Relay Censorship

In blockchain networks, relay nodes are responsible for receiving, validating, and forwarding data—such as transactions and new blocks—to their peers. Relay censorship occurs when an operator of one or more of these critical infrastructure nodes abuses this position to filter the data flow. This can involve blacklisting specific wallet addresses, transaction types (e.g., those interacting with a sanctioned smart contract), or entire blocks from particular validators. The goal is not to alter the data itself, which would be invalid, but to control its visibility and inclusion in the network's mempool or blockchain.

This form of censorship poses a significant threat to network neutrality and liveness. If a sufficiently large portion of the network's relay infrastructure is controlled by a censoring entity, valid transactions can be effectively silenced, creating a split view of the network state. For example, in Proof-of-Stake systems like Ethereum, if relay nodes serving major block builders censor transactions, those transactions may never be included in a proposed block, undermining the chain's credible neutrality. Defenses include diversifying relay infrastructure, using minimal viable gossip protocols like Erlay, and implementing peer-to-peer encryption to obscure transaction content from intermediaries.

A critical real-world concern is the potential for regulatory-driven relay censorship, where node operators in certain jurisdictions are compelled to filter transactions. This contrasts with validator-level censorship, where a block producer excludes transactions. The distinction is important: relay censorship happens before block production, while validator censorship happens during it. Projects like Ethereum's PBS (Proposer-Builder Separation) and encrypted mempools aim to mitigate these risks by separating the roles of transaction selection and block building, making it harder for any single party to control the flow of information.

Relay censorship is the practice where a relay—a specialized node that forwards transactions from users to block builders—selectively withholds or alters transactions based on their content or origin. This is a form of network-level censorship that occurs before a transaction is even considered for inclusion in a block. Relays act as a critical gateway in the proposer-builder separation (PBS) architecture common in modern blockchains like Ethereum, making their behavior a significant concern for network neutrality.

The mechanism typically involves the relay operator implementing a filtering policy. This can be based on: a blocklist of sanctioned addresses (e.g., those identified by regulatory bodies like OFAC), a list of prohibited smart contract interactions, or the geographic origin of the transaction. When a user's transaction is submitted to a censoring relay, the relay's software checks it against these rules. If it matches a forbidden pattern, the relay simply drops the transaction, never forwarding it to the block builders who assemble candidate blocks.

From an economic perspective, relay censorship is often driven by compliance pressures. Builders and validators may choose to connect only to relays that filter transactions to avoid legal risk, creating a market for "compliant" relay services. This creates a censorship vector where a small number of dominant, compliant relays could effectively control transaction flow. Technically, a user can bypass a censoring relay by submitting their transaction directly to a builder or a non-censoring relay, but this requires technical sophistication and awareness of the censorship happening.

The impact is measured by the censorship resistance of the network. If a large majority of block builders rely on a set of censoring relays, a significant portion of blocks may be built without including certain transactions, causing delays or failures for users of those services. This undermines the permissionless and neutral properties of the blockchain. Monitoring tools track metrics like the OFAC compliance rate, which shows the percentage of blocks built without any transactions from OFAC-sanctioned addresses.

Solutions and mitigations are an active area of protocol development. These include crLists (censorship-resistant lists), where validators can force the inclusion of transactions, and builder reputation systems that penalize consistent exclusion. Ultimately, relay censorship highlights the ongoing tension between decentralized network ideals and external regulatory pressures, with its mechanics rooted in the strategic points of control within a blockchain's transaction supply chain.

Relay censorship occurs when the centralized operators of mev-boost relays—the intermediaries between block builders and Ethereum validators—refuse to forward certain transactions, such as those interacting with sanctioned addresses. This creates a central point of failure, undermining the network's permissionless and neutral properties. Mitigations aim to decentralize this critical function, ensuring no single entity can control transaction inclusion. The primary goal is to preserve credible neutrality and liveness for all network participants.

Technical solutions focus on removing the trusted relay intermediary. Permissionless relays allow any validator to connect directly to a public, open network of block builders without a centralized gatekeeper. Protocols like EigenLayer's EigenDA for data availability and SUAVE (Single Unifying Auction for Value Expression) envision a decentralized future for block building and relay functions. Furthermore, in-protocol proposer-builder separation (PBS), a potential future Ethereum upgrade, would formalize the builder-proposer relationship at the consensus layer, eliminating the need for off-chain relays entirely.

Economic and game-theoretic approaches incentivize decentralization. Relay staking models, where operators must bond significant capital (slashing for malicious behavior), increase the cost of censorship. Multi-relay architectures encourage validators to connect to multiple relays, reducing dependence on any single one. The concept of censorship resistance as a service emerges, where specialized providers guarantee transaction inclusion for a fee, creating a competitive market that bypasses censoring relays.

Validator and user-level strategies provide immediate, practical defenses. Validators can implement proposer commitments, pledging to build their own blocks if relays withhold censored transactions, or use minimal viable relays that only perform attestation duties. End-users and applications can employ privacy techniques like mixing or cross-domain transactions to obscure origin, or route transactions through alternative entry points such as Flashbots Protect RPC, which aggregates builders to find a non-censoring path.