Differences between revisions 2 and 3
Revision 2 as of 2019-09-11 18:39:54
Size: 12995
Editor: JustinBRye
Comment: fixing up duff markup
Revision 3 as of 2019-09-11 19:25:36
Size: 13342
Editor: JustinBRye
Comment: first draft, contributions welcome
Deletions are marked like this. Additions are marked like this.
Line 1: Line 1:
This is only a first draft, but the plan is to turn it into something
that the Release Notes can point at as the nearest thing we've got to
an official guide.

todo: links from

 *
NetworkConfiguration (replacing the halfbaked version there)

 *
NetworkFAQ (much of that page needs replacing)
The plan is to turn this into something that the Release Notes can point at as the nearest thing we've got to an official guide.

ToDo: give it a reread in the morning, add links from [[NetworkConfiguration]] (replacing the halfbaked version there) and [[NetworkFAQ]] (much of that page needs replacing)
Line 17: Line 11:

----
Line 42: Line 38:

----
Line 79: Line 77:
of years when you try to undo it. Oh, and beware of [[#initrd|initrd
skew]].
of years when you try to undo it. Oh, and beware of [[#initrd|initrd skew]].
Line 87: Line 84:
== How to disable it ==

If you've currently got a legacy
`/etc/udev/rules.d/75-persistent-net.rules` file but have decided to
switch to the [[#ifnames|new regime]], you can do that just by
disabling the `.rules` file; see the `udev` README.Debian.gz
== How to let go and move on ==

If you've currently got a legacy `/etc/udev/rules.d/75-persistent-net.rules` file but have decided to switch to the [[#ifnames|new regime]], you can do that just by disabling the `.rules` file; see the `udev` README.Debian.gz

----
Line 97: Line 93:
Old releases of RedHat (among others) used a "biosdevname" system,
but that's never been supported under Debian. If you need to know
about it there's bound to be documentation somewhere.
Old releases of RedHat (among others) used a "biosdevname" system, but that's never been supported under Debian. If you need to know about it there's bound to be documentation somewhere.

----
Line 102: Line 98:
= THE NEW REGIME = = THE NEW APPROACH TO NETWORK INTERFACE NAMES =
Line 115: Line 111:
etc., just look at the output of (e.g.)

{{{
       ip a
}}}
etc., just look at the output of (e.g.) `ip a`.
Line 128: Line 120:
== How to switch == == How to switch to this scheme on upgraded systems ==
Line 134: Line 126:
=== STRATEGY A ===

 * wait until `udev` breaks your networking, if it's going to, or
trigger the change by disabling the persistent-names system.
=== strategy A ===

 * wait until `udev` breaks your networking, if it's going to, or trigger the change by disabling the persistent-names system.
Line 151: Line 142:
=== STRATEGY B === === strategy B ===
Line 160: Line 151:
 * consult the list of [[#corner|corner cases]] below and search the
Internet for others
 * consult the list of [[#corner|corner cases]] below and search the Internet for others
Line 181: Line 171:
It's likely to tell you about things like ID_OUI_FROM_DATABASE and an
ID_NET_NAMING_SCHEME, but the lines that matter are the ones (given
It's likely to tell you about things like ID_OUI_FROM_DATABASE and an ID_NET_NAMING_SCHEME, but the lines that matter are the ones (given
Line 191: Line 180:
 * ID_NET_NAME_FROM_DATABASE=:: very rare (possibly mythical) and
not to be confused with ID_OUI_FROM_DATABASE; if present, it gets
maximum priority. Nobody seems to know why - it's mentioned in
[[DebianMan:5/systemd.link|systemd.link(5)]], but otherwise
undocumented. The database is hardcoded into `udev` and has only one
known entry, the spooky-sounding `idrac`.
 * ID_NET_NAME_FROM_DATABASE=:: very rare (possibly mythical) and not to be confused with ID_OUI_FROM_DATABASE; if present, it gets maximum priority. Nobody seems to know why - it's mentioned in [[DebianMan:5/systemd.link|systemd.link(5)]], but otherwise undocumented. The database is hardcoded into `udev` and has only one known entry, the spooky-sounding `idrac`.
Line 200: Line 184:
 * ID_NET_NAME_ONBOARD=:: appears for some but not all kinds of
onboard network card (Ethernet only, or wireless too?) - it's usually
a nice simple name like `eno0`.

 * ID_NET_NAME_SLOT=:: appears for some PCI-hotplug cards. Usually
looks like `ens0` (again, any wifi cases? Does it ever occur along
with _ONBOARD?)

 * ID_NET_NAME_PATH=:: always present; usually something just
complicated enough to be easy to forget, like `wlp3s5` or `enp1s3f0`.
Note that all numbers are in hex.

