I have a working NFS4 setup with a client and a server who both run NixOS.
I have a working kerberos setup as well on the same machines (nfs server is kerberos kdc).

Now I wanted to enable kerberos authentication for NFS, but could not get it to work. Because I didn't think NixOS was to blame, I submitted a call for help with all the details of my setup to:
https://serverfault.com/questions/989749/how-should-i-proceed-debugging-nfs4kerberos

Today I brought in a debian 10 based laptop and set it up as an NFS4 server as well.
Without kerberos, interactions are great again. Debian and NixOS can interact as clients and servers just fine. With kerberos added, a NixOS client can mount the Debian NFS server!
However the debian client cannot access the NixOS NFS server and runs into the same issue as the NixOS client.

So it appears the problem must lie with NixOS' NFS Server interactions with kerberos.

To Reproduce
Steps to reproduce the behavior:

1. setup an nfs client and server
2. setup kerberos
3. try to mount an nfs share that needs kerberos authentication

Metadata

• system: "x86_64-linux"
• host os: Linux 5.3.7, NixOS, 19.09.git.28e5506 (Loris)
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 2.3
• channels(root): "nixos-19.09beta606.3ba0d9f75cc"
• nixpkgs: /nix/var/nix/profiles/per-user/root/channels/nixos

I do not usually update the channel but run straight from a git checkout.
Currently based on channel of October 21 2019

Maintainer information:

# a list of nixos modules affected by the problem
module:
- services/network-filesystems/nfsd.nix