Thursday, 9 August 2012

Understanding SOA records

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 14400 IN SOA
  600 ) 14400 IN NS 14400 IN NS 14400 IN NS 14400 IN A 14400 IN A 14400 IN MX 0
mail 14400 IN CNAME
www 14400 IN CNAME
ftp 14400 IN CNAME

In the rest of this article, we’ll analyze the various parts of this DNS record, starting from the top.

SOA Records:

An SOA(Start 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 14400  IN  SOA
  2004123001 ; Serial number
  86000 ; Refresh rate in seconds
  7200 ; Update Retry in seconds
  1209600 ; Expiry in seconds
  600 ; minimum in seconds )

  • name - is the main name in this zone.
  • TTL - 14400 - TTL defines the duration in seconds that the record may be cached by client side programs. If it is set as 0, it indicates that the record should not be cached. The range is defined to be between 0 to 2147483647 (close to 68 years !)
  • Class - IN - The class shows the type of record. IN equates to Internet. Other options are all historic. So as long as your DNS is on the Internet or Intranet, you must use IN.
  • Nameserver - - The nameserver is the server which holds the zone files. It can be either an external server in which case, the entire domain name must be specified followed by a dot. In case it is defined in this zone file, then it can be written as “ns” .
  • Email address - - This is the email of the domain name administrator. Now, this is really confusing, because people expect an @ to be in an email address. However in this case, email is sent to, but written as . And yes, remember to put the dot behind the domain name.
  • Serial number - 2004123001 - This is a sort of a revision numbering system to show the changes made to the DNS Zone. This number has to increment , whenever any change is made to the Zone file. The standard convention is to use the date of update YYYYMMDDnn, where nn is a revision number in case more than one updates are done in a day. So if the first update done today would be 2005301200 and second update would be 2005301201.
  • Refresh - 86000 - This is time(in seconds) when the slave DNS server will refresh from the master. This value represents how often a secondary will poll the primary server to see if the serial number for the zone has increased (so it knows to request a new copy of the data for the zone). It can be written as “23h88M” indicating 23 hours and 88 minutes. If you have a regular Internet server, you can keep it between 6 to 24 hours.
  • Retry - 7200 - Now assume that a slave tried to contact the master server and failed to contact it because it was down. The Retry value (time in seconds) will tell it when to get back. This value is not very important and can be a fraction of the refresh value.
  • Expiry - 1209600 - This is the time (in seconds) that a slave server will keep a cached zone file as valid, if it can’t contact the primary server. If this value were set to say 2 weeks ( in seconds), what it means is that a slave would still be able to give out domain information from its cached zone file for 2 weeks, without anyone knowing the difference. The recommended value is between 2 to 4 weeks.
  • Minimum - 600 - This is the default time(in seconds) that the slave servers should cache the Zone file. This is the most important time field in the SOA Record. If your DNS information keeps changing, keep it down to a day or less. Otherwise if your DNS record doesn’t change regularly, step it up between 1 to 5 days. The benefit of keeping this value high, is that your website speeds increase drastically as a result of reduced lookups. Caching servers around the globe would cache your records and this improves site performance.

Reducing DNS bandwidth

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.

Increasing site speed

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.

How to improve backup

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.

How to test SOA records

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 from all the nameservers, primary and secondary, all you need to do is
# dig +nssearch
 2005122907 3600 300 604800 600 from server in 0 ms.
2005122907 3600 300 604800 600 from server in 0 ms.
 2005122907 3600 300 604800 600 from server in 239 ms.
 2005122907 3600 300 604800 600 from server in 280 ms.

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