Fix #Firefox issues when load-balancing by disabling #DNS prefetching

By | March 17, 2019

There is an annoying feature from Firefox I just found out is causing me huge problems.

DNS Prefetching: This feature allows firefox to try to prefetch and cache DNS entries related to the sites you visit. Firefox attempts to speed up loading new websites by using DNS Prefetching, which can cause page load errors with some system configurations.


I discovered that when using sites with multiple IPs that resolve to the same name (ex: facebook, twitter, google etc.), lots of times, I get long delays and even “site unreachable” errors.

This is stupid considering that I have a highly reliable Internet connection: a router load-balanced Internet link using two different ISPs. Then I realized that this may in fact be the issue.

It may be that the prefetch is done through one ISP and then the actual request to the server goes through the other ISP. This can cause issues when the connection through one ISP can prefer another actual server IP not the one that is present in the prefetched DNS call.

To disable this feature of Firefox see this extract from Mozilla support:

To disable DNS Prefetching:
In the address bar, type about:config and press Enter.

The about:config “This might void your warranty!” warning page may appear.

Click I accept the risk! to continue to the about:config page.

Right-click in the list of preferences, select New, and then select Boolean.

In the Enter the preference name field, enter:

network.dns.disablePrefetch and click OK.

Select true when prompted to set the value and click OK.

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