DNS server

From Email Marketing Wiki
Jump to: navigation, search

Welcome to the world of DNS - it can be a tricky thing to understand. A lot of people wonder where their website has gone when they migrate hosts. Hopefully the following will assist in your understanding of the process.

Please note that the following is not necessarily technically perfect - however it will aid any newcomers to the world of hosting and DNS in understanding why they can't get to their site .

To get down to basics, every domain name 'resolves' to an IP address. A DNS entry is used to achieve this.

So how does this DNS thing work?

All DNS entries have a TTL or Time to Live. The most common TTL for a domain is 86400 seconds, or 24 hours (1 day.)

When you type a domain name into your browser, it sends a request to your ISP's DNS servers to find out the IP address of your domain. If your ISP's DNS servers don't know what IP address your hosting is bound to, your ISP's DNS servers will contact your current name servers to find out what it is. Your ISP's DNS servers will then 'cache' this record for a period of time equal to your domain's TTL.

So, the most common situation when transferring hosts is as follows - you access your web site via its domain name and your ISP caches the entry. You download all the content on your current site and then change the name servers associated with your domain name to MediaCloud's Name Servers]]. Your ISP however still retains the old cached entry until the TTL expires.

DNS Server

The domain name system or domain name server (DNS) is a system that stores and associates many types of information with domain names, but most importantly, it translates the domain name (computer hostnames) into an IP address. It also lists mail exchange (MX) servers accepting e-mail for each domain. In providing a worldwide, keyword-based redirection service, DNS is an essential component of contemporary Internet use.

Useful for several reasons, the DNS is most well-known for making it possible to attach easy-to-remember domain names (such as "mediacloud.net.au") to hard-to-remember IP addresses (such as 202.124.243.36). We take advantage of this when werecite URLs and e-mail addresses rather than strings of numbers. Less recognized, the domain name system also makes it possible for people to assign authoritative names, without needing to communicate with a central registrar each time.

Still struggling to understand DNS?

This is from an article by Serena M Giddens. It is reproduced here with her permission. See the original at http://www.serena1.com/internet_info.html.


Understanding Domain DNS Propagation
By Serena M. Giddens - November 03, 2001
The inner workings and basic make-up of the Internet is sometimes difficult to imagine, even more difficult to describe in a comprehensible manner. This is my attempt at making the concept a bit more easily understood, especially in relation to the process of relocating your domain from one host to another.
Steps in Moving a Domain
  1. You sign up with a new host.
  2. You point your domain's DNS, via your domain registrar, from your old host to your new host. This usually takes 24-48 hours but can take longer depending on your Registrar.
  3. The new host sends you a Welcome mail with all your account details, including how and when you can upload files and access your site. Some hosts provide you a temporary address to both upload to and view your site. Others do not.
  4. If your host allows you to upload and view your site via a temporary address, you upload your files, view your site and check your site's functioning via the temporary address. And you wait for the new DNS to fully propagate so others will be able to access your site via your domain name. If your new host does not provide a temporary address to upload and view your site, unfortunately you will have to wait for the domain's new DNS to fully propagate before you can proceed with uploading, etc.
  5. Your new DNS begins propagating across the Internet. That generally happens fairly quickly (within the 24-48 hours period subsequent to updating your DNS via your Registrar). But it can take much longer, depending on the circumstances causing the delay. I'll get into that further on.)
  6. Finally, your new DNS fully propagates and then everyone connected to the Internet can see your site at it's new DNS.
What does propagate mean?
It means that the new DNS has been communicated to each of the backbones of the Internet and that each backbone has in turn re-mapped its routes to the domain's new DNS location. (DNS = domain name server.) This DNS information does not travel to each of the Internet backbones in a straight line. It travels much like the mapped routes for any given address within the streets, avenues and boulevards included in a map of a country - in a multitude of directions and connecting paths to its states, counties, cities, communities, etc.
Each backbone has to re-map the new DNS and pass it along the routes to be taken through it to the new DNS. This routing information is necessary in order for anyone's computer connected to the Internet to traverse the Internet to a particular domain's site. (Said computers are generally connected to the Internet via an ISP which is another whole topic and has impact on what particular backbone and route your computer will take to a particular Internet location. But I won't get into that aspect of the Internet for now, and not at all if it remains unnecessary to the purpose of this particular article.)
Each backbone must pass the new DNS information to all the other backbones to which it is connected, in order that the connecting backbones can update their mapping and they, in turn, must pass along the new DNS to the backbones connected to them. This process continues until each and every backbone in the Internet has received the new DNS and has re-mapped the route to the domain's new DNS.
Here's an analogy that might help:
Imagine that the Internet, much like a human body, is all connected together by a huge central nervous system. The system transmits signals along its length (backbone and all related connections thereto), through various routes along the way.
The backbone connections in turn take the signal and push it along to sequential connecting points, similar to how a sensation of pain or pleasure travels between the brain and the origination point of the sensation, perhaps the full length of the body all the way to its toes, should you stub a toe.
Within each backbone are various domain hosts. The backbone contains the mapping to those hosts. Without the mapping done by each backbone, no one could travel along Internet routes or view a particular site. In our analogy, if there's a break in the central nervous system, or an impaired area of the central nervous system of the body, it can slow down or even stop the transmission of the signal to the appropriate area of the body.
The actual time it takes a backbone to update a domain's map location (DNS) depends on various factors, such as where along the central nervous system (route) a backbone is located, as well as whether another prior connecting backbone along the route is functioning properly and is able to timely send along the new mapping to this backbone. (Sometimes backbones go down and there's a major outage which affects a multitude of hosts, ISP's, and millions of sites.) The actual time it takes to propagate is impacted by how long it is before each backbone receives the new DNS mapping from the prior connecting backbone. Other factors which impact the process are: which week day and time - and its relational traffic patterns, overall Internet traffic, and the actual response time of the backbone itself to update/re-map. This isn't an all-inclusive list of variables impacting propagation. There are other factors. But you get the general idea.)
How long it takes for a site's new DNS location to propagate across the Internet such that you are able to see a particular site depends on all the above factors and more. Considering the sheer magnitude of the Internet's overall size and the relational requirements involved, it's rather miraculous that it works at all, let alone as rapidly as it normally does.

See more of Serenas' works at http://www.serena1.com

What can I do while waiting for my cached record to expire?

One way around this is to edit your hosts file and include a mapping for your new IP address.

Another way is to simply use the new IP until the cached record expires. Here's an example: Your FTP hostname is usually "ftp.yourDomainName", however you can replace this with your new IP address which looks something like this "602.78.218.165". The correct IP address is sent to you when you sign up to MediaCloud webhosting, along with your user name and password.

If you want to look at your uploaded site before propagation is complete, use the following URL: http://yourNew-IP-Address/~yourUserName.

There is also the option of asking your old hosting provider/domain registrar to change the TTL on your domain name to something low, like 5 minutes. Once migrated, you can then change it back.

If you want to check what the TTL is for your domain - check out your DNS report. You can also find some other useful DNS-related tools at DNS Stuff.

See also

External links