Kernel-only network configuration on Linux

  1. blog

What if I told you there is a way to configure the network on any Linux server that:

  1. works across all distributions
  2. doesn't require any software installed apart from the kernel and a boot loader (no systemd-networkd, ifupdown, NetworkManager, nothing)
  3. is backwards compatible all the way back to Linux 2.0, in 1996

It has literally 8 different caveats on top of that, but is still totally worth your time.

  1. Kernel-only network configuration on Linux
  2. Known options in Debian
  3. A "new" network configuration system
  4. What are you doing.
  5. Examples
    1. GRUB
    2. systemd-boot and UKI setups
    3. Other systems
    4. dropbear-initramfs
  6. Why?
  7. Caveats
  8. Cleanup
  9. Credits

Known options in Debian

People following Debian development might have noticed there are now four ways of configuring the network Debian system. At least that is what the Debian wiki claims, namely:

At this point, I feel ifupdown is on its way out, possibly replaced by systemd-networkd. NetworkManager already manages most desktop configurations.

A "new" network configuration system

The method is this:

So by "new" I mean "new to me". This option is really old. The nfsroot.txt where it is documented predates the git import of the Linux kernel: it's part of the 2005 git import of 2.6.12-rc2. That's already 20+ years old already.

The oldest trace I found is in this 2002 commit, which imports the whole file at once, but the option might goes back as far as 1996-1997, if the copyright on the file is correct and the option was present back then.

What are you doing.

The trick is to add an ip= parameter to the kernel's command-line. The syntax, as mentioned above, is in nfsroot.txt and looks like this:

ip=<client-ip>:<server-ip>:<gw-ip>:<netmask>:<hostname>:<device>:<autoconf>:<dns0-ip>:<dns1-ip>:<ntp0-ip>

Most settings are pretty self-explanatory, if you ignore the useless ones:

Note that the Red Hat manual has a different opinion:

ip=[<server-id>]:<gateway-IP-number>:<netmask>:<client-hostname>:inteface:[dhcp|dhcp6|auto6|on|any|none|off]

It's essentially the same (although server-id is weird), and the autoconf variable has other settings, so that's a bit odd.

Examples

For example, this command-line setting:

ip=192.0.2.42::192.0.2.1:255.255.255.0:::off

... will set the IP address to 192.0.2.42/24 and the gateway to 192.0.2.1. This will properly guess the network interface if there's a single one.

A DHCP only configuration will look like this:

ip=::::::dhcp

Of course, you don't want to type this by hand every time you boot the machine. That wouldn't work. You need to configure the kernel commandline, and that depends on your boot loader.

GRUB

With GRUB, you need to edit (on Debian), the file /etc/default/grub (ugh) and find a line like:

GRUB_CMDLINE_LINUX=

and change it to:

GRUB_CMDLINE_LINUX=ip=::::::dhcp

systemd-boot and UKI setups

For systemd-boot UKI setups, it's simpler: just add the setting to the /etc/kernel/cmdline file. Don't forget to include anything that's non-default from /proc/cmdline.

This assumes that is the Cmdline=@ setting in /etc/kernel/uki.conf. See 2025-08-20-luks-ukify-conversion for my minimal documentation on this.

Other systems

This is perhaps where this is much less portable than it might first look, because of course each distribution has its own way of configuring those options. Here are some that I know of:

It's interesting that /etc/default/grub is consistent across all distributions above, while the systemd-boot setups are all over the place (except for the UKI case), while I would have expected those be more standard than GRUB.

dropbear-initramfs

If dropbear-initramfs is setup, it already requires you to have such a configuration, and it might not work out of the box.

This is because, by default, it disables the interfaces configured in the kernel after completing its tasks (typically unlocking the encrypted disks).

To fix this, you need to disable that "feature":

IFDOWN="none"

This will keep dropbear-initramfs from disabling the configured interface.

Why?

Traditionally, I've always setup my servers with ifupdown on servers and NetworkManager on laptops, because that's essentially the default. But on some machines, I've started using systemd-networkd because ifupdown has ... issues, particularly with reloading network configurations. ifupdown is a old hack, feels like legacy, and is Debian-specific.

Not excited about configuring another service, I figured I would try something else: just configure the network at boot, through the kernel command-line.

I was already doing such configurations for dropbear-initramfs (see this documentation), which requires the network the be up for unlocking the full-disk encryption keys.

So in a sense, this is a "Don't Repeat Yourself" solution.

Caveats

Also known as: "wait, that works?" Yes, it does! That said...

  1. This is useful for servers where the network configuration will not change after boot. Of course, this won't work on laptops or any mobile device.

  2. This only works for single interface configurations. If you have multiple interfaces, bridges, VLANs, wifi, none of this will work.

  3. It does support IPv6 and feels like the best way to configure IPv6 hosts: true zero configuration.

  4. It likely does not work with a dual-stack IPv4/IPv6 static configuration. It might work with a dynamic dual stack configuration, but I doubt it.

  5. I don't know what happens when a DHCP lease expires. No daemon seems to be running so I assume leases are not renewed, so this is more useful for static configurations, which includes server-side reserved fixed IP addresses. (A non-renewed lease risks getting reallocated to another machine, which would cause an addressing conflict.)

  6. It will not automatically reconfigure the interface on link changes, but ifupdown does not either.

  7. It will not write a good resolv.conf for you, that you need to configure separately. Maybe passing those dns0-ip settings will work? Untested, but DNS is, after all, a mostly user-level implementation (typically in libc), the kernel doesn't (again, typically) care about DNS.

  8. I have not really tested this at scale: only a single, test server at home.

Yes, that's a lot of caveats, but it happens to cover a lot of machines for me, and it works surprisingly well. My main doubts are about long-term DHCP behaviour, but I don't see why that would be a problem with a statically defined lease.

Cleanup

Once you have this configuration, you don't need any "user" level network system, so you can get rid of everything:

apt purge systemd-networkd ifupdown network-manager netplan.io

Note that ifupdown (and probably others) leave stray files in (e.g.) /etc/network which you might want to cleanup, or keep in case all this fails and I have put you in utter misery. Configuration files for other packages might also be left behind, I haven't tested this, no warranty.

Credits

This whole idea came from the A/I folks (not to be confused with AI) who have been doing this forever, thanks!

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