NFS seems to be not able to restrict IPV6 address range

[astrojhgu]

Describe the bug
A clear and concise description of what the bug is.
In the services.nfs.server.exports option, I specified the IPV4 range as 192.168.1.0/24 and did not specify the ipv6 address. I thought it will automatically forbid access through ipv6. However, I find that I can mount the exported point from a machine with a global IPV6 address and this machine is not in the same subnet with the nfs server.

To Reproduce
Steps to reproduce the behavior:

1. Set the nfs related options as

services.nfs.server.enable = true;
  	services.nfs.server.exports = ''
    	/export/data1                 192.168.1.0/24 (rw,insecure,no_subtree_check) 
''

2. check the ipv6 address of the nfs6 server with ip addr
3. on an external machine that has enabled ipv6, run command

sudo mount '[ipv6 address]:/export/data1' /some/mounting/point

4. The mounting will succeed.

Expected behavior
A clear and concise description of what you expected to happen.
As I didn't specify the ipv6 options in servers.nfs.server.export, shouldn't it disallow me from mounting the exported fs with a ipv6 address?

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Additional context
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Metadata
Please run nix-shell -p nix-info --run "nix-info -m" and paste the result.

 - system: `"x86_64-linux"`
 - host os: `Linux 5.4.62, NixOS, 20.03.2913.4bd1938e03e (Markhor)`
 - multi-user?: `yes`
 - sandbox: `yes`
 - version: `nix-env (Nix) 2.3.6`
 - channels(root): `"nixos-20.03"`
 - nixpkgs: `/nix/var/nix/profiles/per-user/root/channels/nixos`

Maintainer information:

# a list of nixpkgs attributes affected by the problem
attribute:
# a list of nixos modules affected by the problem
module:

[peterhoeg]

What happens if you specify an ipv6 network from which to limit access?

[astrojhgu]

/export/data1                 192.168.1.0/24 (rw,insecure,no_subtree_check)

I finally find that the space between the subnet address and the options is the root of everything!

I remove this space so that this line becomes

/export/data1                 192.168.1.0/24(rw,insecure,no_subtree_check)

Then everything is OK now.

So dangerous this tricky is!

What happens if you specify an ipv6 network from which to limit access?

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/how-to-set-access-control-for-nfs-on-ipv6/9110/2