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Servers
AOL name servers honor all TTLs of resource records that are cached via recursion, as expected. As a DNS server can be restarted (and have its cache flushed) at any time, re-caching a record before the TTL expires and restarting the expiration timer is not uncommon. Some name servers have their cache flushed every 4 hours, or as needed.
Clients
There are a number of different implementations of client DNS resolvers, and not all of them handle TTLs in the expected manner. Each layer of software (OS, libraries, and applications) may chose to do some type of DNS caching. Some examples of non-standard DNS implementations and their effects:
Operating Systems
Microsoft Windows 2000 default installations:
A registry setting defaults positive TTLs to 86400.
http://support.microsoft.com/default.aspx?scid=KB;en-us;276324& and http://www.gslbnetwork.com/w2k.dns.cache.issue.pdf
Sun Solaris 2.x:
If NSCD (name service caching daemon) is enabled, the default setting for positive-time-to-live is 3600 seconds (1 hour). This setting may cause nscd to ignore the TTL associated with the RR during the initial lookup.
Web Browsers
Mozilla:
DNS lookups are cached for a default time of 900 seconds (15 minutes). http://www.mozilla.org/docs/netlib/dns.html
Internet Explorer:
"In earlier versions of Internet Explorer (Internet Explorer 3.x), DNS host entries are cached for 24 hours by default. This, in many cases, is too long. During this period, some host entries stop working because of change in the IP address of the remote server that was initially resolved.
Internet Explorer 4.x and later modifies how DNS host entries are cached by decreasing the default time-out value to 30 minutes."
http://support.microsoft.com/default.aspx?scid=KB;en-us;263558&
Mail Servers
Exchange 5.5:
"When the Internet Mail Service is forwarding to a host, it resolves the host name at startup, and never again queries DNS for the value, again ignoring TTL."
http://support.microsoft.com/default.aspx?scid=KB;en-us;285023&
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