NFS

Server

Let's say that we've got one server-machine with 4 directories that we want to share: /mnt/kotomi, /mnt/mafuyu, /mnt/sen and /mnt/tomoyo.

First, we have to create a dedicated directory from which our NFS server will access the data:

$ mkdir /export

Then we have to either move our already-existing directories inside /export (using mv from the command line) or bind-mount them there:

{
  fileSystems."/export/mafuyu" = {
    device = "/mnt/mafuyu";
    options = [ "bind" ];
  };

  fileSystems."/export/sen" = {
    device = "/mnt/sen";
    options = [ "bind" ];
  };

  fileSystems."/export/tomoyo" = {
    device = "/mnt/tomoyo";
    options = [ "bind" ];
  };

  fileSystems."/export/kotomi" = {
    device = "/mnt/kotomi";
    options = [ "bind" ];
  };
}

Having the filesystem ready, we can proceed to configure the NFS server itself:

{
  services.nfs.server.enable = true;
  services.nfs.server.exports = ''
    /export         192.168.1.10(rw,fsid=0,no_subtree_check) 192.168.1.15(rw,fsid=0,no_subtree_check)
    /export/kotomi  192.168.1.10(rw,nohide,insecure,no_subtree_check) 192.168.1.15(rw,nohide,insecure,no_subtree_check)
    /export/mafuyu  192.168.1.10(rw,nohide,insecure,no_subtree_check) 192.168.1.15(rw,nohide,insecure,no_subtree_check)
    /export/sen     192.168.1.10(rw,nohide,insecure,no_subtree_check) 192.168.1.15(rw,nohide,insecure,no_subtree_check)
    /export/tomoyo  192.168.1.10(rw,nohide,insecure,no_subtree_check) 192.168.1.15(rw,nohide,insecure,no_subtree_check)
  '';
}

This configuration exposes all our shares to 2 local IPs; you can find more examples at Gentoo's wiki [1].

Other options are available on the NixOS option page or via the nixos-option command.

If your server-machine has a firewall turned on (as NixOS does by default, for instance), don't forget to open appropriate ports; e.g. for NFSv4:

networking.firewall.allowedTCPPorts = [ 2049 ];

Client

Continuing the server example, mounting the now-exposed tomoyo share on another box (on a client) is as simple as:

{
  fileSystems."/mnt/tomoyo" = {
    device = "server:/tomoyo";
    fsType = "nfs";
  };
}

Note that clients see exposed shares as if they were exposed at the root level - i.e. /export/foo becomes /foo (in the device option). Other, regular fileSystems options apply.

Tip: you can specify NFS version by adding the "nfsvers=" option:

{
  fileSystems."/mnt/tomoyo" = {
    # ...
    options = [ "nfsvers=4.2" ];
  };
}

Tip: by default, all shares will be mounted right when your machine starts - apart from being simply unwanted sometimes, this may also cause issues when your computer doesn't have a stable network connection or uses WiFi; you can fix this by telling systemd to mount your shares the first time they are accessed (instead of keeping them mounted at all times):

{
  fileSystems."/mnt/tomoyo" = {
    # ...
    options = ["x-systemd.automount" "noauto"];
  };
}

Tip: you can also tell systemd to disconnect your NFS-client from the NFS-server when the directory has not been accessed for some time:

{
  fileSystems."/mnt/tomoyo" = {
    # ...
    options = ["x-systemd.idle-timeout=600"]; # disconnects after 10 minutes (i.e. 600 seconds)
  };
}

Nix store on NFS

In a single-user setup (not on Nixos) the Nix store can be also exported over NFS (common in HPC clusters) to share package over the networks. The only requirement is to also pass local_lock=flock or local_lock=all as mount option to allow the nix packages to take locks on modifications. Example entry in fstab:

<host_or_ip>/nix /nix nfs nofail,x-systemd.device-timeout=4,local_lock=all 0 0