Ethereum Node Record (ENR)

An Ethereum Node Record (ENR) is a cryptographically signed, self-contained data structure that defines a node's identity and capabilities within the Ethereum network. It serves as the successor to the older multiaddr-based format, providing a more flexible and extensible way for nodes to advertise their connection information, supported protocols, and other metadata to potential peers. Each ENR contains essential details like the node's public key, IP address, TCP and UDP ports, and the Ethereum fork identifier (eth field).

The core innovation of the ENR is its use of a signed record, which prevents spoofing and ensures the integrity of the advertised information. A node generates its ENR by signing a set of key-value pairs with its private key, creating a verifiable attestation of its network identity. Other nodes can then verify this signature using the included public key, establishing trust in the connection details. This mechanism is fundamental to secure peer discovery in protocols like Discv5, Ethereum's main node discovery protocol.

ENRs are highly extensible through their key-value pairs, allowing them to carry application-specific data beyond basic networking info. For example, they can include fields for Layer 2 rollup support, light client data, or custom chain identifiers. This design enables the Ethereum network to evolve without requiring changes to the core discovery protocol, as new client capabilities can be signaled simply by adding new entries to the ENR.

In practice, when a node starts up, it generates or loads its ENR and shares it during the discovery process. Peers exchange these records to learn how to connect to each other directly. The enode URL, a legacy identifier, has largely been superseded by the ENR for modern clients. The shift to ENRs has been crucial for improving the resilience, security, and future-proofing of Ethereum's decentralized peer-to-peer layer.

An Ethereum Node Record (ENR) is a cryptographically signed data structure that serves as a self-contained identity card for a network participant. It contains essential metadata such as the node's public key, IP address, TCP and UDP ports, and supported network protocols. The core innovation is the inclusion of a digital signature, created using the node's private key, which allows any other peer to verify the authenticity and integrity of the information. This prevents spoofing and ensures that connection details are trustworthy.

The ENR's structure is defined by a simple key-value format, where keys are arbitrary strings and values are arbitrary byte sequences, allowing for extensibility. Common standardized keys include id (identity scheme, e.g., v4 for secp256k1), secp256k1 (the compressed public key), and ip/tcp/udp for network addresses. When a node starts, it generates or loads its ENR. This record is then shared during the Node Discovery Protocol via mechanisms like Discv5, where it is propagated through the network in FINDNODE and NODES messages.

The verification process is fundamental to the ENR's security. When a peer receives an ENR, it first decodes the record from its RLP-encoded format. It then extracts the public key from the secp256k1 field and uses it to verify the attached ECDSA signature against the signed portion of the record. If the signature is valid, the peer can trust the enclosed metadata. This creates a secure basis for building a peer-to-peer network's routing table without relying on a central authority.

ENRs are dynamic and can be updated. If a node's information changes—for instance, its IP address is reassigned—it creates a new ENR with an incremented sequence number and re-signs it. The sequence number is a critical field that allows peers to identify the most recent, and therefore authoritative, version of a node's record. This update mechanism is efficiently handled within the discovery protocol, ensuring the network's view of nodes remains current without excessive overhead.

In practice, ENRs enable the Ethereum Node Discovery Protocol (Discv5) to function as a secure, self-organizing directory. Nodes do not need a pre-configured list of bootstrap peers; they can find each other by gossiping ENRs. This system underpins the resilience of Ethereum's peer-to-peer (p2p) layer, allowing clients like Geth, Nethermind, and Besu to form a robust, decentralized mesh network essential for block and transaction propagation across the globe.

The Ethereum Node Record (ENR) is a self-certifying, flexible data structure that replaces the static, handshake-based node identity system used in the older RLPx protocol. An ENR is a signed, key-value record containing essential node information like its public keys, IP address, and supported network protocols. This signature, created using the node's private key, allows any peer to cryptographically verify the record's authenticity and integrity, establishing a secure foundation for peer discovery without a prior handshake.

The legacy RLPx protocol relied on a fixed, 512-bit secp256k1 public key as a node's sole identifier, exchanged during a dedicated cryptographic handshake. This design was rigid, as it could not natively convey additional metadata (like a node's listening TCP or UDP ports) or support alternative cryptographic identities (like BLS signatures). The shift to ENRs decouples node identity from the connection protocol, allowing a single record to declare support for multiple transport and discovery protocols, such as discv5, using different key types.

This evolution enables significant protocol agility. A node's ENR can be updated dynamically to reflect changes in its network address or capabilities, and the information is efficiently propagated through the discovery protocol. The structured, extensible format is crucial for supporting Ethereum's roadmap, including light clients and Ethereum 2.0 (the consensus layer), where nodes need to advertise their roles and attestation subnet subscriptions. ENRs form the backbone of modern peer-to-peer networking in the Ethereum ecosystem.

The Ethereum Node Record (ENR) is a standardized, self-certifying data structure that serves as the fundamental identity document for a node participating in the Discv5 discovery protocol. It replaces the older, less flexible ENode format by containing essential network information—such as the node's public key, IP address, and TCP/UDP ports—in a cryptographically signed, extensible format. This signature ensures the data's authenticity, preventing spoofing, while the extensible key-value pairs allow nodes to advertise additional capabilities, like supported network protocols or chain identifiers, without requiring protocol upgrades.

Within the Discv5 protocol, the ENR acts as both a query response and a session token. When a node receives a FINDNODE query, it returns a list of ENRs for peers closest to the target, enabling efficient distributed hash table (DHT) routing. Crucially, the ENR's signature allows any node to verify the information without a prior connection, establishing a trustless basis for the peer-to-peer network. This mechanism is vital for bootstrapping: a new node only needs a few initial bootstrap ENRs to begin discovering and validating the rest of the network's participants.

The extensibility of the ENR is key to its role in multi-network environments. Nodes can include specific key-value pairs, such as eth for Ethereum mainnet fork data or snap to signal support for the snap sync protocol. This allows Discv5 to serve multiple chains and sub-protocols simultaneously. When a node's information changes—for instance, its IP address—it generates a new ENR with an updated sequence number. Other nodes accept only the ENR with the highest sequence number, ensuring the network maintains the most current and accurate routing information for each participant.