How to Design Custom Block Policies

A block policy is a set of programmable constraints that govern transaction inclusion and block construction on a blockchain. Unlike static protocol rules, custom policies allow validators or node operators to enforce specific logic, such as filtering transactions from certain addresses, capping gas fees, or prioritizing transactions based on custom heuristics. This is a powerful tool for enhancing network security, optimizing performance, and implementing application-specific requirements directly at the consensus layer.

Designing an effective policy starts with defining its trigger and execution context. The trigger determines when the policy logic runs: it could be on receiving a new transaction, during block proposal, or at block finalization. The execution context provides the data the policy can inspect, such as the transaction's sender, calldata, gas price, or the current state of the mempool. For example, a policy to prevent MEV extraction might trigger on block proposal and inspect the pending transactions for sandwich attack patterns.

The core of any policy is its validation logic. This is where you implement the custom rules using a domain-specific language or, in more advanced systems, a WebAssembly (WASM) module. A simple policy written in a hypothetical DSL might look like:

codeCopy
policy NoHighGas {
  trigger: onTransactionReceived(tx)
  rule: tx.maxFeePerGas < 100 gwei
  action: reject
}

This policy rejects any transaction with a maxFeePerGas above 100 gwei. More complex logic can involve reading from smart contracts or performing multi-transaction analysis.

After defining the logic, you must decide on the enforcement action. Common actions include accept, reject, delay, or modify the transaction. A policy could delay low-priority transactions for a few blocks or modify a transaction's gas limit before inclusion. It's critical to ensure your policy's actions do not violate the underlying chain's consensus rules or create invalid blocks, as this could lead to slashing or chain forks.

Finally, consider the deployment and management lifecycle. Policies are typically deployed as on-chain modules or configured in a node's client software. On networks like Polygon, policies are managed via a PolicyManager smart contract. You must also plan for upgrades and revocation. A well-designed policy system includes versioning, pausing mechanisms, and clear governance for who can update the rules, ensuring the network remains adaptable and secure over time.

Designing custom block policies requires a solid grasp of blockchain fundamentals. You should understand how blocks are structured, what data they contain (headers, transactions, state roots), and the role of consensus mechanisms like Proof-of-Work or Proof-of-Stake. Familiarity with concepts such as block finality, reorgs (reorganizations), and network forks is essential, as your policy will define rules for accepting or rejecting blocks based on these properties. This knowledge is critical for writing logic that accurately assesses the validity and security of incoming chain data.

You must be proficient in a programming language suitable for writing policy logic, typically Go or Rust, as these are common in blockchain infrastructure. Your policy will be implemented as a deterministic function that evaluates a block and returns a PASS or REJECT verdict. You'll need to work with common data structures like block headers, cryptographic hashes (e.g., SHA-256, Keccak-256), and digital signatures. Experience with a blockchain client's RPC interface (e.g., Ethereum's eth_getBlockByNumber) is also necessary to fetch and inspect live chain data for testing.

A deep understanding of the specific chain you are writing a policy for is non-negotiable. This includes its unique consensus rules, gas mechanics, precompiles, and any historical hard forks that changed validation logic. For example, a policy for Ethereum must account for the Merge (transition to Proof-of-Stake), EIP-1559's base fee, and beacon chain finality. You should reference the chain's official documentation, such as the Ethereum Yellow Paper or a client's specification, to ensure your policy's rules are correct and complete.

Finally, you need a development and testing environment. This involves setting up a local node (like Geth or Erigon for Ethereum) or connecting to a reliable RPC provider to access chain data. You should write unit tests against historical block data to verify your policy's behavior during known network events—such as a deep reorg or a consensus fault. Using a framework like the Chainscore Policy SDK can streamline this process by providing type-safe bindings and testing utilities specific to policy development.

