by Sangeetha Naik
I've learned that there is nothing more peaceful than a sleeping child - Anonymous, Age 30
To an Internet Administrator, there is nothing more peaceful than a stable and optimized DNS server. The moment there is a wrong configuration, the server wakes up and starts crying, sites and email goes down. An important part of keeping DNS that way is properly setting up the SOA records.
What are DNS Records. DNS records or Zone files are used for mapping URLs to an IPs. Located on servers called the DNS servers, these records are typically the connection of your website with the outside world. Requests for your website are forwarded to your DNS servers and then get pointed to the WebServers that serve the website or to Email servers that handle the incoming email.
This is how a typical Zone file (containing many common DNS records) looks like.
; ; Zone file for mydomain.com mydomain.com. 14400 IN SOA ns.mynameserver.com. root.ns.mynameserver.com.( 2004123001 86000 7200 3600000 600 ) mydomain.com. 14400 IN NS ns.mynameserver.com. mydomain.com. 14400 IN NS ns2.mynameserver.com. mydomain.com. 14400 IN NS ns3.mynameserver.com. mydomain.com. 14400 IN A 184.108.40.206 localhost.mydomain.com. 14400 IN A 127.0.0.1 mydomain.com. 14400 IN MX 0 mydomain.com. mail 14400 IN CNAME mydomain.com. www 14400 IN CNAME mydomain.com. ftp 14400 IN CNAME mydomain.com.
In the rest of this article, we'll analyze the various parts of this DNS record, starting from the top.
An SOA(State of Authority) Record is the most essential part of a Zone file. The SOA record is a way for the Domain Administrator to give out simple information about the domain like, how often it is updated, when it was last updated, when to check back for more info, what is the admins email address and so on. A Zone file can contain only one SOA Record.
A properly optimized and updated SOA record can reduce bandwidth between nameservers, increase the speed of website access and ensure the site is alive even when the primary DNS server is down.
Here is the SOA record. Notice the starting bracket ``(``. This has to be on the same line, otherwise the record gets broken.
; name TTL class rr Nameserver email-address mydomain.com. 14400 IN SOA ns.mynameserver.com. root.ns.mynameserver.com. ( 2004123001 ; Serial number 86000 ; Refresh rate in seconds 7200 ; Update Retry in seconds 3600000 ; Expiry in seconds 600 ; minimum in seconds )
mydomain.com. 14400 IN SOA ns.mynameserver.com. root.ns.mynameserver.com.(
can also be written as
@ 14400 IN SOA ns.mynameserver.com. root.ns.mynameserver.com. (
14400 IN SOA ns.mynameserver.com. root.ns.mynameserver.com. (
There is constant bandwidth usage between primary and secondary(backup DNS) servers. This depends a lot on the Refresh value. If the refresh value is say 3 hours, your secondary server is polling your primary server every 3 hours and updating the cache. Lets assume you have a 1000 zone files, each with 3 hours refresh rate. You can imagine the bandwidth that must be getting used. This is especially true if the servers are on 2 separate physical servers.
An increase in the Refresh rate can effectively reduce bandwidth usage between the primary and secondary server.
The time it takes to access a website on a browser includes the time it takes to look it up on the domain name server. By increasing the ``Minimum'' value, we're telling the contacting clients to keep their copies of the zone file for a longer time. In effect, reducing the lookups to the nameserver. By reducing the number of times a client has to lookup, we're increasing the site speed.
However, this also means that if you make changes to the DNS record, it will take longer to propagate. If you require to make frequent updates to your DNS records, make sure your Minimum value is lesser than 1 day. That means longer lookup times, but accurate information for the clients
If you are planning a major update on the DNS zone file(say moving to another server or hosting service), reduce the Minimum value a couple of days prior to the change. Then make the change and then jack up the minimum value again. This way the caching clients all over the world will pick up the changes quicker and yet you do not need to sacrifice on site speed thereafter.
Always keep a secondary DNS server and keep a higher Expiry value. This will mean that even if the Primary server goes down, the secondary will have the cached copy(for as long as the Expiry value stands) and it will keep serving lookups. Keeping a secondary server but a low expiry value defeats the purpose of a Backup.
You have set the new SOA values, and you want to know whether the update has taken place. ``Dig'' is a good tool to troubleshoot and check for DNS information.
For example to check out the SOA records of yahoo.com from all the nameservers, primary and secondary, all you need to do is
# dig yahoo.com +nssearch SOA ns1.yahoo.com. hostmaster.yahoo-inc.com. 2005122907 3600 300 604800 600 from server ns2.yahoo.com in 0 ms. SOA ns1.yahoo.com. hostmaster.yahoo-inc.com. 2005122907 3600 300 604800 600 from server ns3.yahoo.com in 0 ms. SOA ns1.yahoo.com. hostmaster.yahoo-inc.com. 2005122907 3600 300 604800 600 from server ns1.yahoo.com in 239 ms. SOA ns1.yahoo.com. hostmaster.yahoo-inc.com. 2005122907 3600 300 604800 600 from server ns4.yahoo.com in 280 ms.
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