nixpkgs/lib/fetchers.nix

# snippets that can be shared by mutliple fetchers (pkgs/build-support)
{

  proxyImpureEnvVars = [
    # We borrow these environment variables from the caller to allow
    # easy proxy configuration.  This is impure, but a fixed-output
    # derivation like fetchurl is allowed to do so since its result is
    # by definition pure.
    "http_proxy" "https_proxy" "ftp_proxy" "all_proxy" "no_proxy"
  ];

}