I have a machine with an NVIDIA GeForce which I don't use for display purposes (i.e. the monitor is not connected to it), and some lackluster on-board graphics chip. (Below you'll find the relevant lshw listing.)

My X sessions work just fine; but when most of my X apps (which require any sort of fancy GFX or a toolkit) run, they emit the following error message:

Xlib: extension "GLX" missing on display ":0".

My question is: How can I make my apps notice the non-NVIDIA GLX library and use it (without removing the NVIDIA card and without switching the monitor to its output ports of course)?

I'm using Debian/Linux Stretch 64bit, kernel version 4.2.6, with LXDE.

Output of lshw -c display:

 *-display 
 description: VGA compatible controller
 product: GK106 [GeForce GTX 650 Ti Boost]
 vendor: NVIDIA Corporation
 physical id: 0
 bus info: pci@0000:02:00.0
 version: a1
 width: 64 bits
 clock: 33MHz
 capabilities: pm msi pciexpress vga_controller bus_master cap_list rom
 configuration: driver=nvidia latency=0
 resources: irq:17 memory:f6000000-f6ffffff memory:e0000000-e7ffffff memory:e8000000-e9ffffff ioport:e000(size=128) memory:f7000000-f707ffff
 *-display
 description: VGA compatible controller
 product: Xeon E3-1200 v2/3rd Gen Core processor Graphics Controller
 vendor: Intel Corporation
 physical id: 2
 bus info: pci@0000:00:02.0
 version: 09
 width: 64 bits
 clock: 33MHz
 capabilities: msi pm vga_controller bus_master cap_list rom
 configuration: driver=i915 latency=0
 resources: irq:30 memory:f7400000-f77fffff memory:d0000000-dfffffff ioport:f000(size=64)

Output of glxinfo:

Xlib: extension "GLX" missing on display ":0".
Xlib: extension "GLX" missing on display ":0".
Xlib: extension "GLX" missing on display ":0".
Xlib: extension "GLX" missing on display ":0".
Xlib: extension "GLX" missing on display ":0".
Xlib: extension "GLX" missing on display ":0".
Xlib: extension "GLX" missing on display ":0".
Error: couldn't find RGB GLX visual or fbconfig
Xlib: extension "GLX" missing on display ":0".
Xlib: extension "GLX" missing on display ":0".
Xlib: extension "GLX" missing on display ":0".
Xlib: extension "GLX" missing on display ":0".
Xlib: extension "GLX" missing on display ":0".
Xlib: extension "GLX" missing on display ":0".
Xlib: extension "GLX" missing on display ":0".
Xlib: extension "GLX" missing on display ":0".
Xlib: extension "GLX" missing on display ":0".
Xlib: extension "GLX" missing on display ":0".

Output of cat /var/log/Xorg.0.log | grep glx:

[ 19.287] (II) LoadModule: "glx"
[ 19.787] (II) Loading /usr/lib/xorg/modules/extensions/libglx.so
[ 22.727] (II) Module glx: vendor="X.Org Foundation"

Note: I do not have the 'glx-alternative-nvidia' installed.

I arrived here after installing nvidia-current from the package repository, and got stuck on a login loop. I looked at the log on ~/.xsession-errors and found the mentioned error:

Xlib: extension "GLX" missing on display ":0".

I already had uninstalled nvidia-current and the problem persisted. I also didn't have glx-alternative-nvidia installed. As a last resort, I simply uninstalled everything from nvidia on my Ubuntu (including CUDA stuff), and it worked after the restart.

First identify which glx module is in use:

$ cat /var/log/Xorg.0.log | grep glx
[ 3.622] (II) LoadModule: "glx"
[ 3.624] (II) Loading /usr/lib/xorg/modules/extensions/libglx.so
[ 3.705] (II) Module glx: vendor="NVIDIA Corporation"

In Debian 8 Jessie my solution was to remove glx-alternative-nvidia package. So after reboot:

$ cat /var/log/Xorg.0.log | grep glx
[ 3.581] (II) LoadModule: "glx"
[ 3.582] (II) Loading /usr/lib/xorg/modules/extensions/libglx.so
[ 3.592] (II) Module glx: vendor="X.Org Foundation"

$ glxinfo | grep "OpenGL renderer"
OpenGL renderer string: Mesa DRI Intel(R) Haswell Desktop

Install OpenGL in your computer, then as mentioned here, add the following to your /etc/X11/xorg.conf:

Section "Files"
 ModulePath "/usr/lib/nvidia-VERSION/xorg"
 ModulePath "/usr/lib/xorg/modules"
EndSection

(Replace nvidia-VERSION with your directory name)
This worked for me.

I solved it by uninstalling glx-alternative-nvidia

apt-get remove glx-alternative-nvidia

Got it "extension "GLX" missing" when I "dist-upgraded" my debian (8 -> 9). The solution was pretty simple.

Download your driver installer on NVIDIA

service lightdm stop

(ALT+F1) + your credentials

sudo nvidia-installer --uninstall
sudo ./NVIDIA-Linux-x86_64-340.102-custom.run #my vga GT218M [GeForce 310M]

After the installer did its job, the GLX went back.

ps: If you user repos, I think an dpkg-reconfigure "driver_package_name" would do the same.