Note : This post assumes familiarity with NixOS https://nixos.org
One of the problems I faced when using NixOS is network access when moving my laptop between work environment and home environment. The office network needs a proxy for internet access while the home network is a direct connection.
Normally this is not a problem. I can set the proxy variable in the shell I am in.
$ export HTTP_PROXY=http://corporate.proxy.com:port
$ wget www.google.com
However this gets problematic when trying to install a new package. Package updates are performed by the Nix daemon. The proxy settings for Nix daemon are derived from nixos configuration .
The first solution that I tried was updating nixos configuration after switching networks and rebuilding the configuration. This got tedious quickly. On top of it, rebuilding configuration without network access could fail .
After digging a bit more, I came across 3 more options.
Child configurations - These are new configurations that are built each time a nixos configuration is built. These are treated as a separate configuration from the base configuration, almost as a peer to it.
See here https://nixos.org/nixos/manual/options.html#opt-nesting.clone for more information.
Clone configurations - These are also new configurations that are built each time a nixos configuration is built but are treated as variations on top of a base configuration. This is the most suitable option for my usecase.
See here https://nixos.org/nixos/manual/options.html#opt-nesting.children for more information.
Profiles - Profiles are system configurations that need to be explicitly built and are maintained separately from the default boot configuration. This is not suitable for my usecase as the configuration needs to be manually built to keep it up to date.
I decided to try the clone configuration as it was most suited to my usecase.
Adding the clone configuration to
/etc/nixos/configuration.nix was straight forward.
boot.loader.grub.configurationName = "Default";
nesting.clone = [
boot.loader.grub.configurationName = lib.mkForce "Work";
networking.proxy.default = "http://proxy.work.com:80";
networking.proxy.noProxy = "127.0.0.1,localhost,work.com";
After building the configuration once with
$ sudo nixos-rebuild switch
I could switch the proxy settings easily by switching from the parent configuration to the cloned configuration.
$ sudo /run/current-system/fine-tune/child-1/bin/switch-to-configuration test
One minor inconvenience remained. I had no way of selecting the proxy configurations on boot. I discovered that this functionality existed at some point and was temporarily disabled during refactoring. I enabled this feature and filed a PR on Nixpkgs.
This PR took almost an year to get merged due to the iterations on test cases and delays due to my work commitments. You should be able to select different proxy configurations from the boot loader menu from NixOS 19.09.