Name Resolution

NameRegistry
The NameRegistry contract provides forward and reverse resolution.
Clients retrieve the current address of NameRegistry by reading it from the storage of the proxy contract NameRegistry.CheckAddress (as explained in the Proxy Contracts chapter).
Resolution by off-chain clients with view support (recommended)
Clients that have the ability to invoke TZIP-16 views should use the following to resolve names and addresses.
View: resolve-name
Resolves a name to an address, optionally other domain data, and expiry information for reference. If no such record exists or it has expired, it returns None.
Before passing a name for resolution, it should first be normalized using the encode algorithm.
Parameter Type
Description
bytes
The UTF-8 encoded name to resolve.
View: resolve-address
Resolves an address to a name, optionally other domain data, and expiry information for reference. If no such record exists or it has expired, it returns None.
Parameter Type
Description
address
The address to resolve.
Return type
The return type for both resolve-name and resolve-address is as follows:
CamelLIGO
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type resolved_domain = [@layout:comb] {
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    // The name of the resolved domain
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    name: bytes;
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​
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    // The address of the resolved domain, if any
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    address: address option;
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​
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    // A map of any additional data users wish to store with the domain
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    data: data_map;
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​
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    // The expiration date of the domain, if any.
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    // It is already
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    expiry: timestamp option;
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}
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​
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type return_type = resolved_domain option;
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Instructions for Off-chain Clients lacking view support
Clients that lack TZIP-16 view support can read from the contract storage directly. The NameRegistry contract has the following storage structure:
CameLIGO
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(* data map with all possible types that can be stored with an entity *)
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type data_map = (string, bytes) map
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​
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type record = {
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    (* The optional address the record resolves to *)
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    address: address option;
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​
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    (* The owner of the record allowed to make changes *)
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    owner: address;
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​
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    (* A map of any additional data clients wish to store with the domain *)
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    data: data_map
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​
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    (* Validator contract reference used for validating names of new subrecords *)
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    validator: nat option;
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​
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    (* Key to the expiry map containing the validity of this record *)
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    expiry_key: bytes option
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​
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    (* ... more fields outside of this interoperability spec *)
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}
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​
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type reverse_record = {
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    (* UTF-8 encoded name *)
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    name: bytes option;
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    (* The owner of the record allowed to make changes *)
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    owner: address;
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​
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    (* A map of any additional data clients wish to store with the record *)
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    data: data_map;
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    (* ... more fields outside of this interoperability spec *)
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}
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​
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type storage = {
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    (* Map of UTF-8 encoded names to forward records *)
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    records: (bytes, record) big_map;
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​
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    (* Map of addresses to reverse records *)
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    reverse_records: (address, reverse_record) big_map;
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​
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    (* Map containing expiry for every second-level domain *)
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    expiry_map: (bytes, timestamp) big_map;
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​
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    (* ... more fields outside of this interoperability spec *)
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}
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​
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type main_storage = {
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    (* Inner storage of the contract *)
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    storage: storage;
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​
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    (* ... more fields outside of this interoperability spec *)
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}
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Clients must not rely on a particular storage or record layout. They should always use annotations to find the correct value.
Forward Resolution (name to address)
The resolution algorithm is as follows:
1.Normalize and validate the full domain name using the encode algorithm. See the section Name Validation and Normalization for more details.
2.Look up the name in the records bigmap. If the bigmap contains no such key, the given domain is not resolvable.
3.Use the expiry_key value of the record to look up the validity in the expiry_map. If a timestamp is found and is lower or equal to the current time, the given domain is not resolvable.
4.Extract the optional address value from the record. If the optional value is None, the given domain is not resolvable. Otherwise use the address value.
Reverse Resolution (address to name)
The resolution algorithm is as follows:
1.Look up the address in the reverse_records bigmap. If the bigmap contains no such key, the given address is not resolvable.
2.Extract the optional name value. If the optional value is None, the given address is not resolvable. 
3.Use the name value to look up the corresponding forward record.  Use it's expiry_key value to look up the validity of the record in the expiry_map. If a timestamp is found and is lower or equal to the current timestamp, the given address is not resolvable
4.Otherwise, use the name value.
Instructions for Contracts
The resolution of names by contracts is currently not supported. Sometimes, it can be useful to validate on-chain that a name corresponds to an address. For example, a wallet might group a transaction sending money with another transaction that performs this check. If the check fails, both transactions fail and no money changes hands.
Both on-chain and off-chain clients can do this by calling the check_address entry-point on NameRegistry.CheckAddress. 
CameLIGO
Michelson
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type check_address_param = [@layout:comb] {
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    (* UTF-8 encoded name *)
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    name: bytes;
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​
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    (* expected address *)
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    address: address
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}
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​
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(* Checks that a name corresponds to an address. *)
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| Check_address of check_address_param
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parameter (or
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    (pair %check_address (bytes %name) (address %address)
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    # ... more entrypoints outside of this interoperability spec
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);
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The transaction will either do nothing (if the address is indeed correct) or fail with the message NAME_ADDRESS_MISMATCH if the address is incorrect.
Name Validation and Normalization
Domain names generally have to conform to the IDNA 2008 mechanism (RFC 5891, 5892, 5893). The Unicode Standard UTS 46 is a more specific (and stricter) application standard which is to be used for validation and normalization of domain names to achieve IDNA conformance.
Length Limit
The length limitations are currently:
1 to 100 characters in a single label,
up to 400 bytes in a full domain name.
It is not necessary for clients to check length when performing lookup or reverse lookup, as the names are already validated on-chain. It is however strongly recommended to do so when creating new records to avoid transaction failures.
Name Encode Algorithm
This algorithm is used both for domain creation and lookup purposes when communicating with contracts. It permits the dot character (.) so it can be used both for encoding names and individual labels.
1.The ToUnicode algorithm is used to produce a normalized and validated name string. The UTS 46 version of ToUnicode validates the string, making it notably different from ToUnicode defined in IDNA, which does not have to fail on invalid labels. The algorithm is invoked with the following parameters:
CheckHyphens = true
CheckBidi = true
CheckJoiners = true
UseSTD3ASCIIRules = true
Transitional_Processing = false
2.The name or label is encoded using UTF-8 into bytes.
Libraries
Some implementations of UTS 46 include:
​idna-uts46-hx for JavaScript
​idna for Python
Libraries that implement IDNA but not UTS 46 can alternatively be used. The string has to be first validated and normalized using ToAscii and the result converted back with ToUnicode. Some implementations of IDNA (that don't include validation and normalization as part of the ToUnicode algorithm and have to be called using ToUnicode(ToAscii(name))):
​System.Globalization.IdnMapping in .NET
​idna for Rust
​java.net.IDN in Java (only implements IDNA 2003)
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Contents
NameRegistry
Resolution by off-chain clients with view support (recommended)
Instructions for Off-chain Clients lacking view support
Instructions for Contracts
Name Validation and Normalization
Length Limit
Name Encode Algorithm
Libraries