In blockchain architecture, validation and policy are complementary but distinct layers of logic. Validation refers to the core, immutable rules that define a state transition as valid or invalid. These are hard-coded consensus rules, like verifying a digital signature or ensuring a transaction doesn't double-spend. A transaction that fails validation is rejected by the network entirely. In contrast, a policy is a set of programmable, often mutable rules that determine if a valid state transition is desirable or permissible for a specific application. Think of validation as the law of physics for the chain, while policy is the local building code.

A practical example is a decentralized exchange (DEX). Validation ensures a swap transaction has a correct signature and the sender has sufficient token balance. A custom block policy, however, could be implemented to reject swaps that involve tokens from a sanctioned address list, even if the transaction is technically valid. This separation allows for application-specific governance and risk management without altering the underlying blockchain protocol. Policies are typically enforced by nodes or specialized middleware, like Chainscore's policy engine, which can inspect and filter transactions and blocks based on configurable logic.

Designing an effective custom policy starts with a clear policy objective. Common objectives include compliance (e.g., OFAC filtering), risk mitigation (e.g., blocking transactions from hacked contracts), and operational control (e.g., rate-limiting). You must then identify the precise data sources and trigger conditions. Will your policy inspect transaction calldata, specific event logs, sender/receiver addresses, or gas prices? The condition could be a simple list check or a complex off-chain computation whose result is verified on-chain via an oracle.

Here’s a conceptual framework for a policy contract snippet using a Solidity-like syntax. This policy checks if a transaction's recipient is on a blocked list maintained by a governance contract.

solidityCopy
// Example Policy Contract Skeleton
contract AddressBlockPolicy {
    address public governance;
    mapping(address => bool) public blockedAddresses;

    constructor(address _governance) {
        governance = _governance;
    }

    // Policy evaluation function
    function checkPolicy(
        address sender,
        address to,
        bytes calldata data
    ) external view returns (bool policyPassed) {
        // The core policy logic: block transactions to listed addresses
        if (blockedAddresses[to]) {
            return false; // Policy violated
        }
        return true; // Policy passed
    }

    // Function to update list (callable only by governance)
    function updateBlocklist(address addr, bool isBlocked) external {
        require(msg.sender == governance, "Unauthorized");
        blockedAddresses[addr] = isBlocked;
    }
}

This structure shows the separation: the checkPolicy function contains the business logic, while governance controls the mutable list.

Finally, integrate your policy with an execution layer. In systems like Chainscore, you would deploy your policy contract and register its address with a policy engine. This engine sits between the user and the network (or between the sequencer and the base layer), evaluating every transaction against all active policies. Transactions that pass validation but fail policy are dropped or queued for review, depending on the system's configuration. This architecture enables dynamic, community-governed security and compliance layers atop any EVM-compatible chain without requiring a hard fork.

A block policy in Geth is a set of rules that determines whether a block is considered valid by your node, beyond the core consensus rules of the Ethereum protocol. These policies are implemented in the eth/ethconfig/config.go file as a BlockPolicy struct. The primary function, CheckBlock, is called during block import and allows you to inspect header and body data. Common use cases include rejecting blocks from specific miners, enforcing gas limit ceilings, or requiring certain transaction types. This mechanism is distinct from a fork; it's a local filter for your node's view of the chain.

To implement a policy, you must define a struct that satisfies the BlockPolicy interface. The core method is func (p *YourPolicy) CheckBlock(block *types.Block) error. Within this method, you can access block properties like block.Header().Coinbase (the miner's address), block.Header().GasLimit, and block.Transactions(). If your validation logic deems the block invalid, return a descriptive error. Otherwise, return nil. For example, a policy to reject blocks from a known bad actor would check if the Coinbase matches a blacklisted address and return an error if it does.

After defining your policy struct, you must integrate it into Geth's node configuration. This is done by modifying the ethconfig.Defaults or by programmatically setting the Eth.BlockPolicy field when constructing your node stack via the node.New function. The policy is then injected into the Ethereum protocol manager. It's crucial to test your policy on a private testnet or dev mode first. Use geth --dev to mine blocks that trigger your conditions, ensuring your CheckBlock logic correctly accepts valid blocks and rejects those that violate your custom rules without causing chain synchronization issues.

