DNS Caching
 Table of Contents
Every time your web browser wants to load a web page, it needs to contact
a DNS server first, which will translate the www.domain.com
address into a machine readable format - the IP address. The same thing
happens when PostCast Server tries to send a message to a recipient. The
domain name in the e-mail address is translated into the address of the
server that receives messages for that domain. If for some reason, PostCast
Server or any other program on your computer is unable to contact the
DNS server, you will not be able to view the web page, send a message,
or do anything on the Internet.
DNS Caching
Even though the communication only takes up a few hundred milliseconds
each time a message is sent, these can add up over time, as the number
of requests can easily reach hundreds or thousands of DNS queries. DNS
servers can sometimes be unavailable and the responses to queries can
be very slow.
An easy way to speed up this process is to cache the information locally
thereby eliminating the need for repetitive queries to the remote DNS
server. This is done by telling PostCast Server to contact a local database
on your computer, rather than the remote DNS server. This local connection
can be performed much faster and does not depend on the response time
of the remote DNS server across the Internet.
PostCast Server contacts the remote DNS server once, and then caches
(memorizes) the addresses returned from the query. So the next time you
are requesting the same domain.com address, it instantly returns the answer,
without having to contact your ISP's DNS server to ask it for the translation.
The process is much faster, since it does not have to connect to any remote
servers every time.
Basically, the logic behind the DNS cache feature is to include both
bad and good domains in the list and use the ?Valid? column to mark them
as valid. The domain is considered bad only if DNS server returns the
?Unresolved? error, which means that the DNS entry for that domain does
not exist.
MX Records
PostCast Server requires an MX record for each domain it needs to send
messages to. MX records specify that e-mail sent to a domain is handled
by another machine.
For example, we want the machine mail.domain.com to handle the mail
sent to @domain.com. All we need to do is have an MX for domain.com pointing
to mail.domain.com, then we tell the machine mail.domain.com that it handles
mail for @domain.com, otherwise it will reject it. All the SMTP servers
out there will now connect to mail.domain.com to deliver mail to @domain.com.
Backup MXes and MX Priority
The purpose of MXes does not end there; it is also possible to have
multiple MX records. In our simple example, we omitted to mention that
MX records all have a preference assigned to them. For example, the MX
saying that mail to @domain.com goes to mail.domain.com has preference
20. There is a second MX with preference 50 saying that mail to @domain.com
should go to mail2.domain.com.
When there are multiple MX records, remote mail servers will first try
to deliver mail to the server with the lowest preference. That means that
if a mail server wants to send mail to @domain.com, it looks up the MXes.
It sees the one with preference 20 is the lowest one, it tries to connect
to mail.domain.com. If mail.domain.com doesn't respond, then it will look
for the next-lowest preference MX, namely the one with preference 50,
and thus connect to mail2.domain.com. mail2.domain.com is a machine outside
our main network, so even if our entire main network was experiencing
problems, it would probably still be up. In this case, mail2.domain.com
would be a backup MX.
DNS Caching Dialog Box
Use the DNS Caching dialog box to configure the caching interval and
to manually modify the cached MX records. You can also create new MX records
for domains by pressing the "Add" button and specifying MX servers
and their priorities.
DNS entries never expire
Select this option if you do not want to refresh DNS information for
domains. This is useful because it speeds up creating and maintaining
DNS cache database, but the DNS entries may become obsolete if MX records
for some domain change. If that happens, the program will not be able
to connect to the server that handles the incoming e-mail for the recipient.
Expire after
This option forces the program to regularly get the latest MX records
for a domain from the DNS server. You can specify the reoccurrence interval
and configure the program to refresh its database in certain number of
days, weeks, or months.
Domain name is valid
This indicates whether the domain name has valid DNS records. If the
DNS server returned an error after querying the data, the domain will
be automatically marked as invalid. You can use this option to change
this setting.
Address
This column contains the addresses of servers that accept e-mail messages
for the domain.
Priority
Determines the priority of this mail server. The less the number the
higher the priority (0 means the highest priority, 65535 - the lowest).
Thus, mail servers with lower priorities are regarded as secondary and
will be used only if all mail servers with higher priorities are inaccessible
or out of order for some reasons. |