Peer Blacklist

The most fundamental trigger is a violation of the network's core consensus or communication protocol. This includes:

• Invalid block propagation: Sending blocks that fail validation rules.
• Invalid transaction propagation: Broadcasting transactions with malformed data or invalid signatures.
• Consensus rule breaches: Attempting to finalize a block that breaks the chain's state transition logic (e.g., double-spends).

Nodes can be blacklisted for exhausting network or peer resources, which is a form of Denial-of-Service (DoS) attack. Common triggers include:

• Message flooding: Sending an excessive volume of messages (e.g., transactions, requests) to overwhelm peers.
• Connection spam: Rapidly opening and closing many connections to a single peer.
• Unresponsive behavior: Consuming bandwidth and connection slots without providing useful data or responding to requests.

Propagating data known to be harmful or incorrect can trigger blacklisting to prevent network-wide corruption. This includes:

• Eclipse attack attempts: Selectively sending peers a manipulated view of the network to isolate them.
• Propagating known invalid data: Persistently sharing transactions or blocks that have already been globally marked as invalid.
• Withholding attacks: Behaving in a way consistent with intentionally withholding block data (a precursor to certain consensus attacks).

Attempts to create multiple fake identities (Sybil nodes) to gain disproportionate influence are a key defense target. Triggers include:

• Duplicate identity presentation: Multiple connections from the same IP or using the same node ID.
• IP address spoofing: Detected manipulation of network addresses.
• Rapid identity churn: A single entity rapidly introducing many new "peers" into the network's peer discovery pool.

While not malicious, chronically unreliable peers degrade network quality and may be blacklisted to optimize routing. This is often a temporary penalty and includes:

• High latency: Consistently slow response times that delay block/transaction propagation.
• Frequent disconnections: An unstable connection that repeatedly drops, forcing costly re-syncing.
• Stale data provision: Serving old chain data long after newer data is available.

In some networks, blacklisting can be initiated through on-chain governance votes or by authorized entities (e.g., foundation multisigs) in response to:

• Confirmed exploits: A node operator is conclusively linked to a theft or attack.
• Persistent protocol non-compliance: Ignoring mandated upgrades or soft forks.
• Legal/regulatory action: A court order or sanction requires the exclusion of an entity's nodes from the network.

A peer blacklist is a locally or network-maintained list of node identifiers (e.g., IP addresses, public keys, or peer IDs) that are prohibited from connecting to the network or to a specific client. Its primary purpose is to mitigate attacks such as:

• Eclipse attacks, where a node is surrounded by malicious peers.
• Sybil attacks, where an adversary creates many fake identities.
• Spam and resource exhaustion from abusive nodes. It functions as a first-line reputation filter.

Blacklists can be implemented at different levels with varying scopes:

• Client-Level: Managed locally by a single node (e.g., in a config file). Effective for personal defense but lacks network-wide impact.
• Network-Level: Propagated and enforced by protocol rules or a decentralized governance mechanism. More powerful but complex to coordinate.
• Dynamic vs. Static: Static lists are manually curated, while dynamic lists are updated automatically based on node behavior (e.g., violating protocol rules).

Blacklists have significant security limitations:

• IP/Identity Rotation: Malicious actors can easily change their IP address or create new peer IDs, rendering IP-based blacklists ineffective.
• Decentralization Trade-off: Aggressive network-wide blacklisting can centralize trust in the entity maintaining the list, contradicting P2P principles.
• False Positives: Legitimate nodes may be incorrectly blacklisted, leading to network partitioning or censorship.
• Resource Cost: Maintaining and distributing a large, current blacklist consumes network bandwidth and storage.

Blacklists are rarely sufficient alone and are used alongside other mechanisms:

• Peer Scoring: Systems like Ethereum's eth/65 or libp2p's gossipsub use reputation scores to demote rather than outright ban, allowing for recovery from mistakes.
• Proof-of-Work for Connection: Requiring a small proof-of-work for initial connection to increase Sybil attack cost.
• Graph-based Defense: Algorithms that analyze the connectivity graph to detect and isolate eclipse attack patterns.
• Allowlists (Whitelists): The inverse approach, only permitting connections from known-good peers, used in private networks.

A peer blacklist is analogous to a firewall's deny rule. It explicitly blocks traffic from specific sources. However, just as a firewall cannot stop an attacker who uses a new IP address, a P2P blacklist cannot stop a determined Sybil attack. Effective security requires a defense-in-depth approach, combining the firewall (blacklist) with intrusion detection systems (peer scoring), strong authentication (cryptographic handshakes), and network design (random peer selection).

A gossip protocol is a communication method where network nodes periodically exchange data with a random subset of peers, ensuring eventual consistency. Peer blacklists are often propagated via these protocols.

• Mechanism: When a node identifies a bad actor, it can gossip the blacklist entry to its peers.
• Example: In blockchain networks like Ethereum, libp2p uses gossip to share peer reputation data, including blacklists for peers sending invalid blocks.

A node reputation system assigns and tracks a score for each peer based on its behavior (e.g., uptime, valid transactions). A peer blacklist is the extreme enforcement action when a node's reputation falls below a critical threshold.

• Dynamic vs. Static: Reputation is dynamic; a blacklist is often a static, binary ban.
• Behaviors Penalized: Spamming, propagating invalid data, or being offline can degrade reputation leading to blacklisting.

Consensus rules are the immutable protocol rules all nodes must follow to validate blocks and transactions. A peer blacklist is a tool for enforcing these rules at the network layer by banning peers that consistently broadcast invalid data.

• Example: A peer that propagates a block violating the gas limit or containing a double-spend may be blacklisted by honest nodes.
• Distinction: This is separate from a chain reorganization, which is the blockchain itself rejecting invalid data.

Peer discovery is the process by which a node finds and connects to other nodes in the network, often starting from a list of bootnodes (hardcoded, trusted entry points). Peer blacklists interact with this system.

• Bootnodes: Provide initial peer lists; they are typically not blacklistable by design.
• Discovery Protocols: Protocols like Kademlia DHT (used in Ethereum) help find new peers, but nodes will cross-reference results against their local blacklist to avoid connecting to known bad actors.