Consider performance implications. The CheckBlock function is called for every new block your node processes, including during fast sync and during normal operation. Keep validation logic efficient to avoid slowing down block processing. For complex checks, consider caching results or using bloom filters. Remember, a block policy is a local rule; other nodes on the network may accept blocks your node rejects. This is useful for creating compliant nodes that adhere to regulatory requirements or for research nodes filtering specific data, but it does not affect network-wide consensus.

Erigon's block execution engine uses a modular policy system to validate incoming blocks before they are added to the chain. The core interface for this is the HeaderValidator and BlockValidator. A custom policy is a Go struct that implements one or more methods from these interfaces, such as ValidateHeader or ValidateBody. This design allows you to intercept the validation flow and inject your own logic—like checking for specific transaction patterns, gas limits, or miner addresses—without modifying Erigon's core consensus code.

To implement a policy, start by creating a new Go package. Your main struct should embed the default validator to maintain baseline Ethereum consensus rules. For example, a policy that rejects blocks from a specific miner would override ValidateHeader. You would extract the Coinbase address from the block header and compare it against a denylist. The method must return an error if the check fails, halting the block's import. All validation occurs within the context of the existing chain state, accessible via parameters like the ChainReader interface.

Integrating the policy requires modifying Erigon's initialization. In cmd/integration.go or your custom main.go, you must wrap the default engine with your policy. This is typically done by creating a new instance of your policy struct, passing the base engine as a dependency, and then setting this wrapped engine on the Ethereum backend. This ensures your custom checks run as part of the standard block import pipeline, including during sync and for new blocks propagated over the P2P network.

Thorough testing is critical. Write unit tests for your policy's validation logic using mocked headers and chain states. Furthermore, integrate it into a private development network using Erigon's --datadir and --private.api.addr flags to test block rejection and acceptance in a live environment. Monitor logs for your policy's error messages. Remember that an overly restrictive policy could cause your node to fork from the network, so logic should be carefully audited.

Practical use cases for custom policies include enterprise compliance (blacklisting addresses), research (filtering blocks for data analysis), or creating specialized sidechains with enhanced rules. By leveraging this interface, developers can tailor Erigon's behavior for specific needs while relying on its performant execution core for all standard operations. The complete source code for Erigon's validation interfaces can be found in the eth/consensus.go and core/block_validator.go files on the Erigon GitHub repository.

You now understand the key components of a custom block policy: the policy contract that defines the rules, the policy registry for on-chain management, and the policy engine for off-chain validation. By implementing these, you can enforce transaction-level controls like gas limits, whitelisted contracts, or compliance checks before a block is finalized. This moves security from the application layer to the consensus layer itself.

For practical implementation, start by defining your policy's logic in a Solidity contract that implements a standard interface, such as IBlockPolicyManager. Use tools like the Foundry testing framework to simulate policy enforcement against mock blocks. A critical next step is to integrate your policy with a client like Erigon or Geth by modifying the consensus engine to query your on-chain registry before importing a block.

Consider these advanced applications: a MEV-resistant policy that rejects blocks with sandwich attacks, a compliance policy for enterprise chains that validates regulatory flags, or a gas optimization policy for L2 rollups. Each requires careful benchmarking for performance impact, as every validation adds latency to block propagation.

The future of block policies is closely tied to modular blockchain architectures. With the rise of rollups and app-chains, custom execution environments will demand tailored consensus rules. Explore frameworks like the EigenLayer restaking primitive, which allows for the creation of actively validated services (AVSs) that could implement novel block validation logic.

To continue your learning, review the source code for existing implementations like Polygon's Bor client or Optimism's fault proof system. Engage with the research on verifiable block functions and participate in forums for client development, such as the Ethereum Magicians. Building a custom block policy is an advanced undertaking, but it provides unparalleled control over your chain's security and functionality.