How to use to setup a DNS service to websites and to mail servers.
By Joe Mehaffey


DNS is the method by which your request to view <say> using your browser is initially setup.  When you make a request through your browser to make a connection to,  your browser and computer initiate a service request that goes to a DNS (DOMAIN NAME SERVER) associated with your Internet Service Provider (ISP).  This DNS system  maintains current tables of every registered domain (including in the world.  In the normal case,  the DNS receives your <> request,  the DNS responds back to your computer with the then current IP address of your desired destination (  A DNS offers many special features such as the ability to furnish directions to sub-domains <such as or or even just>.  A DNS can also provide proper destination addresses for mail servers and backup mail servers.  Also,  DNS can also be used to simply redirect an access to one HOST NAME (such as to some other HOST NAME such as <>.

As an example,  suppose you want to setup a domain name <> and a second domain <>.  Then you would like to have a mail server <>
residing at your home site that could act as a regular mail server where people can send email in and then the recipients can use as a
POP/IMAP mail server address and locally or remotely retrieve mail from the mail server.  A user could also add a mail service on the same machine using the same mail server software.  Suppose also that you have a website at and you wish to access the site by the “alias” of being able to enter or perhaps or maybe  Note: There is no practical difference between the subdomain <www> and for instance <support> excepting as defined by an A Record on the DNS system.  Both are serviced to their destination simply by the definition of the DNS server's A Record.  Examples of this follow.


Most home users have an IP address that can change hour to hour and this complicates building a home website and making it accessible to everyone on the
internet.  A service called DYNAMIC DNS allows users anywhere on the Internet to “find” your website(s) and/or mail server(s) with their constantly
changing IP address.   The function of a DNS is to provide a FIXED name for your website to "home to" with a fixed address for remote users on the internet to use to “find” your “Dynamically Changing” IP address.  The way this is done is for YOUR remote computers to forward a new IP address to the DDNS (Dynamic Domain Name Server) whenever your assigned (by your local Internet Service Provider) IP address changes.

The DDNS server system can accommodate multiple HOST NAMES for a single website if necessary.  You might have some combination of the
following Address records (called A Records): can  point to your main website. could also point to your main website or to some other place depending on your A Record setup. could point to your internet camera. could point to your mail server. could point to your backup or secondary mail server. could point to your friend George’s webpage on his own site. could point to could point to

The arrangement is very flexible and allows many variations. In a real sense,  DNS is like a TELEPHONE BOOK for the Internet.  DNS does not SEND you anyplace   You furnish DNS with a NAME (such as  DNS returns to you the IP address (analogus to a telephone number) of your specified domain name or subdomain so YOUR computer knows the IP address to which to direct its service request.

The DDNS server system also supports MX records.  These can be used for pointing incoming mail to your domains (such as to the proper
mail server.  Note that the DESTINATION ADDRESS of an MX record MUST be a domain name and NOT simply an IP address.  (The “why not”
appears to be part of an effort to prevent spam by increasing traceability of email.)

Examples of actual A records for our example using the formats

HOST (names)            CURRENT IP ADDRESS       OFFLINE URL (***)     *)                     *)          *)                        (blank)    *)                        (blank)  **)                        (blank)            (blank)                                 (blank)                                  (blank)                       

Note:  (*) above denotes a DYNAMIC IP address which is changed under control of a DDNS client on your computer system and (**) above denotes a FIXED or static IP address which is only changed under your manual control by a visit to the s, see below.  (***) Represents some remote URL (or local A-Record or C-Name that the DNS is providing the "alias" host name for.

In this example, the IP addresses represent John’s home computer IP address.  This would be updated periodically by a DDNS CLIENT (more on this later) running on John’s home computer or perhaps updated as a feature in John’s home firewall/router (such as NexLand or Zyxel and others).  The IP represents the IP address of John’s camera server which may be remote from John’s home computer and have an entirely different ISP and IP address.  It might also be the same as the IP address as for <> if the camera server was located behind the same firewall and router at John’s house.  The mail server is obviously located also at John’s house since it shares the IP address with other HOSTs located at John’s house.  The mail server has an entirely different IP address and so is likely located remotely from John’s home and perhaps is his backup email server in case of failure of his primary email server

The OFFLINE URL is a backup feature in case you want to direct your HOST to some URL address instead of a fixed or dynamic IP address.   You can put any URL you wish in the field and IF you check the “Is Offline” box in the A Record setup, the particular A Record address will be directed to your OFFLINE URL.  If your offline URL is not available, your internet browser or other client will give you an error message (404 not found or similar).

The STATIC IP ADDRESS above in (**) represents a mail server that does not have a “real name” or “host name” assigned to it.  The address is assumed to be static (fixed at all times in the future) and so you will not have your DDNS CLIENT program update this particular A Record.

MX RECORDS (eMail director files)

Incoming eMail to a domain arrives with its own distinctive protocol and port identity.  An email request is handled separately from other service requests because of some special needs and features of email processing.  For instance, you might want a backup mail server in case your primary mail server went down.  You might also want to have multiple mail servers to share the load in case you had a large volume of email.  MX RECORDS provide part of the solution for these needs.