 * ID_NET_NAME_MAC=:: also always present, but with a low enough
priority that by default it won't be used; e.g. `wlx800e1319c734`

You'd think in a sane world we'd be able to just ask for

{{{
        
udevadm ifnames
}}}
 * ID_NET_NAME_ONBOARD=:: appears for some but not all kinds of onboard network card (Ethernet only, or wireless too?) - it's usually a nice simple name like `eno0`.

 * ID_NET_NAME_SLOT=:: appears for some PCI-hotplug cards. Usually looks like `ens0` (again, any wifi cases? Does it ever occur along with _ONBOARD?)

 * ID_NET_NAME_PATH=:: always present; usually something just complicated enough to be easy to forget, like `wlp3s5` or `enp1s3f0`. Note that all numbers are in hex.

 * ID_NET_NAME_MAC=:: also always present, but with a low enough priority that by default it won't be used; e.g. `wlx800e1319c734`

/* You'd think in a sane world we'd be able to just ask for `udevadm ifnames`
Line 225: Line 197:
But no, that would be too simple. But that would be too simple. */
Line 228: Line 200:
== Corner cases ==

(Please try to avoid ballooning this section with tales of "I
don't know how this happened but it all went wrong for me"...)

 * TRIGGERING THE SWITCH: you don't need to ''delete''
`/etc/udev/rules.d/70-persistent-net.rules` to test what network
interface names you'll get without it. Just renaming it (e.g. to
`70-persistent-net.rules.old`) or commenting out particular lines
should be enough (as long as you update your [[#initrd|initramfs]]
before you reboot).

 * THE ID_NET_NAME_ HIERARCHY:: if you're ignoring
ID_NET_NAME_BORING on the assumption that anything you don't
understand probably isn't important, you need to reread the above -
the general rule is, if you don't recognise it, it'll mess you up.
== Complications and corner cases ==

(Additions welcome, but please try to avoid ballooning this section with tales of "I don't know how this happened but it all went wrong for me"...)

 * TRIGGERING THE SWITCH:: you don't need to ''delete'' `/etc/udev/rules.d/70-persistent-net.rules` to test what network interface names you'll get without it. Just renaming it (e.g. to `70-persistent-net.rules.old`) or commenting out particular lines should be enough (as long as you update your [[#initrd|initrd]] before you reboot).

 * THE ID_NET_NAME_ HIERARCHY:: if you're ignoring ID_NET_NAME_SOMETHING on the assumption that anything you don't understand probably isn't important, you need to reread the above - the general rule is, if you don't recognise it, it'll mess you up.
Line 246: Line 209:
 * INITRD SKEW:: it's all very well having everything sorted out in
`/etc`; but to make sure your initramfs doesn't contain out-of-date
versions of important `systemd` unit files, regenerate it with

{{{
        
sudo update-initramfs -u
}}}


 * LEFTOVERS:: the old system first started being publicly deprecated
in `NEWS` files back in mid-2015, so this upgrade has been hanging
ominously over people's heads for quite long enough now that you're
entitled to forgive yourself if you've forgotten something you did
last time the subject came up. Did you set up a `net.ifnames=0` kernel
parameter, and/or mask some `systemd` config file? Check your
administrative logbooks. What do you mean, you don't keep logs?

 * USB DEVICES:: since they might get plugged into a different socket
each time, these use ID_NET_NAME_MAC - automated via
`/lib/udev/rules.d/73-usb-net-by-mac.rules`.

 * VIRTUAL MACHINES:: on virtual machines (according to the `udev`
README) you will need to remove the files
`/etc/systemd/network/99-default.link` and (if using virtio network
devices) `/etc/systemd/network/50-virtio-kernel-names.link`,
then [[#initrd|rebuild the initrd]].

 * UNPREDICTABILITY:: it turns out even after all this there are still
reported cases of interfaces changing their name on a reboot. All that
needs to happen is that some buggy BIOS, or some new, less buggy
version of a driver module, changes its mind about whether or not your
hardware counts as the kind that should have an ONBOARD or SLOT name.
 * INITRD SKEW:: it's all very well having everything sorted out in `/etc`; but to make sure your initrd doesn't contain out-of-date versions of important `systemd` unit files, regenerate it with `sudo update-initramfs -u`

 * LEFTOVERS:: the old persistent-names system first started being publicly deprecated in `NEWS` files back in mid-2015, so this upgrade has been hanging ominously over people's heads for a long time. Are you sure you didn't do something about it the last time the subject came up, like setting up a `net.ifnames=0` kernel parameter, and/or masking some `systemd` config file? If so, this may result in confusing symptoms when you try to go over to the new system. Check your administrative logbooks. What do you mean, you don't keep logs?

 * USB DEVICES:: since they might get plugged into a different socket each time, these use ID_NET_NAME_MAC - automated via `/lib/udev/rules.d/73-usb-net-by-mac.rules`.

