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

https://discourse.nixos.org/t/predictable-network-interface-names-in-initrd/4055/4

This needs a thorough review and testing. But I think we should try to get this into 19.09 as now the network setup stuff is a bit flakey

Also not sure if this is the right approach. I'm still very confused how all this udev magic comes together. Because currently if networking.useNetworkd = false then stage-2 doesn't have an /etc/systemd/network/99-default.link file , but from what I read from the manpages from systemd, we need this file present regardless if we're using networkd or not, as it is a file that is read by udev, not networkd. (See https://www.freedesktop.org/software/systemd/man/systemd.link.html for more info.

So I think we should also copy this file over to the stage-2 /etc directory otherwise no renaming happens in stage-2 either.

But that begs the question, in the current scenario, why is stage-2 udev trying to rename if there is no 99-default.link to read the config from?

Edit:

I think udev will also look in $out/systemd/network for link files, at least in stage-2. Though this is a hunch. I'm building a version of master with debug logging enabled for udev to investigate a bit more.

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

https://discourse.nixos.org/t/predictable-network-interface-names-in-initrd/4055/5

@arianvp With this PR, even after fixing the typo mentioned by @emilazy I was experiencing the problems you mentioned in the NixOS Discourse thread, namely:

1. in stage-1 the network is still not renamed
2. in stage-2 the renaming fails because the interface is busy

However, after I applied the changes in the following commit on top of your PR, everything seems to be finally fixed for me on NixOS 19.09:
wizeman@5d9d8fa

I don't even want to pretend to understand how udev works -- the idea about adding 75-net-descriptions.rules was taken from PR #47664.

Note that I have only done limited testing on 2 machines: one using a static network config with networking enabled in the initrd and another one using NetworkManager (with networking disabled in the initrd). AFAIK I'm not using systemd-networkd.

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

https://discourse.nixos.org/t/predictable-network-interface-names-in-initrd/4055/9

@fpletz has a branch lying around that is a better implementation of this one. We worked on it at All Systems Go. @fpletz did you push this work somewhere?

It looks like even with my changes, one of my machines still failed to rename the network interface, even in stage 2. I don't know what's different about this machine that causes this to fail...

@fpletz ping?

Would love to see this fixed for 20.03.

@arianvp this seems somewhat redundant to #47664.

@erikarvstedt, can we close the other one, or should more be moved into here?

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

https://discourse.nixos.org/t/disk-encryption-on-nixos-servers-how-when-to-unlock/5030/8

For nixos-19.09pre-git (Loris), I've come up with this module config (importable in a configuration.nix) with explanations inlined (beware that I'm no udev's nor systemd's well informed user — I've only run a few tests on a single machine (APU2) and read a few docs and comments, instead of going to bed — so I may have missed or misunderstood things, but it works for me™ to decrypt my ZFS root over SSH):

# Use predictable interface names in stage-1 and stage-2.
# DOC: https://www.freedesktop.org/wiki/Software/systemd/PredictableNetworkInterfaceNames/
#
# Tip: names that can be given using ID_NET_NAME_* envvars
# can be checked before hand with:
# udevadm test-builtin net_id /sys/class/net/*

{ pkgs, lib, config, ... }:
let udevNetSetupLinkRules = pkgs.writeTextFile {
    name = "80-net-setup-link.rules";
    destination = "/etc/udev/rules.d/80-net-setup-link.rules";
    text = ''
      SUBSYSTEM!="net", GOTO="net_setup_link_end"

      IMPORT{builtin}="path_id"

      ACTION!="add", GOTO="net_setup_link_end"

      # Load net_setup_link to setup the ID_NET_NAME_* envvars
      IMPORT{builtin}="net_setup_link"

      # Rename eth* using the "path" name policy (eg. enp1s0),
      # Note that in stage-1 the envvar ID_NET_NAME is not set,
      # hence not usable as in $${pkgs.systemd}/lib/udev/rules.d/80-net-setup-link.rules
      # Because in stage-1 there is no /etc/systemd/network/*.link
      # nor **/systemd/network/99-default.link
      # to set NamePolicy= which is responsible to set ID_NET_NAME.
      # Not sure if ATTR{type}=="1" and KERNEL=="eth*" are equivalent or not.
      ATTR{type}=="1", KERNEL=="eth*", NAME="$env{ID_NET_NAME_PATH}"

      LABEL="net_setup_link_end"
    '';
  };
in
{
  networking = {
    # Currently no-op.
    # false would set boot.kernelParams = [ "net.ifnames=0" ];
    # to disable NamePolicy= in *.link.
    usePredictableInterfaceNames = true;
  };

  boot.initrd = {
    extraUdevRulesCommands = ''
      # Query hwdb to set some more ID_* in case someone need them for their rules.
      cp -v ${pkgs.systemd}/lib/udev/rules.d/75-net-description.rules $out/

      # The name set here in stage-1 by 80-net-setup-link.rules
      # will stay in stage-2 (at least until the device is removed/added).
      cp -v ${udevNetSetupLinkRules}/etc/udev/rules.d/80-net-setup-link.rules $out/
    '';
  };

  services.udev.packages = [
    # Only useful here in stage-2 if the device is removed and re-added
    # (eg. the network module is rmmod-ed then modprobe-d).
    # The stage-1 (or initrd) is only a pivot_root after all,
    # it does not reload the kernel, hence passing to stage-2
    # does not trigger ACTION=="add" for the net devices.
    udevNetSetupLinkRules
  ];

  /* Useless block, only here for explanations.

  # NixOS put this .link only in the root filesystem, not in the initrd
  # hence it's only active in stage-2, not stage-1.
  # And even in stage-2, the 80-net-setup-link.rules has priority.
  # DOC: https://www.freedesktop.org/software/systemd/man/systemd.link.html
  environment.etc."systemd/network/79-net-setup.link".text = ''
    [Match]
    OriginalName=*

    [Link]
    #NamePolicy=keep kernel database onboard slot path
    NamePolicy=mac
    MACAddressPolicy=persistent
  '';
  */
}

This PR is unfinished and probably not correct.

I and @fpletz briefly hacked on this during All-systems-go conf and our work is in this branch https://github.com/NixOS/nixpkgs/tree/fix-predictable-ifnames-in-initrd

@ju1m This works perfectly for me, thank you! I'd love to see this cleaned up and merged upstream -- presumably some part of NixOS's existing udev setup is incorrect?

Can this be closed in favor of #79532?