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TTL
| Posted by JFSG, 06-15-2009, 08:23 PM |
| Why are there many people setting their TTL to higher values rather than lower ones? By setting it lower, won't it propagate faster?
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| Posted by Rekhatitus, 06-15-2009, 08:29 PM |
| The TTL instructs non-authoritative DNS servers how long to cache records retrieved from the zone file. The longer this period, the longer it will take to propagate changes to the zone files. The shorter the TTL, the harder your DNS servers will work beacuse non-authoritative servers will have to ask it the same question more frequently. Values from a few hours to a day are reasonable.
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| Posted by Collabora, 06-15-2009, 10:41 PM |
| Just a little sidebar regarding TTL and your windows desktop cache --
Windows dns cache handles negative responses differently than positive responses.
The TTL for positive responses is the lesser of the following values: The number of seconds specified in the query response the resolver received (configured in DNS server)The value of the MaxCacheTtl registry setting. Default = 86,400 sec (24 hours)The TTL for negative responses is the number of seconds specified in the MaxNegativeCacheTtl registry setting. The defualt value is 15 minutes.
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| Posted by JFSG, 06-15-2009, 11:54 PM |
| Does a DNS server consume much bandwidth? How many processors, RAM & Port (10 or 100Mbps?) is recommended? So even if the TTL is a bit low (something like 15 to 45 min), as long as the DNS servers have enough resources to support the DNS queries, it should be fine right?
I don't really understand what you mean.. Can you simplify it?
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| Posted by Collabora, 06-16-2009, 01:40 AM |
| Up to this point the discussion has been about DNS servers. But there are also
DNS clients. There is nothing you can do about the clients, but sometimes it is useful to know what happens on the client side.
In windows, whenever a hostname is resolved, as with a web browser, windows will store the query results in its DNS cache (aka the Resolver Cache). This optimizes the performance of DNS name resolution by storing previously resolved names in memory.
When the Windows resolver receives a positive or negative response to a query, it adds that positive or negative response to its cache, and as a result, creates a DNS resource record. The resolver always checks the cache before querying any DNS server. If a DNS resource record is in the cache, the resolver uses the record from the cache instead of querying a server. This behavior expedites queries and decreases network traffic for DNS queries.
What I had posted was Window's default settings for the resolver cache and how a local user might change them. For example, if you change your server TTL to something greater than 86,400s, Windows will always use TTL=86,400s in its cache (by default) no matter high you set it.
You can view the cache using ipconfig /displaydns at the command prompt
(Similarly there are also DNS caching servers on the internet that may be storing your resource record with a TTL different than the config on your dns server)
Last edited by Collabora; 06-16-2009 at 01:45 AM.
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| Posted by JFSG, 06-16-2009, 02:21 AM |
| Sorry but I'm still a little bit confused about all these. I'll Google up some of those. Thanks!
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