DNS Component

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The DNS component is used to query DNS for record information.

Syntax

TipwDNS

Remarks

The DNS component queries a DNS server for records as specified in RFC 1035 and other relevant RFCs. The component property QueryType indicates the record to be queried. Calling Query will initiate the query.

Once the query is complete, a single Response event will fire and the record properties will be filled. StatusCode is the server's success or fail code, and Status is a standard string interpretation of the code.

The records are contained in the Records properties. The programming interface is designed to allow a full walk of the records in the DNS response, including provisions for handling new record types.

The component operates synchronously by default (i.e., it waits for a response before returning control to the caller); however, the component also may operate asynchronously (i.e., return control immediately), by setting Timeout to 0. Please see the Timeout property for more information.

Property List

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The following is the full list of the properties of the component with short descriptions. Click on the links for further details.

Method List

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The following is the full list of the methods of the component with short descriptions. Click on the links for further details.

Event List

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The following is the full list of the events fired by the component with short descriptions. Click on the links for further details.

Config Settings

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The following is a list of config settings for the component with short descriptions. Click on the links for further details.

DNSPort Property (DNS Component)

This is the port to connect to on the DNS server.

Syntax

property DNSPort: Integer read get_DNSPort write set_DNSPort;

Default Value

53

Remarks

This property contains the port to connect to on the DNS server. A valid port number (a value between 1 and 65535) is required for the connection to take place. The property must be set before a connection is attempted and cannot be changed once a connection is established. Any attempt to change this property while connected will fail with an error.

This property is not available at design time.

DNSServer Property (DNS Component)

This is the address of the DNS server.

Syntax

property DNSServer: String read get_DNSServer write set_DNSServer;

Default Value

''

Remarks

This property contains the address of the DNS server. The component attempts to find the default DNS server for the machine where it is installed, and provide it in the DNSServer property. You may change it to any particular DNS server you want to query. While any internet host running a DNS service will suffice, it is preferable to use either the DNS server for your network or the DNS server for the domain that you are querying.

If the component cannot determine the default DNS server address, the property value will be an empty string.

Note: Automatic discovery of the default DNS server is not supported in the Java edition.

This property is not available at design time.

Idle Property (DNS Component)

The current status of the component.

Syntax

property Idle: Boolean read get_Idle;

Default Value

true

Remarks

This property will be False if the component is currently busy (communicating or waiting for an answer), and True at all other times.

This property is read-only.

LocalHost Property (DNS Component)

The name of the local host or user-assigned IP interface through which connections are initiated or accepted.

Syntax

property LocalHost: String read get_LocalHost write set_LocalHost;

Default Value

''

Remarks

This property contains the name of the local host as obtained by the gethostname() system call, or if the user has assigned an IP address, the value of that address.

In multihomed hosts (machines with more than one IP interface) setting LocalHost to the IP address of an interface will make the component initiate connections (or accept in the case of server components) only through that interface. It is recommended to provide an IP address rather than a hostname when setting this property to ensure the desired interface is used.

If the component is connected, the LocalHost property shows the IP address of the interface through which the connection is made in internet dotted format (aaa.bbb.ccc.ddd). In most cases, this is the address of the local host, except for multihomed hosts (machines with more than one IP interface).

Note: LocalHost is not persistent. You must always set it in code, and never in the property window.

Protocol Property (DNS Component)

This protocol is used for the DNS request.

Syntax

property Protocol: TipwDNSProtocols read get_Protocol write set_Protocol;

TipwDNSProtocols = (
  ptUDP,
  ptTCP,
  ptTLS,
  ptHTTPS
);

Default Value

ptUDP

Remarks

This property specifies the protocol used for the DNS request when Query is called. Possible values include the following:

QueryType Property (DNS Component)

This is the type of record to query.

Syntax

property QueryType: TipwTQueryTypes read get_QueryType write set_QueryType;

TipwTQueryTypes = (
  qtAddress,
  qtNS,
  qtMD,
  qtMF,
  qtCName,
  qtSOA,
  qtMailBox,
  qtMailGroup,
  qtMR,
  qtNULL,
  qtWKS,
  qtPointer,
  qtHostInfo,
  qtMailInfo,
  qtMX,
  qtText,
  qtRP,
  qtAFSDB,
  qtX25,
  qtISDN,
  qtRT,
  qtAAAA,
  qtSRV,
  qtNAPTR,
  qtCERT,
  qtRevLookup
);

Default Value

qtAddress

Remarks

Before a query is attempted, this property must be set to the desired record type. The records are defined as follows:

This property is not available at design time.

Records Property (DNS Component)

This is the collection of records in the last response.

Syntax

property Records: TipwDNSRecordList read get_Records;

Remarks

Once the query is complete, a single Response event will fire. The following properties will contain the results:

Example (loop through all fields of all records)

For i = 0 to DNSControl.RecordsCount - 1 For j = 0 to DNSControl.RecordFieldCount(i) - 1 DNSControl.Records(i).RecordFieldIndex(i) = j DoSomethingWith(DNSControl.RecordFieldName(i), DNSControl.RecordFieldValue(i)) Next j Next i

This property is read-only and not available at design time.

