- Qt 5.15
- Qt Network
- C++ Classes
- QDnsLookup
- Qt 5.15.19 Reference Documentation

# QDnsLookup Class

The QDnsLookup class represents a DNS lookup. More...

Header: | #include <QDnsLookup>
qmake: | QT += network
Since: | Qt 5.0

This class was introduced in Qt 5.0.

- List of all members, including inherited members

## Public Types

enum | Error { NoError, ResolverError, OperationCancelledError, InvalidRequestError, InvalidReplyError, …, NotFoundError }
enum | Type { A, AAAA, ANY, CNAME, MX, …, TXT }

## Properties

- error : const Error
- errorString : const QString
- name : QString

- nameserver : QHostAddress
- type : Type

## Public Functions

| QDnsLookup(Type type, const QString &name, const QHostAddress &nameserver, QObject *parent = nullptr)
| QDnsLookup(Type type, const QString &name, QObject *parent = nullptr)
| QDnsLookup(QObject *parent = nullptr)
virtual | ~QDnsLookup()
QList<QDnsDomainNameRecord> | canonicalNameRecords() const
Error | error() const
QString | errorString() const
QList<QDnsHostAddressRecord> | hostAddressRecords() const
bool | isFinished() const
QList<QDnsMailExchangeRecord> | mailExchangeRecords() const
QString | name() const
QList<QDnsDomainNameRecord> | nameServerRecords() const
QHostAddress | nameserver() const
QList<QDnsDomainNameRecord> | pointerRecords() const
QList<QDnsServiceRecord> | serviceRecords() const
void | setName(const QString &name)
void | setNameserver(const QHostAddress &nameserver)
void | setType(QDnsLookup::Type)
QList<QDnsTextRecord> | textRecords() const
Type | type() const

## Public Slots

void | abort()
void | lookup()

## Signals

void | finished()
void | nameChanged(const QString &name)
void | nameserverChanged(const QHostAddress &nameserver)
void | typeChanged(Type type)

## Detailed Description

QDnsLookup uses the mechanisms provided by the operating system to perform DNS lookups. To perform a lookup you need to specify a name and type then invoke the lookup() slot. The finished() signal will be emitted upon completion.

For example, you can determine which servers an XMPP chat client should connect to for a given domain with:

     void MyObject::lookupServers()
     {
         // Create a DNS lookup.
         dns = new QDnsLookup(this);
         connect(dns, SIGNAL(finished()),
                 this, SLOT(handleServers()));

         // Find the XMPP servers for gmail.com
         dns->setType(QDnsLookup::SRV);
         dns->setName("_xmpp-client._tcp.gmail.com");
         dns->lookup();
     }

Once the request finishes you can handle the results with:

     void MyObject::handleServers()
     {
         // Check the lookup succeeded.
         if (dns->error() != QDnsLookup::NoError) {
             qWarning("DNS lookup failed");
             dns->deleteLater();
             return;
         }

         // Handle the results.
         const auto records = dns->serviceRecords();
         for (const QDnsServiceRecord &record : records) {
             ...
         }
         dns->deleteLater();
     }

Note: If you simply want to find the IP address(es) associated with a host name, or the host name associated with an IP address you should use QHostInfo instead.

## Member Type Documentation

### enum QDnsLookup::Error

Indicates all possible error conditions found during the processing of the DNS lookup.

Constant | Value | Description
--- | --- | ---
`QDnsLookup::NoError` | `0` | no error condition.
`QDnsLookup::ResolverError` | `1` | there was an error initializing the system's DNS resolver.
`QDnsLookup::OperationCancelledError` | `2` | the lookup was aborted using the abort() method.
`QDnsLookup::InvalidRequestError` | `3` | the requested DNS lookup was invalid.
`QDnsLookup::InvalidReplyError` | `4` | the reply returned by the server was invalid.
`QDnsLookup::ServerFailureError` | `5` | the server encountered an internal failure while processing the request (SERVFAIL).
`QDnsLookup::ServerRefusedError` | `6` | the server refused to process the request for security or policy reasons (REFUSED).
`QDnsLookup::NotFoundError` | `7` | the requested domain name does not exist (NXDOMAIN).

### enum QDnsLookup::Type

Indicates the type of DNS lookup that was performed.

