Storage of DNS Records in ENS

Abstract

This EIP defines a resolver profile for ENS that provides features for storage and lookup of DNS records. This allows ENS to be used as a store of authoritative DNS information.

Motivation

ENS is a highly desirable store for DNS information. It provides the distributed authority of DNS without conflating ownership and authoritative serving of information. With ENS, the owner of a domain has full control over their own DNS records. Also, ENS has the ability (through smart contracts) for a domain's subdomains to be irrevocably assigned to another entity.

Specification

The resolver profile to support DNS on ENS follows the resolver specification as defined in ERC-137.

Traditionally, DNS is a zone-based system in that all of the records for a zone are kept together in the same file. This has the benefit of simplicity and atomicity of zone updates, but when transposed to ENS can result in significant gas costs for simple changes. As a result, the resolver works on the basis of record sets. A record set is uniquely defined by the tuple (domain, name, resource record type), for example the tuple (example.com, www.example.com, A) defines the record set of A records for the name www.example.com in the domain example.com. A record set can contain 0 or more values, for example if www.example.com has A records 1.2.3.4 and 5.6.7.8 then the aforementioned tuple will have two values.

The choice to work at the level of record sets rather than zones means that this specification cannot completely support some features of DNS, such as zone transfers and DNSSEC. It would be possible to build a different resolver profile that works at the zone level, however it would be very expensive to carry out updates and so is not considered further for this EIP.

The DNS resolver interface consists of two functions to set DNS information and two functions to query DNS information.

setDNSRecords(bytes32 node, bytes data)

setDNSRecords() sets, updates or clears 1 or more DNS records for a given node. It has function signature 0x0af179d7.

The arguments for the function are as follows:

• node: the namehash of the fully-qualified domain in ENS for which to set the records. Namehashes are defined in ERC-137
• data: 1 or more DNS records in DNS wire format. Any record that is supplied without a value will be cleared. Note that all records in the same RRset should be contiguous within the data; if not then the later RRsets will overwrite the earlier one(s)

clearDNSZone(bytes32 node)

clearDNSZone() removes all DNS records for the domain. It has function signature 0xad5780af.

Although it is possible to clear records individually with setDNSRecords() as described above this requires the owner to know all of the records that have been set (as the resolver has no methods to iterate over the records for a given domain), and might require multiple transactions. clearDNSZone() removes all zone information in a single operation.

The arguments for the function is as follows:

• node: the namehash of the fully-qualified domain in ENS for which to clear the records. Namehashes are defined in ERC-137

dnsRecords(bytes32 node, bytes32 name, uint16 resource) view returns (bytes)

dnsRecords() obtains the DNS records for a given node, name and resource. It has function signature 0x2461e851.

The arguments for the function are as follows:

• node: the namehash of the fully-qualified domain in ENS for which to set the records. Namehashes are defined in ERC-137
• name: the keccak256() hash of the name of the record in DNS wire format.
• resource: the resource record ID. Resource record IDs are defined in RFC1035 and subsequent RFCs.

The function returns all matching records in DNS wire format. If there are no records present the function will return nothing.

hasDNSRecords(bytes32 node, bytes32 name) view returns (bool)

hasDNSRecords() reports if there are any records for the provided name in the domain. It has function signature 0x4cbf6ba4.

This function is needed by DNS resolvers when working with wildcard resources as defined in RFC4592.

The arguments for the function are as follows:

• node: the namehash of the fully-qualified domain in ENS for which to set the records. Namehashes are defined in ERC-137
• name: the keccak256() hash of the name of the record in DNS wire format.

The function returns true if there are any records for the provided node and name, otherwise false.

Rationale

DNS is a federated system of naming, and the higher-level entities control availability of everything beneath them (e.g. .org controls the availability of ethereum.org). A decentralized version of DNS would not have this constraint, and allow lookups directly for any domain with relevant records within ENS.

Backwards Compatibility

Not applicable.

Reference Implementation

The reference implementation of the DNS resolver is as follows:

pragma solidity ^0.7.4;
import "../ResolverBase.sol";
import "@ensdomains/dnssec-oracle/contracts/RRUtils.sol";

abstract contract DNSResolver is ResolverBase {
    using RRUtils for *;
    using BytesUtils for bytes;

    bytes4 constant private DNS_RECORD_INTERFACE_ID = 0xa8fa5682;
    bytes4 constant private DNS_ZONE_INTERFACE_ID = 0x5c47637c;

    // DNSRecordChanged is emitted whenever a given node/name/resource's RRSET is updated.
    event DNSRecordChanged(bytes32 indexed node, bytes name, uint16 resource, bytes record);
    // DNSRecordDeleted is emitted whenever a given node/name/resource's RRSET is deleted.
    event DNSRecordDeleted(bytes32 indexed node, bytes name, uint16 resource);
    // DNSZoneCleared is emitted whenever a given node's zone information is cleared.
    event DNSZoneCleared(bytes32 indexed node);

    // DNSZonehashChanged is emitted whenever a given node's zone hash is updated.
    event DNSZonehashChanged(bytes32 indexed node, bytes lastzonehash, bytes zonehash);

    // Zone hashes for the domains.
    // A zone hash is an ERC-1577 content hash in binary format that should point to a
    // resource containing a single zonefile.
    // node => contenthash
    mapping(bytes32=>bytes) private zonehashes;

    // Version the mapping for each zone.  This allows users who have lost
    // track of their entries to effectively delete an entire zone by bumping
    // the version number.
    // node => version
    mapping(bytes32=>uint256) private versions;