 * VIRTUAL MACHINES:: on virtual machines (according to the `udev` README) you will need to remove the files `/etc/systemd/network/99-default.link` and (if using virtio network devices) `/etc/systemd/network/50-virtio-kernel-names.link`, then [[#initrd|rebuild the initrd]].

 * UNPREDICTABILITY:: it turns out even after all this there are still reported cases of interfaces changing their name on a reboot. All that needs to happen is that some buggy BIOS, or some new, less buggy version of a driver module, changes its mind about whether or not your hardware counts as the kind that should have an ONBOARD or SLOT name.

 * ANTIQUE SYSTEMS:: on Debian 9 "Stretch" or newer, merely booting without an `ifnames=0` override should be enough to let you to test the new scheme, but on Debian 8 "Jessie" or older you'll need to actively set it to `ifnames=1`.

----
Line 279: Line 224:
= THE ALTERNATIVE APPROACH = = ALTERNATIVE APPROACHES =
Line 292: Line 237:
----
Line 295: Line 242:
[[https://www.debian.org/releases/stretch/amd64/release-notes/ch-whats-new.en.html#new-interface-names|Stretch
Release Notes]]

[[https://www.debian.org/releases/buster/amd64/release-notes/ch-information.en.html#migrate-interface-names|Buster
Release Notes]]
[[https://www.debian.org/releases/stretch/amd64/release-notes/ch-whats-new.en.html#new-interface-names|Stretch]] and [[https://www.debian.org/releases/buster/amd64/release-notes/ch-information.en.html#migrate-interface-names|Buster]] Release Notes
Line 303: Line 246:
[[https://www.freedesktop.org/wiki/Software/systemd/PredictableNetworkInterfaceNames/]]

The big problem with the latter is that it delegates all its technical
details to a link pointing at the sourcecode:
[[https://www.freedesktop.org/wiki/Software/systemd/PredictableNetworkInterfaceNames/]]. The big problem with the latter is that it delegates all its technical details to a link pointing at the sourcecode:
Line 316: Line 256:
General guides to overriding [[Systemd]] configuration: General guides to overriding [[systemd]] configuration:

The plan is to turn this into something that the Release Notes can point at as the nearest thing we've got to an official guide.

ToDo: give it a reread in the morning, add links from NetworkConfiguration (replacing the halfbaked version there) and NetworkFAQ (much of that page needs replacing)

Warning: Anything that changes the names of your network interfaces may result in the machine suddenly not being reachable over SSH, so if you're editing settings on a remote server, plan your changes carefully and doublecheck your safety nets.


THE ORIGINAL SIMPLE SCHEME

Back in the nineties, eth0, eth1, etc were simply assigned by the kernel.

Why it was abandoned

At least in theory, if module probes completed in a different order, eth0 and eth1 might switch places on successive boots. As boot processes became less linear and interfaces became more hotpluggable this became more of a concern.

How to get it back

If you wipe out all other mechanisms (booting with the ifnames mechanism deactivated and no /etc/udev/rules.d/75-persistent-net.rules file, or maybe just a udev that has finally stopped supporting it) then I suppose you'll be back to this one.

(Is this account missing old approaches to interface-renaming that still need to be dealt with? For instance, does the package ifrename still work?)


THE "PERSISTENT NAMES" APPROACH

This scheme, introduced somewhere around Debian 5 "Lenny", used udev to identify interfaces by MAC address and assign a fixed interface number to any interface it recognized (writing the rules to /etc/udev/rules.d/75-persistent-net.rules). This could have annoying side-effects (e.g. if you were replacing a machine's sole NIC, you'd also have to take special care to ensure it took over as network interface number 0) but these were minor and predictable.

Why this one was abandoned

This still had subtle race conditions, required /etc to be on a writable file system, and had problems with virtualization, so it's no longer supported upstream; the plan (still taken for granted in most of the documentation) was for it not to be supported in Debian 10 "Buster", but in the end it has scraped through into another release cycle.

How to cling to it for now

First, make sure you've got a working "legacy" /etc/udev/rules.d/75-persistent-net.rules file. Then deactivate the replacement scheme. The simple way of doing that latter (which you might want to try for one-off testing) is just to boot with the kernel parameter net.ifnames=0, which can be set in an interactive grub session at boot (or made persistent by editing /etc/default/grub and running update-grub).

Alternatively, you can override /lib/systemd/network/99-default.link, with a custom version in /etc/systemd/network/, or similarly override /lib/udev/rules.d/80-net-setup-link.rules, or mask the latter by using a /dev/null symlink instead of a custom version, or... there seem to be lots of ways of doing this, so make sure you haven't done it in more than one way or it'll trip you up in a couple of years when you try to undo it. Oh, and beware of initrd skew.

Unfortunately, none of this will carry on working beyond the point when udev stops supporting the legacy 75-persistent-net.rules mechanism, so you should be ready to switch to a different system when that happens.