Constant | Value | Description
--- | --- | ---
`QDnsLookup::A` | `1` | IPv4 address records.
`QDnsLookup::AAAA` | `28` | IPv6 address records.
`QDnsLookup::ANY` | `255` | any records.
`QDnsLookup::CNAME` | `5` | canonical name records.
`QDnsLookup::MX` | `15` | mail exchange records.
`QDnsLookup::NS` | `2` | name server records.
`QDnsLookup::PTR` | `12` | pointer records.
`QDnsLookup::SRV` | `33` | service records.
`QDnsLookup::TXT` | `16` | text records.

## Property Documentation

### error : const Error

This property holds the type of error that occurred if the DNS lookup failed, or NoError.

Access functions:

Error | error() const

Notifier signal:

void | finished()

### errorString : const QString

This property holds a human-readable description of the error if the DNS lookup failed.

Access functions:

QString | errorString() const

Notifier signal:

void | finished()

### name : QString

This property holds the name to lookup.

Note: The name will be encoded using IDNA, which means it's unsuitable for querying SRV records compatible with the DNS-SD specification.

Access functions:

QString | name() const
void | setName(const QString &name)

Notifier signal:

void | nameChanged(const QString &name)

### nameserver : QHostAddress

This property holds the nameserver to use for DNS lookup.

Access functions:

QHostAddress | nameserver() const
void | setNameserver(const QHostAddress &nameserver)

Notifier signal:

void | nameserverChanged(const QHostAddress &nameserver)

### type : Type

This property holds the type of DNS lookup.

Access functions:

Type | type() const
void | setType(QDnsLookup::Type)

Notifier signal:

void | typeChanged(Type type)

## Member Function Documentation

### QDnsLookup::QDnsLookup(Type type, const QString &name, const QHostAddress &nameserver, QObject *parent = nullptr)

Constructs a QDnsLookup object for the given type, name and nameserver and sets parent as the parent object.

This function was introduced in Qt 5.4.

### QDnsLookup::QDnsLookup(Type type, const QString &name, QObject *parent = nullptr)

Constructs a QDnsLookup object for the given type and name and sets parent as the parent object.

### QDnsLookup::QDnsLookup(QObject *parent = nullptr)

Constructs a QDnsLookup object and sets parent as the parent object.

The type property will default to QDnsLookup::A.

### `[slot]` void QDnsLookup::abort()

Aborts the DNS lookup operation.

If the lookup is already finished, does nothing.

### `[signal]` void QDnsLookup::finished()

This signal is emitted when the reply has finished processing.

Note: Notifier signal for property error. Notifier signal for property errorString.

### `[slot]` void QDnsLookup::lookup()

Performs the DNS lookup.

The finished() signal is emitted upon completion.

### `[signal]` void QDnsLookup::nameChanged(const QString &name)

This signal is emitted when the lookup name changes. name is the new lookup name.

Note: Notifier signal for property name.

### `[signal]` void QDnsLookup::typeChanged(Type type)

This signal is emitted when the lookup type changes. type is the new lookup type.

Note: Notifier signal for property type.

### `[virtual]` QDnsLookup::~QDnsLookup()

Destroys the QDnsLookup object.

It is safe to delete a QDnsLookup object even if it is not finished, you will simply never receive its results.

### QList<QDnsDomainNameRecord> QDnsLookup::canonicalNameRecords() const

Returns the list of canonical name records associated with this lookup.

### QList<QDnsHostAddressRecord> QDnsLookup::hostAddressRecords() const

Returns the list of host address records associated with this lookup.

### bool QDnsLookup::isFinished() const

Returns whether the reply has finished or was aborted.

### QList<QDnsMailExchangeRecord> QDnsLookup::mailExchangeRecords() const

Returns the list of mail exchange records associated with this lookup.

The records are sorted according to RFC 5321, so if you use them to connect to servers, you should try them in the order they are listed.

### QList<QDnsDomainNameRecord> QDnsLookup::nameServerRecords() const

Returns the list of name server records associated with this lookup.

### QList<QDnsDomainNameRecord> QDnsLookup::pointerRecords() const

Returns the list of pointer records associated with this lookup.

### QList<QDnsServiceRecord> QDnsLookup::serviceRecords() const

Returns the list of service records associated with this lookup.

The records are sorted according to RFC 2782, so if you use them to connect to servers, you should try them in the order they are listed.

### QList<QDnsTextRecord> QDnsLookup::textRecords() const

Returns the list of text records associated with this lookup.