I've scoured the web for several days now, essentially every single time I encounter this. I hope someone here knows a solution to this.
- I'm using the latest stable release of Firefox at the time of writing.
- I'm running on Dual Stack (native IPv4 and native IPv6).
- Any tab exhibits the symptom as soon as you refresh or open a new URL.
- No other browser or client is affected. I can use Opera, Internet Explorer, PuTTY etc without any such symptoms while Firefox continues to show me "Server not found".
- NB: The DNS cache service on my box is turned off (meaning
ipconfig /flushdnsis out of the picture).
This remains unsolved as of July 2016. I still experience the DNS "outages" which seem to affect only Firefox.
I recently switched to Firefox and I am experiencing "Server not found" errors in intervals from less than an hour to several days.
This happens out of the blue, except that when I disconnect Cisco AnyConnect, this seems to be a sure-shot way of exposing this behavior. The issue, however, also occurs sporadically without any VPN software even running.
Stuff I researched and tried
I tried F5 obviously, but that never worked. The only thing that works reliably is to restart Firefox. Personally, that would mean I would have to switch browser again, as this behavior is simply unacceptable.
I found this several year old thread which suggests turning off IPv6, which is also recommended over at mozillaZine. In fact this prompted me several days ago to turn both IPv6 and DNS prefetching off by setting the following options in
Yet I have seen the symptoms again in the days since the configuration change (I have since reset those two options).
Now what I found flabbergasting was that when I looked at the outgoing requests from my machine using Wireshark while attempting to refresh the tabs that showed those symptoms, I saw not a single DNS request: zero, nada. What I saw instead were NBNS requests, i.e. "NetBIOS Name Service". And in fact the requests contained the very names of domains I was trying to access from Firefox.
Please note that after restarting Firefox, I can see these NBNS requests also for cases were a standard DNS lookup fails (not the "connection", but the lookup fails, I can see DNS responses). That is
wpad for example fails to resolve via DNS in the intranet and then internet and then ends up as an attempt via NBNS.
How can I either prevent this from happening in the first place or when it happens recover from it without having to restart Firefox? This being: Firefox losing its ability to resolve server names via DNS.