Simple DNS Resolver v1.5


Author: Emmanuel KARTMANN.

Last Update: June 1st, 2002


Using the SimpleDNSResolver in Internet Explorer

OVERVIEW


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).


WHAT’S NEW


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).



CONTENTS



INTRODUCTION TO DNS

    What is DNS?

      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.


    The DNS Protocol


      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).



The DNS Protocol



      Queries and Responses are usually sent via UDP (datagrams), in one (or more) packets (some implementations use
      TCP instead of UDP).



    Resource Records


      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.


      General Resource Record Format



        All RRs have the same top level format shown below:

        DNS Resource Record Format

        Where:




















        NAMEan owner name, i.e., the name of the node to which this resource record pertains.
        TYPEtwo 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…

        CLASStwo 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)


        TTLa 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.
        RDLENGTHan unsigned 16 bit integer that specifies the length in
        octets of the RDATA field.
        RDATAa 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.


      Standard Resource Record Formats


    Well Known DNS Implementations


    • 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).

    Resolver Tool: nslookup


      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:


        C:\> nslookup
        Default Server: mynameserver.mydomain.com
        Address: XXX.XXX.XXX.XXX

        > myhost.mydomain.com
        Server: mynameserver.mydomain.com
        Address: XXX.XXX.XXX.XXX

        Name: myhost.mydomain.com
        Address: YYY.YYY.YYY.YYY

        > yourhost.mydomain.com
        *** mynameserver.mydomain.com can’t find yourhost.mydomain.com: Non-existent domain



      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 using default 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
      >



    DNS Magic: Who’s my email server?

      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 “[email protected]”, 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:\> nslookup
        Default 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.

      For more about email and DNS, please see the reference documentation.



COMPONENT FEATURES

This component:


  • implements the basics of DNS, as defined in
    RFC1034 and
    RFC1035,
  • uses the resolver cache, queries first with UDP, then retries with TCP if the response is truncated
  • asks the server to perform recursive resolution on behalf of the client to resolve the query
  • finds DNS server addresses in local configuration (using API or Windows Registry)
  • provides extended error information (ISupportErrorInfo and IErrorInfo are implemented)
  • provides very small executable: 52 KB (MinSize) to 60 KB (MinDependency)
  • requires no Graphical User Interface: the component can be used in non-GUI applications, like a Windows NT Service.
  • is integrated with SimpleEmailClient (another component): the latter calls method GetEmailServers to automatically find SMTP servers
  • runs on Windows 2000 (relies on Windows DNS API from the Platform SDK, August 2001)
  • compiles with VC++ 6.0 SP5


USAGE

To use this component:


  • create an instance of the component,
  • (optionally) put/get properties from interface ISimpleDNSClient:

    • ServerAddresses
    • Separator

  • call a method from interface ISimpleDNSClient:

  • handle errors (try/catch in C++, On Error Resume Next in VBScript)


SAMPLE CODE (VBScript)


    Dim oDNS
    ' Create object instance
    Set oDNS = CreateObject("Emmanuel.SimpleDNSClient.1")

    ' Declare output variable
    Dim found_names

    ' Set the server address(es)
    oDNS.ServerAddresses = "99.99.99.99"

    ' Set separator for output variable (if multiple results are found)
    oDNS.Separator = ", "

    ' (1) Find IP address of hostname "www.microsoft.com" (Internet class, type A)
    On Error Resume Next
    oDNS.Resolve "www.microsoft.com", found_names, "C_IN", "T_A"
    If Err <> 0 Then
        MsgBox Err.Description
    Else
        ' Show resolved names (within dialog box)
        MsgBox "Found names:" & vbCrLf & vbCrLf & found_names
    End If

    ' (2) Find Email Servers for domain "microsoft.com"
    On Error Resume Next
    oDNS.GetEmailServers "microsoft.com", found_names
    If Err <> 0 Then
        MsgBox Err.Description
    Else
        ' Show resolved names (within dialog box)
        MsgBox "Found names:" & vbCrLf & vbCrLf & found_names
    End If

Please refer to the test HTML files “TestSimpleDNSResolver.htm
and “TestDNSMagic.htm” for a full VBSsript example (see GUI below).

Using the SimpleDNSResolver in Internet Explorer


IMPLEMENTATION

  • 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).


You can download the Microsoft Platform SDK from the Microsoft Web Site:

http://www.microsoft.com/msdownload/platformsdk/sdkupdate



TO DO LIST


  • Test All DNS Resource Records (although all RR types are implemented, most of them couldn’t be tested)
  • Support DNS Security extensions (RFC 2535, “Domain Name System Security Extensions”)



REFERENCE DOCUMENTATION


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