Please refer to the DNSRecord type for a complete list of fields.

RecordSource Property (DNS Component)

The source of records provided in the Records properties.

Syntax

property RecordSource: TipwTRecordSources read get_RecordSource write set_RecordSource;

TipwTRecordSources = (
  rsAnswerSection,
  rsNameServerSection,
  rsAdditionalRecordsSection
);

Default Value

rsAnswerSection

Remarks

This property contains the source of records provided in the Records properties and can have one of the following values:

This property is not available at design time.

Recursive Property (DNS Component)

This determines whether to ask the DNSServer to recursively query other servers.

Syntax

property Recursive: Boolean read get_Recursive write set_Recursive;

Default Value

true

Remarks

If this property is True, the component will tell the DNS server to perform the query recursively. This means that if the DNS server does not have information about the domain being queried, it will send the request to another DNS server.

This property is not available at design time.

RequestId Property (DNS Component)

This is the Id of the current DNS request.

Syntax

property RequestId: Integer read get_RequestId write set_RequestId;

Default Value

1

Remarks

This property is the identifier of the request that generated the current response values. Each time the component makes a DNS query, it generates a unique request identifier to track the response.

If a custom value is needed for RequestId, the property must be set before sending a request. The component increments RequestId automatically after sending each request.

Note: The component raises an exception when a value that is not between 1 and 32,767 is specified.

This property is not available at design time.

Status Property (DNS Component)

This is a standard string interpretation of StatusCode .

Syntax

property Status: String read get_Status;

Default Value

''

Remarks

This property contains a standard string interpretation of StatusCode. For a list of valid error codes and their descriptions, please see the Error Codes section.

This property is read-only and not available at design time.

StatusCode Property (DNS Component)

This contains the success or error code for the DNS query.

Syntax

property StatusCode: Integer read get_StatusCode;

Default Value

0

Remarks

This property contains the success or error code for the DNS query. For a list of valid error codes and their descriptions, please see the Error Codes section.

This property is read-only and not available at design time.

Timeout Property (DNS Component)

This property includes the timeout for the component.

Syntax

property Timeout: Integer read get_Timeout write set_Timeout;

Default Value

60

Remarks

If the Timeout property is set to 0, all operations return immediately, potentially failing with a WOULDBLOCK error if data cannot be sent immediately.

If Timeout is set to a positive value, data is sent in a blocking manner and the component will wait for the operation to complete before returning control. The component will handle any potential WOULDBLOCK errors internally and automatically retry the operation for a maximum of Timeout seconds.

The component will use DoEvents to enter an efficient wait loop during any potential waiting period, making sure that all system events are processed immediately as they arrive. This ensures that the host application does not freeze and remains responsive.

If Timeout expires, and the operation is not yet complete, the component raises an exception.

Note: By default, all timeouts are inactivity timeouts, that is, the timeout period is extended by Timeout seconds when any amount of data is successfully sent or received.

The default value for the Timeout property is 60 seconds.

Config Method (DNS Component)

Sets or retrieves a configuration setting.

Syntax

function Config(ConfigurationString: String): String;

Remarks

Config is a generic method available in every component. It is used to set and retrieve configuration settings for the component.

These settings are similar in functionality to properties, but they are rarely used. In order to avoid "polluting" the property namespace of the component, access to these internal properties is provided through the Config method.

To set a configuration setting named PROPERTY, you must call Config("PROPERTY=VALUE"), where VALUE is the value of the setting expressed as a string. For boolean values, use the strings "True", "False", "0", "1", "Yes", or "No" (case does not matter).

To read (query) the value of a configuration setting, you must call Config("PROPERTY"). The value will be returned as a string.

DoEvents Method (DNS Component)

This method processes events from the internal message queue.

Syntax

procedure DoEvents();

Remarks

When DoEvents is called, the component processes any available events. If no events are available, it waits for a preset period of time, and then returns.

GetFieldValue Method (DNS Component)

This gets the value of the field indicated from a record.

Syntax

function GetFieldValue(RecordIndex: Integer; FieldName: String): String;

Remarks

This method will get the value of the DNS record field specified by FieldName from the DNS record at RecordIndex. If no such field exists for that record type, the component raises an exception.

Interrupt Method (DNS Component)

This method interrupts the current method.

Syntax

procedure Interrupt();

Remarks

If there is no method in progress, Interrupt simply returns, doing nothing.

Query Method (DNS Component)

This method queries a domain.

Syntax

procedure Query(Domain: String);

Remarks

This method will query the host Domain for any records of the type in QueryType. After the query, RequestId is the identifier corresponding to the request. Once the query is complete, a single Response event will fire. The following properties will contain the results:

Reset Method (DNS Component)

This method will reset the component.

