It has recently come to my attention that setting up multiple A records for a hostname can be used not only for round-robin load-balancing but also for automatic failover.
So I tried testing it:
- I loaded a page from our domain
- Noted which of our servers had served the page
- Turned off the web server on that host
- Reloaded the page
And indeed the browser automatically tried a different server to load the page. This worked in Opera, Safari, IE, and Firefox. Only Chrome failed to try a different server.
But after leaving that server offline for a few minutes and looking at the access logs, I found that the number of requests to the other servers had not significantly increased. With 1 out of 3 servers offline, I had expected accesses to each of the remaining 2 servers to roughly increase by 50%, but instead I only saw 7-10%. That can only mean in-browser DNS failover does not work for the majority of browsers/visitors, which directly contradicts what I had just tested.
Does anyone have an idea what is up with browsers' DNS failover behavior? What possible reason could there be why automatic failover works for me but not the majority of our visitors?
edit: To make myself clear, I made absolutely no change to our DNS settings; there's no TTL or propagation issue here, it's all about how the client handles the multiple A records.
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