Debian – How is systemd supposed to work regarding network interfaces

debianethernetsystemd

It is well-known that systemd changes the network interface names from e.g. eth0 to enp0s25, and this is called "Predictable Network Interface Names".

On my system, eth0 is called "enp0s31f6".

And I'm sorry to say this, but I too agree with this user that says "I see little irony in word "predictable"" https://askubuntu.com/questions/704361/why-is-my-network-interface-named-enp0s25-instead-of-eth0

Back on topic:

On my system (Debian 9.8 with one physical PCI Ethernet adapter), I suddenly discovered that DHCP doesn't work any longer.

I need to manually run dhclient to get an IP.

Then, I look into /etc/network/interfaces and see:

auto eth0
iface eth0 inet dhcp

Shouldn't this be enp0s31f6?

However, how am I supposed to know what to put there, what if it changes again, e.g. if I move the Ethernet PCI card to a different slot?

Am I really supposed to edit this file manually, and change eth0 to whatever systemd decides to call my interface?

Surely I must be missing some crucial piece of information here regarding how either systemd works, how Debian works, or how Predictable Network Interfaces work. Perhaps a combination of those.

Am I supposed to refer to an abstract network interface in /etc/network/interfaces? Some abstract string that always means "the first network interface card" (isn't this just eth0?)? Or by some UID? By its PCI domain:bus:device?

This is very confusing, and a lot of documentation refers to what I would consider a mess of legacy scripts, modern systemd, and many distros seem to use a combination of old style scripts and systemd functionality which is hard to grasp.

Questions:

Why did it stop working?
What did I do wrong?
Why was it ever eth0 in my /etc/network/interfaces, when Debian uses systemd and Predictable Network Interfaces?
What is the correct way to fix this?

Best Answer

Why did it stop working? What did I do wrong?

It's hard to tell what happened "suddenly". Review your /var/log/syslog or /var/log/messages for DHCP logs around the time it stopped working and you may find out. Also check the modification time in stat /etc/network/interfaces.

Why was it ever eth0 in my /etc/network/interfaces, when Debian uses systemd and Predictable Network Interfaces?

This is the new default since Debian stretch. If you installed your system earlier and your /etc/network/interfaces originates from jessie (or wheezy...) then it was created by the installer (netcfg) using the old naming scheme.

What is the correct way to fix this?

Whatever suits you best.

If you expect to ever have multiple Ethernet interfaces in this machine, then think about how you'd like to tell them apart. Remember that their ethX order would not be stable/predictable. PredictableNetworkInterfaceNames gives you several options, choose according to your use case.
If this machine will only ever have a single Ethernet interface, then you can either
- fall back to using the kernel name, which will always be eth0 (the above page gives you several options, my favorite is ln -s /dev/null /etc/systemd/network/99-default.link, just don't forget to regenerate your initramfs afterwards with update-initramfs -u);
- or don't care about the name of your interface, ditch /etc/network/interfaces entirely and switch to systemd-networkd with a match-all config like

[Network]
DHCP=ipv4

in /etc/systemd/network/dhcp.network (choose an arbitrary file name). Sorry, I can't get the indentation right...

First method (bruteforce):

So the first method I came up with was simple to bruteforce the driver reload with init.d script.

Following init script is tailored for Debian 6.0, but the same principle should work on almost any distribution using proper init.d scripts.

#!/bin/sh -e

### BEGIN INIT INFO
# Provides:          reorder-nics
# Required-Start:
# Required-Stop:
# Default-Start:     S
# Default-Stop:
# Short-Description: Reloads the nics in correct order
### END INIT INFO

#
# This script should reload the nic drivers in corrected order.
# Basically it just unloads and then loads the drivers in different order.
#

echo "Reloading NICs!"

# unload the drivers
modprobe -r driver_0        # eth0 nic interface
modprobe -r driver_1        # eth1 nic interface

# load the drivers in corrected order
modprobe driver_1
modprobe driver_0

#EOF

Then the script must be added to proper runlevel directory. This can be done easily on Debian with "update-rc.d" command. For example: update-rc.d reorder-nics start S

Second method (Better I think):

I also found a bit more elegant way (at least for Debian & Ubuntu systems).

First Make sure that kernel doesn't automatically load the NIC drivers. This can be done by creating a blacklist file in /etc/modprobe.d/. I created a file named "disable-nics.conf". Note that files in /etc/modprobe.d/ must have .conf suffix. Also naming modules in /etc/modprobe.d/blacklist.conf do not affect autoloading of modules by the kernel, so you have to make your own file.

# Disable automatic loading of kernel driver modules
# Disable NIC drivers

blacklist driver_0     # eth0 by default
blacklist driver_1     # eth1 by default

Then run 'depmod -ae' as root

Recreate your initrd with 'update-initramfs -u'

And finally add the driver names in corrected order into /etc/modules file.

# /etc/modules: kernel modules to load at boot time.
#
# This file contains the names of kernel modules that should be loaded
# at boot time, one per line. Lines beginning with "#" are ignored.
# Parameters can be specified after the module name.

# drivers in wanted order
driver_1    # this one should be loaded as eth0
driver_0    # this one should be loaded as eth1

Changes should come in effect after the next boot.

Reboot is not necessary though; it's easy to switch the devices with following command (as root, of course):

modprobe -r driver_0; modprobe -r driver_1; modprobe driver_1; modprobe driver_0

Some useful links I found while searching the solution:

Linux – How to Find Available Network Interfaces

The simplest method I know to list all of your interfaces is

ifconfig -a

EDIT

If you're on a system where that has been made obsolete, you can use

ip link show

Best Answer

Related Solutions

Debian – How to change the order of the network cards (eth1 <-> eth0) on linux

First method (bruteforce):

Second method (Better I think):

Linux – How to Find Available Network Interfaces

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