This ATL COM component provides very simple Internet name resolving functionality (Domain Name System or DNS).
For example, it can automagically find the email (SMTP) servers available for your machine (see
"DNS Magic: Who's my email server?" for details).
This version has several bug fixes and finds DNS domain better than before (it uses the
complete "search list" as defined in the TCP/IP configuration).
Version 1.4 was a complete rewriting of the DNS component. Instead of using a porting of the
BIND 8 library (from UNIX to NT), it relies on the Microsoft Platform SDK (August 2001), which,
at last, contains a decent DNS API. Please refer to the implementation
section for more details.
For those of you who are using Windows 95/98/Millenium (where the Microsoft DNS API
is not available), you have two solutions: get the older versions of my
component (ask me) or simply copy the DNSAPI.DLL file from a Windows 2000
installation to the Windows\System directory of your system (Thanks to
Hans for this tip).
The Domain Name System (DNS) is a distributed host information database used in the Internet.
Most (if not all) Internet software (e.g. ping, telnet, ftp, web browsers, etc...)
use the DNS database to resolve IP addresses so that you, the user, can type
the name of a machine instead of its IP address (user-friendly, isn't it?).
Let me give you an example: when you type a Web address in your favorite browser (e.g. "www.kartmann.org"), the
browser fetches the corresponding IP address in the DNS database and uses this
address to connect to the Web server.
Information held in the DNS database can be:
an ip address (e.g. "209.132.1.74")
a hostname (e.g. "www.kartmann.org")
a canonical name (i.e. the real name of an IP alias)
a mail server name
etc...
Information in the DNS is held in Resource Records (RR). RRs come in several types,
which correspond to the varieties of data that can be contained in the DNS. Many RFCs
(most of them still experimental) propose additional RR types, like the geographical location (RFC 1712),
digital certificates (RFC 2538), cryptographic keys (RFC 2536), etc...
A application or library (or COM object) acting as a DNS client is called a resolver.
Application programs can use the domain name system via a resolver library (or COM object in our case).
The resolver sends queries corresponding to the library function, and waits for responses from the local name server.
The local name server can either:
Reply immediately if it knows the answer (i.e. if the query is about data in its name space)
Reply immediately if the answer is in its cache (DNS data has a Time-To-Live; the data in the cache must not
have expired yet).
Reply with a alternate server name for the request (non-recursive queries). The resolver must then
send the same query to the alternate server.
Send queries to foreign name servers, wait for answers and transmit them to the resolver (recursive queries).
Queries and Responses are usually sent via UDP (datagrams), in one (or more) packets (some implementations use
TCP instead of UDP).
Information in the DNS is held in Resource Records (RR); when a server replies to a resolver, it sends
resource records in its response. RRs come in different types and formats, as describes in this section.
All RRs have the same top level format shown below:
Where:
NAME
an owner name, i.e., the name of the node to which this resource record pertains.
TYPE
two octets containing one of the RR TYPE codes. Valid types include:
"A" (value 1) a host address
"CNAME" (value 5) the canonical name for an alias
"PTR" (value 12) a domain name pointer
"MX" (value 15) a mail exchanger
etc...
CLASS
two octets containing one of the RR CLASS codes. Valid classes are:
"IN" (value 1) the Internet
"CS" (value 2) the CSNET class (Obsolete)
"CH" (value 3) the CHAOS class (MIT)
"HS" (value 4) Hesiod (MIT)
TTL
a 32 bit signed integer that specifies the time interval (in seconds)
that the resource record may be cached before the source
of the information should again be consulted. Zero
values are interpreted to mean that the RR can only be
used for the transaction in progress, and should not be
cached.
RDLENGTH
an unsigned 16 bit integer that specifies the length in
octets of the RDATA field.
RDATA
a variable length string of octets that describes the
resource. The format of this information varies
according to the TYPE and CLASS of the resource record. See below for
list of the most common types.
berkeley sockets: the berkeley C library provides a very basic implementation of
the DNS via functions gethostbyname (resolve an hostname into its ip address) and
gethostbyaddr (reverse: from the ip address, finds the corresponding hostname).
Winsock: on Windows, you have the same functions (and their asynchronous versions
WSAAsyncGetHostByName and WSAAsyncGetHostByAddr).
BIND: the Berkeley Internet Name Domain is the most popular
implementation of the DNS specifications (full implementation with client
and server software).
Windows NT, Windows 2000 and UNIX systems provide a DNS resolver via the command-line program nslookup
(probably a port of the nslookup program shipped with BIND). With this program,
you can read information from the DNS database.
