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Re: DNS cache?

From: Martin Pala
Subject: Re: DNS cache?
Date: Thu, 27 Jan 2005 23:14:45 +0100
User-agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.7.5) Gecko/20050105 Debian/1.7.5-1

Yet one hint - even if you use DNS round-robbin, it could be better to monitor both physical addresses/hostnames then virtual/multihost name.

This is because clients are directed randomly to one of your servers. In the case that one of them fails, ca. half of the requests will fail as well (unless you have dynamic DNS updates) - when you monitor both hosts directly, you will know the error as soon as possible. When you monitor loadballanced hostname, the error report can be delayed until you get failed node address from DNS.

In any case, it is useful to know that particular machine failed and that fix is needed, to solve temporary single point of failure.


Jonathan Trott wrote:
I am monitoring a remote web cluster using monit. The cluster is connected using multiple ADSL connections and the load is distributed using round robin DNS with a TTL of 0. The problem is that when I test http using the DNS name in monit, it appears to cache the DNS response the first time it starts up and doesn't refresh the record. This means that if the link that monit has cached the IP address of goes down, monit returns an alert even though the other connections are fine and the website is still reachable. Can we get monit to follow the TTL of DNS records and lookup the record again when that TTL has expired? This would also be useful for testing servers/services hosted on dynamic DNS for instance.

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