Nice. LGTM in principle, but in practice enabling this will break a lot of configs. E.g. I use initrd networking it a bunch of NixOS configs and applying this would break those configs silently until the next reboot, which would fail to boot.

I think NixOS at the very least needs configVersion (like stateVersion) to merge this safely. And it just so happens that ATM I'm trying to push configVersion to NixOS, but it depends on #39048 (and the next step is https://github.com/oxij/nixpkgs/commits/nixos/warn-missing-stateversion, and only then the configVersion patchset).

Non-breaking interim fix in #39340. Marked this as [WIP] until we can safely get it into 18.09.

@xeji any updates on this?

also cc release managers @vcunat @samueldr, since this is the real fix to workarounds for predictable interface naming now not only for networkd but also when using dhcpcd (see #45421)
and cc @fpletz who is working on improving networking in general

I would like to get this ready now - shouldn't take long, mainly adapt to systemd 239 and test.

Before proceeding we still need to address concerns about breaking existing configs, see discussion above. In the worst case, servers using initrd networking could become unreachable after update.

I see two options:

1. accept this breakage and document it in the release notes
2. make this change conditional on stateVersion - safer but leads to uglier code. (@oxij mentioned a configVersion mechanism but I don't think that has been implemented yet)

How should be proceed here?

In both cases we need to document this in the release notes. The choice is now:

1. Accept this breakage
2. Make conditional on stateVersion

As this realistically could break boot, without resort (hey, it's been a week of uptime, I nix-collect-garbage -d already my older boot options), I think gating behind stateVersion would be better.

At least the change isn't too invasive, and if one would like to keep stateVersion < 18.09, they could realistically apply the same change locally if the predictable naming is desired; ${config.boot.initrd.extraUdevRulesCommands} even looks like a workable place to put the fix locally.

Made the change conditional on stateVersion and added documentation. First tests look ok, but needs more testing.

We could go a step further and dump our old non-standard 80-net-setup-link.rules in favor of the standard systemd udev rule, which uses an udev builtin to rename interfaces to predictable names based on the net.ifnames kernel parameter. Then usePredictableInterfaceNames becomes essentially an alias for that kernel parameter. But this will make it difficult to support the previous behavior for older configs, so I think we should do that maybe in 19.03 when everyone has had a chance to update their configs.

Timed out, unknown build status on aarch64-linux (full log)

Attempted: tests.initrd-network-ssh.predictable

client: exit status 1
client: running command: ping -c 1 server
client: exit status 1
client: running command: ping -c 1 server
client: exit status 1
client: running command: ping -c 1 server
client: exit status 1
client: running command: ping -c 1 server
building of '/nix/store/081zagf5asipv62a26vwjylhjykzr4wz-vm-test-run-initrd-network-sshpredictable.drv' timed out after 3600 seconds
error: build of '/nix/store/081zagf5asipv62a26vwjylhjykzr4wz-vm-test-run-initrd-network-sshpredictable.drv' failed

Running on the aarch64 community box I am seeing issues, and they seem consistent.

