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The Domain Name Service (DNS) protocol is what applications use to
resolve domain names into Internet addresses, as well as provide other
information about a domain, such as the name of the mail servers which
are responsible for receiving email for users in that domain. All of
the SocketTools components provide basic domain name resolution
functionality, but the Domain Name Services control gives an
application direct control over what servers are queried, the amount of
time spent waiting for a response and the type of information that is
returned.
The following properties, methods and events are available for use
by your application:
Initialize
Initialize the control and validate the runtime license key for the
current process. This method is normally not used if the control is
placed on a form in languages such as Visual Basic. However, if the
control is being created dynamically using a function similar to
CreateObject, then the application must call this method to
initialize the component before setting any properties or calling any
other methods in the control.
Reset
Reset the internal state of the control and re-initialize the
component to use the default nameserver configuration for the local
host. This can be useful if your application wishes to discard any
settings made by a user and return to using the local system
configuration.
Uninitialize
Unload the Windows Sockets library and release any resources that
have been allocated for the current process. This is the last method
call that the application should make prior to terminating. This is
only necessary if the application has previously called the
Initialize method.
Host Tables
When resolving a host name or IP address, the library will first
search the local system's host table, a file that is used to map host
names to addresses. On Windows 95/98 and Windows Me, if the file exists
it is usually found in C:\Windows\hosts. On Windows NT and later
versions, it is found in C:\Windows\system32\drivers\etc\hosts. Note
that the file does not have an extension.
HostFile
Return the full path of the file that contains the default host table
for the local system. This can be useful if you wish to temporarily
switch between the default host file and another host file specific
to your application.
Host Name Resolution
The control can be used to resolve host names into IP addresses, as
well as perform reverse DNS lookups converting IP addresses into the
host names that are assigned to them. The control will search the local
system's host table first, and then perform a nameserver query if
required.
HostAddress
A property which returns the IP address of the host name specified in
the HostName property. Setting this property to an IP address will
cause the control to perform a reverse DNS lookup to attempt to
determine the name of the host that was assigned that address. If
successful, the host name for the specified IP address can be
determined by reading the value of the HostName property.
HostName
A property which returns the name of the host associated with the IP
address specified in the HostAddress property. Setting this property
to a host name will cause the control to perform a DNS lookup to
determine the IP address of that host. If successful, the IP address
for the host can be determined by reading the value of the
HostAddress property.
Resolve
A method which resolves a host name into an IP address, returned as a
string in dotted notation. The control first checks the system's
local host table, and if the name is not found there, it will perform
a nameserver query for the A (address) record for that host.
Query
Perform a general nameserver query for a specific record type. This
method can be used to perform queries for the common record types
such as A and PTR records, as well as for other record types such as
TXT (text) records. Refer to the Technical Reference for more
information about the specific types of records that can be
returned.
Mail Exchange Records
When a system needs to deliver a mail message to someone, it needs
to determine what server is responsible for accepting mail for that
user. This is done by looking up the mail exchange (MX) record for the
domain. For example, if a message was addressed to joe@example.com, to
determine the name of the mail server that would accept mail for that
recipient, you would perform an MX record query against the domain
example.com. A domain may have more than one mail server, in which case
multiple MX records will be returned.
MailExchange
A property array which returns the mail exchanges for the domain
specified in the HostName property. This is a zero-based array, with
the maximum number of entries returned by the
MailExchanges
property
Advanced Properties
In addition to providing host name and IP address resolution, the
control can be used to perform advanced queries for other types of
records.
HostInfo
Return additional information about the specified host name. If the
name server has been configured to provide host information for the
domain, this method will return that data. Typically it is used to
indicate what hardware and operating system the host uses.
HostServices
Return information about the UDP and TCP based services that the host
provides. If defined, this will return a list of service names such
as "ftp" and "http". Note that your application
should not depend on this information to be a definitive list of what
services a server provides.
NameServer
A property array which can be used to return the current nameservers
that are configured for the local host, or the values can be changed
to specify new nameservers. The maximum number of nameservers that
can be configured for each instance of the control is four.
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