Namehashing is an algorithm that converts a domain name in a classical format (like www.example.crypto) to ERC-721 token id. All .crypto ecosystem contracts accept a domain name as a method argument in the form of an ERC-721 token. Namehashing is defined as a part of the EIP-137 standard. See the standard for a text description of the algorithm.

To verify an implementation of the namehash algorithm, use the following reference table:

Reverse lookup

Fundamentally, namehashing is a one-way operation. It recursively hashes the labels using the SHA-256 hash function.

If one possesses a precomputed table of all hashes and corresponding domains reverse lookups are possible. This table can be reconstructed using the events on the CNS and UNS Registry NewURI event.

JSON RPC

The UNS and CNS metadata APIs track all domain names with their corresponding namehash. That makes it possible to obtain an original domain name from token metadata which can be retrieved from the token metadata URI.

The token metadata URI can be retrieved via ETH RPC call to ProxyReader#tokenURI. This works for CNS and UNS.

Now, the domain name along with other metadata can be retrieved by performing a simple GET request using the token metadata URI.

For example:

Javascript
let proxyReaderContractInstance = new Contract(address, abi); // Get a proxy reader contract instance using web3 or ethers
  
let tokenUri = await proxyReaderContractInstance.tokenURI("0x756e4e998dbffd803c21d23b06cd855cdc7a4b57706c95964a37e24b47c10fc9"); // call the tokenURI method
​
let metadataResponse = await fetch(tokenUri); // GET data from URI
let metadata = await metadataResponse.json(); // Parse it as json
  
console.log(metadata.name); // Retrieve the domain name from metadata

NewURI events

When a domain is created its name and token ID are logged using a `NewURI` event. This event can be looked up in order to reverse the namehash of a domain. This approach also works for CNS and UNS.

For an implementation example, see the `unhash` function of the resolution library.

Note that regardless of the way a namehash is reversed, it should always be forward-validated by hashing the retrieved domain name. In order to ensure the integrity of the retrieved domain name.

Public API

Unstoppable Domains maintains a public-facing API which can be used to obtain domain information with a namehash. To do this, call https://unstoppabledomains.com/api/v1/{namehash}

CNS Example Request:

$ curl https://unstoppabledomains.com/api/v1/0x0f4a10a4f46c288cea365fcf45cccf0e9d901b945b9829ccdb54c10dc3cb7a6f

CNS Example Response:

{
  "addresses": {},
  "whois": {},
  "ipfs": {},
  "gundb": {},
  "social": {},
  "meta": {
    "owner": "0x000000000000000000000000000000000000dead",
    "type": "CNS",
    "ttl": 0,
    "domain": "crypto",
    "namehash": "0x0f4a10a4f46c288cea365fcf45cccf0e9d901b945b9829ccdb54c10dc3cb7a6f"
  },
  "records": {}
}

UNS Example Request:

$curl https://unstoppabledomains.com/api/v1/0x4cf2206dea828b317ad033d760428ba3b0624b974bd5657bbeaef92d624e7b85

UNS Example Response:

{
"addresses": {},
"multicoinAddresses": {},
"whois": {},
"ipfs": {
"html": "QmQ38zzQHVfqMoLWq2VeiMLHHYki9XktzXxLYTWXt8cydu"
},
"social": {},
"dns": [],
"meta": {
"domain": "myjohnny.wallet",
"namehash": "0x4cf2206dea828b317ad033d760428ba3b0624b974bd5657bbeaef92d624e7b85",
"tokenId": "34803577301218677365125560751744006764397707708613262855939915661234351799173",
"owner": "0xe7474d07fd2fa286e7e0aa23cd107f8379085037",
"resolver": "0x049aba7510f45ba5b64ea9e658e342f904db358d",
"type": "UNS",
"ttl": 0
},
"records": {
"ipfs.html.value": "QmQ38zzQHVfqMoLWq2VeiMLHHYki9XktzXxLYTWXt8cydu"
}
}