server#
server# <<< NixOS Stage 1 >>>
server#
server# loading module virtio_balloon...
server# modprobe: FATAL: Module virtio_balloon not found in directory /lib/modules/4.14.66
server# loading module virtio_console...
server# modprobe: FATAL: Module virtio_console not found in directory /lib/modules/4.14.66
server# loading module virtio_rng...
server# [    1.824348] random: fast init done
server# [    1.828758] random: crng init done
server# loading module dm_mod...
server# [    1.850868] device-mapper: ioctl: 4.37.0-ioctl (2017-09-20) initialised: dm-devel@redhat.com
server# loading module af_packet...
server# modprobe: FATAL: Module af_packet not found in directory /lib/modules/4.14.66
server# running udev...
server# [    1.886406] systemd-udevd[141]: starting version 239
server# [    1.898005] usb 1-1: new high-speed USB device number 2 using ehci-pci
server# [    2.033234] virtio_net virtio0 enp0s1: renamed from eth0
server# [    2.075432] input: QEMU QEMU USB Keyboard as /devices/platform/3f000000.pcie/pci0000:00/0000:00:08.0/usb1/1-1/1-1:1.0/0003:0627:0001.0001/input/input1
server# [    2.086893] virtio_net virtio1 enp0s2: renamed from eth1
server# [    2.162322] hid-generic 0003:0627:0001.0001: input: USB HID v1.11 Keyboard [QEMU QEMU USB Keyboard] on usb-0000:00:08.0-1/input0
server# kbd_mode: KDSKBMODE: Inappropriate ioctl for device
server# %Gipconfig: ens4: SIOCGIFINDEX: No such device
server# ipconfig: no devices to configure
server# bringing up network interface enp0s1...
server# bringing up network interface enp0s2...
server# bringing up network interface lo...
server# [    2.306066] usb 1-2: new high-speed USB device number 3 using ehci-pci
server# acquiring IP address via DHCP...
server# udhcpc: SIOCGIFINDEX: No such device
server# [    2.473022] input: QEMU QEMU USB Tablet as /devices/platform/3f000000.pcie/pci0000:00/0000:00:08.0/usb1/1-2/1-2:1.0/0003:0627:0001.0002/input/input2
server# [    2.489920] hid-generic 0003:0627:0001.0002: input: USB HID v0.01 Mouse [QEMU QEMU USB Tablet] on usb-0000:00:08.0-2/input0
client: exit status 1
client: running command: ping -c 1 server
client: exit status 1
client: running command: ping -c 1 server
client: exit status 1
client: running command: ping -c 1 server
client: exit status 1
client: running command: ping -c 1 server
client: exit status 1
client: running command: ping -c 1 server
client: exit status 1
client: running command: ping -c 1 server
client: exit status 1
client: running command: ping -c 1 server
client: exit status 1
client: running command: ping -c 1 server
client: exit status 1
client: running command: ping -c 1 server
error: interrupted by the user

-A unpredictable is fine though.

I got a "complete" output here, though unsurprisingly it matches with ofborg's output (and IIRC they do run on the same hardware). So it's not strictly because of ofborg, but either the configuration of the specific machine or some random luck factor?

I'm speculating, but this may be caused by older versions of NixOS/nix/qemu on different builders.

I will try to make the predictable-interface-names test more robust by grepping for a pattern instead of a hardcoded interface name. That should work around this issue and fix it on aarch64.

The initrd-networking-ssh.predictable test cannot be fixed this way because the expression must hardcode the interface name to start initrd networking. It will always fail on builders where that name is different. But that's a separate issue regarding uniform setup of our CI infrastructure and/or reproducible results of our qemu-based test framework - out of scope of this PR.

This should not depend on stateVersion because that's not what stateVersion is for. stateVersion is for cases where there is mutable state on disk (like PostgreSQL databases) that cannot be migrated easily. It does not mean "change option defaults to whatever was the case in NixOS version ".

@edolstra then what mechanism do you suggest for preventing breakage of existing configs?

@edolstra other than stateVersion what can be used now to prevent breakage from this change?

Though, maybe in the end there are no risks? Would the system collect the currently booted configuration? There should be references still existing keeping it alive, right? Still annoying to have a broken boot though.

This isn't ready yet, the tests still needs a little fine tuning (see above discussion). I suspended working on it until there's consensus on stateVersion or another approach.

Any updates on this pull request, please?

The nixos/modules/services/hardware/80-net-setup-link.rules file that this copies to initrd has been deleted as of 1f03f6f; what's the correct way forward to have this behaviour in initrd now that it has apparently been obsoleted? The commit message claims that the .rules files are now obsolete and all the renaming is done in C in udev, but my initrd seems to still use eth0 and then the device renaming fails after boot:

Sep 16 14:04:39 patchouli systemd-udevd[1680]: eth0: Failed to rename network interface 2 from 'eth0' to 'enp4s0': Device or resource busy

cc @xeji @andir (author of the commit) @erikarvstedt (author of similar PR #47664)

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/4055/1

I think this fix is included in #79532. @xeji, @oxij can you take a look?

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No reaction after #39329 (comment), and this is probably fixed, so closing.