The format of an MX Record might be as follows:         5         20

The above would indicate that incoming mail to would be first offered to the mail server at and if that server does not respond immediately it would be offered to the alternative server at  The 5 and 20 represent PREFERENCE NUMBERS.  These numbers have no meaning except that lower numbers have preference over larger numbers in sending email traffic to a particular server.

If you wish to disable one server so the other takes all of the email traffic, you simply remove one of the MX Records and the remaining MX Record
will direct all email traffic to your remaining mail server.


In the above example, some of the IP addresses (for,,, and must be kept up to date
with the current IP address whenever the local IP address changes at John’s house.  This is done by a DDNS CLIENT program.  The
IP address is “fixed” and is NOT to be updated by the DDNS CLIENT programs.  In our case, we will require a client program in the computer serving and another in the separate computer serving <>.  Note that even though the mail server at John’s house may be a
different machine from the machine hosting the website, a single client program running in ONE of the computers is all that is required
since the mail server and the webserver share the same internet connection and IP address.  You COULD also run a separate client program in each
machine to service “its own” DDNS hostnames if you wanted to do so.


DDNS Client Programs are very simple in concept and generally in practice.  Basically, what these programs do is to keep track of any changes in your
ISP’s assigned IP address for your connection, then when the IP address changes, send the necessary updates to ALL of your DDNS HOSTNAMES
that need it.  Note:  Your DDNS client program MUST (for send an individual update for each and every separate HOSTNAME on that needs the particular IP address update serviced by your particular client.  In other words,  if you have ten hostnames with endings of
<> on the DNS server,  and you only update 3 of them with your DDNS Client program,  the other 7 will remain with whatever IP
address you manually input.  In the alternative, you can have one DDNS client in one computer responsible for updating 3 DDNS IPs and another client
in another computer responsible for updating another 5 DDNS IPs and leave 2 DDNS IP entries either static or manual change only.


  • 1)      I did not understand that a single update from a DDNS CLIENT program running on my computer would ONLY update the specific “A Record”  pointed to.  I expected it to update all A Records with <say> as the domain name.  This was NOT correct.
  • 2)      I discovered that the DDNS Client in my NexLand 800 Turbo firewall/router would update the “dynamic DNS domain names” but  would NOT update any of the “custom DNS domain names”.  This is a “fault or a feature” of the 800 Turbo unit depending on how you look at things.  There was no workaround available at to assign some sort of "alias" so that a "dynamic DNS host name"  could be updated and then used as the destination host name by several A-Name records.  The remaining option was to use a software client instead of the 800 Turbo DDNS update feature to update multiple A-names on the custom website.
  • 3)      I expected that MX records could have an IP address in the DATA field.  At first,  I was able to input an IP address for my backup mail server  which had a fixed address and things seemed to work for MOST email.  However,  some ISPs will not forward mail to mail servers which have an IP address in the MX record’s data field instead of the required hostname such as  Adding an A Record with and putting in the MX Record’s DATA field fixed the problem. now prevents putting an IP address into the MX record under any circumstances.
  • 4)      I had to open port 110 in my firewall so my mail server could receive mail.  Depending on your mail server and if you wish to use other services such as SSL mail,  IMAP mail and other services,  it may be necessary to open other ports in your firewall.  Open as few as possible to get the job done to maintain maximum security.
  • 5)      My ISP did not allow the use of port 25 for outgoing SMTP mail delivery by my mail server directly to the internet.  I used instead the SMTP  RELAY SERVER feature of my Merak mail server to “just mail” outgoing email with the proper return addresses via the existing SMTP mail  server at my ISP.  This is working 100% OK with no problems.  I was glad that the original information I had (from the FAQ site) that there was no way to send out email via an internal mail server if port 25 was blocked by the ISP proved incorrect.   I am told that some ISPs also prohibit the kind of email relaying used by Merak,  but I have no other information on this.
  • 6)     It may "seem" intuititively obvious that you should be able to put the HOSTNAME used in another  address record (A-Record) into the  OFFLINE URL in an A Record used for a pointer to your MAIL SERVER (such as but IT DOES NOT WORK.  You must have your Dynamic DNS Client update this A record so it will have the correct IP Address.  HOWEVER,  You CAN put a host name (such as into the OFFLINE URL entry IF this host name is not a part of the DNS system.  (I have asked to consider changing this.)
  • 7)    Conversely to #6,  it is OK to use a URL in the OFFLINE URL window for "A Records OTHER than those associated with MX records.  You CAN use another A Record entry in the OFFLINE URL windows if the A Record is NOT associated with an MX record.
  • 8)    The latency time between when you update an entry on the DNS server (or any other such DNS server) and when the change takes effect on the Internet is highly variable.  Be prepared to be slightly confused at first when some of your changes take effect quite quickly while others can take hours.



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