitive for one-time state transitions (false -> true). intra/core/sigcond.go29-32

Sources: intra/dialers/retrier.go1-120 intra/dialers/split_and_desync.go1-55 intra/dialers/direct_split.go1-45 intra/core/sigcond.go1-32

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Architecture Overview

The splitting and desynchronization system consists of three main components wrapped around a standard net.TCPConn. The retrier orchestrates which implementation to use based on the settings.DialerOpts.

Diagram: Splitting and Desync Component Hierarchy

Key Design Points:

Component	Responsibility	Interface
retrier	Strategy selection, retry orchestration, connection lifecycle. intra/dialers/retrier.go73-117	core.DuplexConn, core.RetrierConn intra/dialers/retrier.go119-120
splitter	Direct TCP/TLS splitting on first write. intra/dialers/direct_split.go37-42	core.DuplexConn, core.RetrierConn intra/dialers/direct_split.go44-45
overwriteSplitter	TCB desynchronization with TTL manipulation. intra/dialers/split_and_desync.go45-55	core.DuplexConn, core.RetrierConn intra/dialers/split_and_desync.go57-58
SigCond	First-write coordination between goroutines. intra/core/sigcond.go29-32	One-time signal primitive

Sources: intra/dialers/retrier.go73-120 intra/dialers/direct_split.go37-45 intra/dialers/split_and_desync.go45-58 intra/dialers/retrier.go119-120

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Direct TCP Splitting

Implementation: splitter

The splitter type wraps a net.TCPConn and intercepts the first Write() call to split the data into multiple segments. This defeats DPI systems that rely on matching patterns in the first TCP segment.

Diagram: splitter Write Logic

The splitter ensures that only the first writer performs the split by using s.used.Signal(). intra/dialers/direct_split.go53-56 Subsequent writes or concurrent attempts that fail to signal (because the state already transitioned) fall back to a standard conn.Write(b). intra/dialers/direct_split.go58-59

Sources: intra/dialers/direct_split.go48-73

Split Strategies

The splitter supports two direct splitting strategies via writeSplit():

1. SplitTCP Strategy: Calls writeTCPSplit(w, b) to split the buffer at an arbitrary position. intra/dialers/direct_split.go64-65
2. SplitTCPOrTLS Strategy: Calls writeTCPOrTLSSplit(w, b) which attempts to parse TLS record headers. intra/dialers/direct_split.go66-67 If valid TLS is detected, it fragments the record into two segments by modifying the record length in the header.

Sources: intra/dialers/direct_split.go61-73

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TCB Desynchronization Attack

Implementation: overwriteSplitter

The overwriteSplitter implements a TCB (Transmission Control Block) desynchronization attack. It sends a "fake" packet with a low TTL that reaches the censor but not the destination, followed by the real packet. intra/dialers/split_and_desync.go43-55

Diagram: Desync Attack Workflow

Sources: intra/dialers/split_and_desync.go109-176 intra/dialers/split_and_desync.go183-258

TTL Estimation via UDP Traceroute

The system estimates the TTL distance to the DPI middlebox using tracert(). intra/dialers/split_and_desync.go109-176

1. UDP Probing: Sends UDP packets with increasing TTL (from 2 up to desync_max_ttl) to a range of destination ports. intra/dialers/split_and_desync.go156-174
2. ICMP Collection: Uses unix.Recvmsg with unix.MSG_ERRQUEUE to retrieve ICMP messages from the kernel error queue. intra/dialers/split_and_desync.go217-221
3. TTL Parsing: Identifies hop limits by checking for unix.IP_RECVERR or unix.IPV6_RECVERR in socket control messages via exceedsTTL and exceedsHopLimit. intra/dialers/split_and_desync.go73-105

Measured TTLs are stored in ttlcache, a core.Sieve[netip.Addr, int], for 30 seconds to avoid redundant traceroutes for the same destination. intra/dialers/split_and_desync.go35-41

Sources: intra/dialers/split_and_desync.go35-176 intra/dialers/split_and_desync.go216-258

Attack Mechanism

The attack involves precise socket option manipulation:

• TTL Restriction: The socket TTL is temporarily lowered using unix.IP_TTL or unix.IPV6_UNICAST_HOPS to ensure the fake segment reaches the censor but is dropped before the destination. intra/dialers/split_and_desync.go162-165
• Payload Overwriting: The overwriteSplitter structure manages a payload that must be smaller than the first written packet to effectively desynchronize the TCB state. intra/dialers/split_and_desync.go53-55
• Fake HTTP/1.1 String: The system provides a default fake string desync_http1_1str for HTTP-based desynchronization. intra/dialers/split_and_desync.go29

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Integration and Performance

First-Write Coordination: SigCond

Both splitters use SigCond to ensure only one goroutine performs the split on the first write. intra/dialers/direct_split.go49-59 intra/dialers/split_and_desync.go48-49

• Signal(): Atomically sets the condition to true and closes the internal channel to wake waiters. intra/core/sigcond.go70-76
• Cond(): Checks if the signal has already been fired. intra/core/sigcond.go41-43
• Wait(): Blocks until the condition is signaled. intra/core/sigcond.go47-52
• TryWait(timeout): Blocks with a timeout, returning true if signaled or false on timeout. intra/core/sigcond.go57-67

Data Flow and Performance

• rwext: Wraps a connection and extends read/write deadlines automatically on every I/O operation based on settings.DialerOpts. intra/rwconn.go21-24 intra/rwconn.go52-60 It also handles ReadFrom and WriteTo by disabling deadlines (extending forever) when zero-copy methods are used. intra/rwconn.go63-90
• ippPins: A core.Sieve that maintains a mapping between ip:port and successful dialer IDs to prioritize working paths in multi-dialer scenarios. intra/dialers/retrier.go68 intra/dialers/retrier.go177-196
• calcTimeout: Dynamically calculates retry timeouts based on RTT (SYN to SYNACK) to avoid unnecessary retries while maintaining responsiveness. intra/dialers/retrier.go134-142

Sources: intra/core/sigcond.go29-76 intra/rwconn.go19-124 intra/dialers/retrier.go68-196 intra/dialers/retrier.go134-142