TTL Property of DNS

By -

All DNS records have a TTL property, specifying the maximum amount of time other DNS servers and applications should cache the record.
Setting a DNS record's TTL value to zero, means that applications and DNS servers should not cache the record.

When a DNS record is stored in the cache of a DNS server, the record's TTL is continuously reduced as time go by, and when the TTL finally reaches zero the record is removed from the cache.

When a DNS server passes DNS records from the cache along to applications and other DNS servers, it supplies the current TTL value - not the original. This way the original TTL is guaranteed no matter how many DNS servers the record passes through.

When deciding on the TTL, you need to consider how often the record will be changed.
Because of caching, changes to a DNS record will not reach the entire network until the original TTL has expired - a good reason for setting a short TTL.
However caching helps reduce network traffic. The longer the TTL, the longer the record will live in other DNS server caches around the world, and so fewer requests to the original DNS server are needed - a good reason for setting a long TTL.

Generally, for a record pointing to a server/device with a static IP address and no need for quick updates, a TTL of one day is a good starting point.
However, if the record is for a host with a dynamic IP address or for server which is part of a failover set (see http://www.simplefailover.com), you should be using a TTL value of a few minutes or less.

Most DNS servers will not cache a DNS record for more than one week. This is also the default in Simple DNS Plus, but you can change this through the "Maximum cache time" option - see Options dialog / DNS / Caching section.

Use the Record Properties dialog to modify a record's TTL (select the record in the DNS Records window and click the "Properties" button).

Comments

  1. subhiNovember 18, 2011 at 12:20 PM

    Time to Live (TTL)
    For most resource records, this field is optional. It indicates a length of time used by other DNS servers to determine how long to cache information for a record before expiring and discarding it. For example, most resource records created by the DNS Server service inherit the minimum (default) TTL of 1 hour from the start of authority (SOA) resource record which prevents overlong caching by other DNS servers. For an individual resource record, you can specify a record-specific TTL that overrides the minimum (default) TTL inherited from the SOA RR. TTL values of zero (0) can also be used for resource records that contain volatile data not to be cached for later use after the current DNS query in progress is completed.
    Note
    In the resource record examples below, the TTL field is omitted wherever it is optional. The TTL field is included in the syntax for each record to indicate where it may be added.

    ReplyDelete

Post a Comment

Popular posts from this blog

Boot configuration Data Store --BCDEdit /set

By -
The  BCDEdit /set  command sets a boot entry option value in the Windows boot configuration data store (BCD). Use the  BCDEdit /set  command to configure specific boot entry elements, such as kernel debugger settings, data execution protection (DEP) and processor address extension (PAE) options, and to load alternate hardware abstraction layer (HAL) and kernel files. You can use these boot entry options when you are testing and debugging your driver for Windows Vista, Windows 7, and later versions of Windows. bcdedit /set [{ID}] datatype value Parameters [ { ID }] The  { ID }  is the GUID that is associated with the boot entry. If you do not specify an  { ID } , the command modifies the current operating system boot entry. If a boot entry is specified, the GUID associated with the boot entry must be enclosed in braces  { } . To view the GUID identifiers for all of the active boot entries, use the  bcdedit /enum  command. datatype value The fol
Read more

ADSI Edit

By -
Image
ADSI Edit (adsiedit.msc) Updated: March 19, 2010 Applies To: Windows SBS 2008, Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2, Windows Server 2008, Windows Server 2008 R2 Active Directory® Service Interfaces Editor (ADSI Edit) is a Lightweight Directory Access Protocol (LDAP) editor that you can use to manage objects and attributes in Active Directory. ADSI Edit (adsiedit.msc) provides a view of every object and attribute in an Active Directory forest. You can use ADSI Edit to query, view, and edit attributes that are not exposed through other Active Directory Microsoft Management Console (MMC) snap-ins: Active Directory Users and Computers, Active Directory Sites and Services, Active Directory Domains and Trusts, and Active Directory Schema. This topic includes the following sections: Installing ADSI Edit Using ADSI Edit
Read more

Userenv logging (User Environment logging)

By -
When you enable userenv logging, you can perform debug logging of the user profile and the system policy processes. Userenv log files also contain information about the status of each Group Policy extension, such as Application Deployment, Security, and Folder Redirection. Userenv log files reveal what is occurring in the background as a user logs on   Userenv.log file located in the %SystemRoot%\Debug\UserMode folder Userenv log files contains information about the following: Group Policy settings that are not processed or not applied as expected Folder redirection that does not occur Profile or registry hive load, unload, or deletion failures Logon script, or script not applied as expected Default behaviors occurring because a slow link was detected Roaming profile issues Slow logon issues Whether a given GPO is accessible, and if not, why access was denied. The name of the domain controller that is accessing SYSVOL. The Userenv log has a maximum size of 1 megabyt
Read more