Hardware/OS this is happening on:
AMD Vision / Windows 8.1 Toshiba Satellite C855D / 8GB memory
AMD 64-X2 / Ubuntu 16.04 Home-built desktop / 8GB memory
Intel I3 / UBUNTU 16.04 Toshiba Satellite A505 / 8GB memory
All, running Arduino x64 Version 1.8.1, All with ESP8266 by ESP8266 Community Version 2.3.0
If no responding internet connection, the only thing that shows-up on the boards manager, is "Arduino AVR boards Built In by Arduino Version 1.6.17 (when another board other than the 8266 is chosen, and the IDE has been restarted.)
I'd provide screen captures, if I were using the Linux machines, and at present, the desktop is temporarily down (for an almost similar reason, I did a system update on it out of the machine (extracted the master hard drive, put it in a dock connected to the laptop, and booted it from the laptop.), and UBUNTU is balking at the NVIDIA drivers, locking the graphics log-in, in a crash-back to log-in loop. Without internet access for that machine, I cannot remove the drivers, force it to re-install the generic graphics driver, and restore the NVIDIA drivers off the internet.)