Syntax

procedure Reset();

Remarks

This method will reset the component's properties to their default values.

Error Event (DNS Component)

Fired when information is available about errors during data delivery.

Syntax

type TErrorEvent = procedure (
  Sender: TObject;
  ErrorCode: Integer;
  const Description: String
) of Object;

property OnError: TErrorEvent read FOnError write FOnError;

Remarks

The Error event is fired in case of exceptional conditions during message processing. Normally the component raises an exception.

The ErrorCode parameter contains an error code, and the Description parameter contains a textual description of the error. For a list of valid error codes and their descriptions, please refer to the Error Codes section.

Log Event (DNS Component)

This event fires once for each log message.

Syntax

type TLogEvent = procedure (
  Sender: TObject;
  LogLevel: Integer;
  const Message: String;
  const LogType: String
) of Object;

property OnLog: TLogEvent read FOnLog write FOnLog;

Remarks

This event fires once for each log message generated by the component. The verbosity is controlled by the LogLevel setting.

LogLevel indicates the level of message. Possible values are as follows:

The value 1 (Info) logs basic information, including the URL, HTTP version, and status details.

The value 2 (Verbose) logs additional information about the request and response.

The value 3 (Debug) logs the headers and body for both the request and response, as well as additional debug information (if any).

Message is the log entry.

LogType identifies the type of log entry. Possible values are as follows:

• "Info"
• "RequestHeaders"
• "ResponseHeaders"
• "RequestBody"
• "ResponseBody"
• "ProxyRequest"
• "ProxyResponse"
• "FirewallRequest"
• "FirewallResponse"

Response Event (DNS Component)

This event fires after a DNS query.

Syntax

type TResponseEvent = procedure (
  Sender: TObject;
  RequestId: Integer;
  const Domain: String;
  StatusCode: Integer;
  const Description: String;
  Authoritative: Boolean
) of Object;

property OnResponse: TResponseEvent read FOnResponse write FOnResponse;

Remarks

The Response event will fire after a call to Query. The RequestId parameter contains the request Id associated with the original query. Domain is the domain of the query. The Authoritative parameter is true if the response is from an authoritative name server. If there is an error returned by the server, StatusCode and Description provide them.

Once the query is complete, a single Response event will fire. The following properties will contain the results:

SSLServerAuthentication Event (DNS Component)

This event fires when connecting to the server.

Syntax

type TSSLServerAuthenticationEvent = procedure (
  Sender: TObject;
  const RemoteAddress: String;
  RemotePort: Integer;
  const CertEncoded: String;
  const CertEncodedB: TBytes;
  const CertSubject: String;
  const CertIssuer: String;
  const Status: String;
  var Accept: Boolean
) of Object;

property OnSSLServerAuthentication: TSSLServerAuthenticationEvent read FOnSSLServerAuthentication write FOnSSLServerAuthentication;

Remarks

This event allows the client to decide whether or not to continue with the connection process. The Accept parameter recommends whether to continue or close the connection. This is just a suggestion: application software must use its own logic to determine whether or not to continue.

When Accept is False, Status shows why the verification failed (otherwise, Status contains the string "OK"). If the client decides to continue, you can override and accept the certificate by setting the Accept parameter to True.

RemoteAddress is the IP address of the server.

RemotePort is the source port of the server.

CertEncoded is the Base64-encoded certificate presented by the server.

CertSubject is the subject of the certificate presented by the server.

CertIssuer is the subject of the issuer of the certificate presented by the server.

Status is the status of the certificate.

Accept defines whether the certificate is accepted.

SSLStatus Event (DNS Component)

This shows the progress of the secure connection.

Syntax

type TSSLStatusEvent = procedure (
  Sender: TObject;
  const RemoteAddress: String;
  RemotePort: Integer;
  const Message: String
) of Object;

property OnSSLStatus: TSSLStatusEvent read FOnSSLStatus write FOnSSLStatus;

Remarks

The event is fired for informational and logging purposes only. It is used to track the progress of the connection.

RemoteAddress is the IP address of the remote machine.

RemotePort is the port of the remote machine.

Message is the log message.

DNSRecord Type

This is a record that is returned by the DNS server.

Remarks

When a Query method is performed by the DNS component, the DNS server returns one or more DNSRecords containing information about the domain you specified. This information is contained in the different fields of the record.

Each record may have multiple FieldNames and corresponding FieldValues. You may iterate through these by setting the FieldIndex field.

Fields

Constructors

constructor Create();

Config Settings (DNS Component)

The component accepts one or more of the following configuration settings. Configuration settings are similar in functionality to properties, but they are rarely used. In order to avoid "polluting" the property namespace of the component, access to these internal properties is provided through the Config method.

DNS Config Settings

UDP Config Settings

Socket Config Settings

Base Config Settings

Trappable Errors (DNS Component)

DNS Errors

The component may also return one of the following error codes, which are inherited from other components.

UDP Errors

SSL Errors

TCP/IP Errors