Resolvers are specified in EIP137. A resolver must implement the following method:

function supportsInterface(bytes4 interfaceID) constant returns (bool);

supportsInterface is defined in EIP165, and allows callers to determine if a resolver supports a particular record type. Record types are specified as a set of one or more methods that a resolver must implement together. Currently defined record types include:

supportsInterface must also return true for the interfaceID value 0x01ffc9a7, which is the interface ID of supportsInterface itself.

Additionally, the content interface was used as a defacto standard for Swarm hashes, and has an interface ID of 0xd8389dc5. New implementations should use contenthash instead.

Example Resolver

A simple resolver that supports only the addr type might look something like this:

contract SimpleResolver {
    function supportsInterface(bytes4 interfaceID) constant returns (bool) {
        return interfaceID == 0x3b3b57de;
    }
​
    function addr(bytes32 nodeID) constant returns (address) {
        return address(this);
    }
}

This trivial resolver always returns its own address as answer to all queries. Practical resolvers may use any mechanism they wish to determine what results to return, though they should be constant, and should minimise gas usage wherever possible.