Here's an example sesssion of the nslookup program:
Start a MS-DOS command prompt and type the nslookup command:
In Windows 2000, you'll find an implementation of nslookup. Type 'help' to
get the full syntax of nslookup commands:
> help
Commands: (identifiers are shown in uppercase, [] means optional)
NAME - print info about the host/domain NAME usingdefault server
NAME1 NAME2 - as above, but use NAME2 as server
help or ? - print info on common commands
set OPTION - set an option
all - print options, current server and host
[no]debug - print debugging information
[no]d2 - print exhaustive debugging information
[no]defname - append domain name to each query
[no]recurse - ask for recursive answer to query
[no]search - use domain search list
[no]vc - always use a virtual circuit
domain=NAME - set default domain name to NAME
srchlist=N1[/N2/.../N6] - set domain to N1 and search list to N1,N2, etc.
root=NAME - set root server to NAME
retry=X - set number of retries to X
timeout=X - set initial time-out interval to X seconds
type=X - set query type (ex. A,ANY,CNAME,MX,NS,PTR,SOA,SRV)
querytype=X - same as type
class=X - set query class (ex. IN (Internet), ANY)
[no]msxfr - use MS fast zone transfer
ixfrver=X - current version to use in IXFR transfer request
server NAME - set default server to NAME, using current default server
lserver NAME - set default server to NAME, using initial server
finger [USER] - finger the optional NAME at the current default host
root - set current default server to the root
ls [opt] DOMAIN [> FILE] - list addresses in DOMAIN (optional: output to FILE)
-a - list canonical names and aliases
-d - list all records
-t TYPE - list records of the given type (e.g. A,CNAME,MX,NS,PTR etc.)
view FILE - sort an 'ls' output file and view it with pg
exit - exit the program
>
There's a strong link between Internet email and the DNS. Mail servers use
the DNS information database to route email messages from the originator to the
recipient.
Basically, when the recipient email address is "someone@somewhere.com", the
mail server searches the mail server (or Mail eXchanger) for domain "somewhere.com"
in the DNS database. It then connects to the mail server (on port 25) and sends
the email using SMTP.
Mail servers are registered in the DNS database as "Mail eXchanger" ("MX") records.
Note that there can be several registered mail servers for a given domain (with a
preference assigned to each server).
Using nslookup, you can find the email server of your domain, as shown in the
sample session below.
Start a MS-DOS command prompt and type the nslookup command:
C:\> nslookupDefault Server: youserver.yourdomain
Address: X.X.X.X
> set type=MX
> microsoft.com
microsoft.com MX preference = 10, mail exchanger = mail1.microsoft.com
microsoft.com MX preference = 20, mail exchanger = mail2.microsoft.com
microsoft.com MX preference = 30, mail exchanger = mail3.microsoft.com
microsoft.com MX preference = 40, mail exchanger = mail4.microsoft.com
microsoft.com MX preference = 50, mail exchanger = mail5.microsoft.com
The lowest preference indicates the best (primary) mail server. A mailer would
try it first and if it cannot connect to this server, it would use other servers
(by order of preference).
You can test this "DNS Magic" by using the nslookup program shipped with Windows 2000,
or with the DNS Magic HTML page. This component
provides
method ISimpleDNSClient::GetEmailServers()
to find the registered email servers for a given domain.
Base API is the new Windows 2000 DNS API
This component relies on the Windows DNS API provided by the Windows Platform SDK (August 2001).
The previous version was based on a porting of the BIND resolver library. Due to the high maintenance
cost associated with this library, I decided to drop it when the DNS API came out. See the
Component online documentation for more details.
Automatic Conversion of reverse lookups (PTR)
If your request is a reverse (IP address-> IP name) lookup (type PTR), then the component
silently converts request to a in-arpa format. That is, if you request a resolution of type
PTR for IP address "207.46.230.229", then the component will send a resolution request for
"229.230.46.207.in-addr.arpa" (PTR). The result will be "www.international.microsoft.com".
Find Email Server automatically
The method GetEmailServers sends a request of type MX (Mail eXchanger)
in order to find registered servers for a domain. It's only a shortcut for the
Resolve method.
Ignored Parameter: Resource Class
Due to limitations in the current Windows DNS API, the parameter
BResourceClass is ignored by the Resolve method (you should always use the
default class "C_IN" for Internet Class).
Ignored Property: ServerAddresses
Due to limitations (bugs ?) in the current Windows DNS API, the property
ServerAddresses is ignored by the Resolve method (the
component always uses the local machine DNS configuration to find the DNS servers).
Book: "DNS and BIND", by Paul Albitz & Cricket Liu, O'Reilly & Associates
If you have to deal with DNS, I strongly recommend you to read this book: it's
a very good presentation of DNS, from the protocol itself to the configuration tricks
of a DNS server (using BIND). http://www.oreilly.com/catalog/dns3/index.html
BIND (Berkeley Internet Name Domain), the reference implementation of the Domain Name System (DNS) protocols. http://www.isc.org/products/BIND
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