    // The records themselves.  Stored as binary RRSETs
    // node => version => name => resource => data
    mapping(bytes32=>mapping(uint256=>mapping(bytes32=>mapping(uint16=>bytes)))) private records;

    // Count of number of entries for a given name.  Required for DNS resolvers
    // when resolving wildcards.
    // node => version => name => number of records
    mapping(bytes32=>mapping(uint256=>mapping(bytes32=>uint16))) private nameEntriesCount;

    /**
     * Set one or more DNS records.  Records are supplied in wire-format.
     * Records with the same node/name/resource must be supplied one after the
     * other to ensure the data is updated correctly. For example, if the data
     * was supplied:
     *     a.example.com IN A 1.2.3.4
     *     a.example.com IN A 5.6.7.8
     *     www.example.com IN CNAME a.example.com.
     * then this would store the two A records for a.example.com correctly as a
     * single RRSET, however if the data was supplied:
     *     a.example.com IN A 1.2.3.4
     *     www.example.com IN CNAME a.example.com.
     *     a.example.com IN A 5.6.7.8
     * then this would store the first A record, the CNAME, then the second A
     * record which would overwrite the first.
     *
     * @param node the namehash of the node for which to set the records
     * @param data the DNS wire format records to set
     */
    function setDNSRecords(bytes32 node, bytes calldata data) external authorised(node) {
        uint16 resource = 0;
        uint256 offset = 0;
        bytes memory name;
        bytes memory value;
        bytes32 nameHash;
        // Iterate over the data to add the resource records
        for (RRUtils.RRIterator memory iter = data.iterateRRs(0); !iter.done(); iter.next()) {
            if (resource == 0) {
                resource = iter.dnstype;
                name = iter.name();
                nameHash = keccak256(abi.encodePacked(name));
                value = bytes(iter.rdata());
            } else {
                bytes memory newName = iter.name();
                if (resource != iter.dnstype || !name.equals(newName)) {
                    setDNSRRSet(node, name, resource, data, offset, iter.offset - offset, value.length == 0);
                    resource = iter.dnstype;
                    offset = iter.offset;
                    name = newName;
                    nameHash = keccak256(name);
                    value = bytes(iter.rdata());
                }
            }
        }
        if (name.length > 0) {
            setDNSRRSet(node, name, resource, data, offset, data.length - offset, value.length == 0);
        }
    }

    /**
     * Obtain a DNS record.
     * @param node the namehash of the node for which to fetch the record
     * @param name the keccak-256 hash of the fully-qualified name for which to fetch the record
     * @param resource the ID of the resource as per https://en.wikipedia.org/wiki/List_of_DNS_record_types
     * @return the DNS record in wire format if present, otherwise empty
     */
    function dnsRecord(bytes32 node, bytes32 name, uint16 resource) public view returns (bytes memory) {
        return records[node][versions[node]][name][resource];
    }

    /**
     * Check if a given node has records.
     * @param node the namehash of the node for which to check the records
     * @param name the namehash of the node for which to check the records
     */
    function hasDNSRecords(bytes32 node, bytes32 name) public view returns (bool) {
        return (nameEntriesCount[node][versions[node]][name] != 0);
    }

    /**
     * Clear all information for a DNS zone.
     * @param node the namehash of the node for which to clear the zone
     */
    function clearDNSZone(bytes32 node) public authorised(node) {
        versions[node]++;
        emit DNSZoneCleared(node);
    }

    /**
     * setZonehash sets the hash for the zone.
     * May only be called by the owner of that node in the ENS registry.
     * @param node The node to update.
     * @param hash The zonehash to set
     */
    function setZonehash(bytes32 node, bytes calldata hash) external authorised(node) {
        bytes memory oldhash = zonehashes[node];
        zonehashes[node] = hash;
        emit DNSZonehashChanged(node, oldhash, hash);
    }

    /**
     * zonehash obtains the hash for the zone.
     * @param node The ENS node to query.
     * @return The associated contenthash.
     */
    function zonehash(bytes32 node) external view returns (bytes memory) {
        return zonehashes[node];
    }

    function supportsInterface(bytes4 interfaceID) virtual override public pure returns(bool) {
        return interfaceID == DNS_RECORD_INTERFACE_ID ||
               interfaceID == DNS_ZONE_INTERFACE_ID ||
               super.supportsInterface(interfaceID);
    }

    function setDNSRRSet(
        bytes32 node,
        bytes memory name,
        uint16 resource,
        bytes memory data,
        uint256 offset,
        uint256 size,
        bool deleteRecord) private
    {
        uint256 version = versions[node];
        bytes32 nameHash = keccak256(name);
        bytes memory rrData = data.substring(offset, size);
        if (deleteRecord) {
            if (records[node][version][nameHash][resource].length != 0) {
                nameEntriesCount[node][version][nameHash]--;
            }
            delete(records[node][version][nameHash][resource]);
            emit DNSRecordDeleted(node, name, resource);
        } else {
            if (records[node][version][nameHash][resource].length == 0) {
                nameEntriesCount[node][version][nameHash]++;
            }
            records[node][version][nameHash][resource] = rrData;
            emit DNSRecordChanged(node, name, resource, rrData);
        }
    }
}

Security Considerations

Security of this solution would be dependent on security of the records within the ENS domain. This degenenrates to the security of the key(s) which have authority over that domain.

Copyright

Copyright and related rights waived via CC0.