VerroTech Redundancy Example
When hosting a domain on the Internet for commerce, maximum availability
of your site as well as incoming email is extremely important. In order to
ensure the maximum availablility of your web and mail services, it is common
for larger corporations to load-balance these requirements and utilise redundant
systems. Ideally these systems will also be multi-honed.
Definition: Redundant System
A system not used in normal working that provides the capability to take over the
work of the systems normally performing the task. One level of redundancy is to have
two times the number of systems to handle the requirements of the task.
Definition: Multi-Honed
A system connected to the network from multiple points, usually in different
geographical locations but certainally by different means. For example having
a server in the UK and in the US would make the system multi-honed as would having
two seperate Internet access lines to one site.
We will be using an example domain of company.com in this document to detail examples.
An Internet domain has a minimum of two nameservers associated with it. The nameservers
for the domain provide information about it such as where to send mail to and where
the website is located. In a normally hosted domain this information would point the
client to a single machine where the website is located and a single machine to handle
the domain's mail. Let us consider the registration of example.com to be hosted as
a normal domain with VerroTech.
The domain is to be hosted on our Stavros (UK) system and so we would provide our Stavros
nameserver addresses to the registrar:
ns.verrotech.com \
> Stavros UK Systems
ns2.verrotech.com /
Records would then be created on our Stavros nameserver to this affect and the relevant
records added to allow FTP access, the website to be available and mail to be routed
properly. At this point, the nameserver information for example.com would be as follows:
nameserver ns.verrotech.com (Stavros UK)
nameserver ns2.verrotech.com (Stavros UK)
www -> 212.67.208.152 (Stavros UK)
mail to mail.verrotech.com (Stavros UK)
The example company have been experiencing very heavy web and mail traffic from
all around the world, including mailbomb attacks. To continue to ensure server
availability, it it felt that some form of redundancy should be incorporated.
DNS Redundancy can be provided by adding a secondary DNS server. VerroTech can
simply add a secondary entry for example.com on their Hektor (US) servers, update
the registrar information to show:
nameserver ns.verrotech.com (Stavros UK)
nameserver red.verrotech.net (Hektor US)
In addition to standard secondary DNS support (where the secondary server requests
periodic updates from the 'primary' whilst keeping it's own copy), VerroTech can
use an internal system to tranfer primary DNS zone records between our systems
and keep them synchronised. This would mean that even if an outage affected BOTH
systems simultaniously and only the 'secondary' came back up DNS records would still
be served.
In our example we only list a single incoming mail host for example.com (mail.verrotech.com).
We can, in fact, list any number of incoming mail server and, perhaps more importantly, list
a priority of delivery. In addition to configuring our systems to receive your mail, we can
configure some or all of them to relay it in addition. This means that one system still acts
as the main mail collector but other systems will accept mail for example.com and hold them
in the event the main system is unavailable.
Maximum availabliliy of mail can be assured by VerroTech for example.com as follows:
mail high priority server > mail.verrotech.com (Stavros UK)
mail medium priority server > mail.verrotech.net (Hektor US Relay)
mail low priority server > mail.noc.verrotech.com (UK Relay)
In the first instance a remote mailer will try mail.verrotech.com falling back in order to
Hektor and then the Norwich backup Relay. Mail will therefore be kept regardless of any
transient network conditions or system failures.
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