How to let go and move on

If you've currently got a legacy /etc/udev/rules.d/75-persistent-net.rules file but have decided to switch to the new regime, you can do that just by disabling the .rules file; see the udev README.Debian.gz


ANOTHER APPROACH SOMETIMES SEEN ON NON-DEBIAN SYSTEMS

Old releases of RedHat (among others) used a "biosdevname" system, but that's never been supported under Debian. If you need to know about it there's bound to be documentation somewhere.


THE NEW APPROACH TO NETWORK INTERFACE NAMES

The new "net.ifnames" approach uses names usually derived from the location of the interface in terms of hardware buses etc.

(This system is often advertised as providing "Predictable Names", but the main thing that's predictable about it is that calling it this will cause furious users to pop up disputing the appropriateness of that name. Can we just skip all that here, please?)

How to cope with it on fresh installs

This should be easy enough; before you start configuring firewalls etc., just look at the output of (e.g.) ip a.

Unlike the old days, when the only way to guess which cable was plugged into eth0 and which was eth1 was to keep track of MAC addresses, this system provides extra clues in the interface names.

How to switch to this scheme on upgraded systems

It's advisable to do this as a separate thing in its own right, not as part of a general dist-upgrade. However, if your PC only has one network interface and not much is at stake you can try:

strategy A

  • wait until udev breaks your networking, if it's going to, or trigger the change by disabling the persistent-names system.

  • ask ip what new name it's using, and fix your config files.

You should probably at least check in advance to see what files hard-code interface names, by running something like

        sudo rgrep ''wlan0'' /etc

(Obvious possibilities include /etc/network/interfaces and configuration files for firewalls, wifi, DHCP...)

strategy B

This strategy, more or less compulsory for remote servers, runs along the lines of:

  • devise a migration plan
  • consider a dead-man's-handle etckeeper cronjob or the like

  • consult the list of corner cases below and search the Internet for others

  • once you're sure you're safe, implement your migration plan
  • you've got backups, right?

To find out what names udev would be choosing between if you switched over to the new system, first get a list of the network devices the system knows about:

        echo /sys/class/net/*

For each device (other than lo), ask udevadm what NET_IDs it knows:

        udevadm test-builtin net_id /sys/class/net/''enp0s1'' 2>/dev/null

It's likely to tell you about things like ID_OUI_FROM_DATABASE and an ID_NET_NAMING_SCHEME, but the lines that matter are the ones (given in unhelpfully random order) starting with ID_NET_NAME_. One of these is the name that udev will give priority to - the list of candidates may be so short that all you need to know is that ..._PATH beats ..._MAC, but there are also some rarer possibilities, and in general if something unusual shows up then it will take priority.

From highest priority to lowest, the list is:

* ID_NET_NAME_FROM_DATABASE=

very rare (possibly mythical) and not to be confused with ID_OUI_FROM_DATABASE; if present, it gets maximum priority. Nobody seems to know why - it's mentioned in systemd.link(5), but otherwise undocumented. The database is hardcoded into udev and has only one known entry, the spooky-sounding idrac.

* ID_NET_NAME_ONBOARD=

appears for some but not all kinds of onboard network card (Ethernet only, or wireless too?) - it's usually a nice simple name like eno0.

* ID_NET_NAME_SLOT=

appears for some PCI-hotplug cards. Usually looks like ens0 (again, any wifi cases? Does it ever occur along with _ONBOARD?)

* ID_NET_NAME_PATH=

always present; usually something just complicated enough to be easy to forget, like wlp3s5 or enp1s3f0. Note that all numbers are in hex.

* ID_NET_NAME_MAC=

also always present, but with a low enough priority that by default it won't be used; e.g. wlx800e1319c734

Complications and corner cases

(Additions welcome, but please try to avoid ballooning this section with tales of "I don't know how this happened but it all went wrong for me"...)

* TRIGGERING THE SWITCH

you don't need to delete /etc/udev/rules.d/70-persistent-net.rules to test what network interface names you'll get without it. Just renaming it (e.g. to 70-persistent-net.rules.old) or commenting out particular lines should be enough (as long as you update your initrd before you reboot).

* THE ID_NET_NAME_ HIERARCHY
if you're ignoring ID_NET_NAME_SOMETHING on the assumption that anything you don't understand probably isn't important, you need to reread the above - the general rule is, if you don't recognise it, it'll mess you up.

* INITRD SKEW

it's all very well having everything sorted out in /etc; but to make sure your initrd doesn't contain out-of-date versions of important systemd unit files, regenerate it with sudo update-initramfs -u

* LEFTOVERS

the old persistent-names system first started being publicly deprecated in NEWS files back in mid-2015, so this upgrade has been hanging ominously over people's heads for a long time. Are you sure you didn't do something about it the last time the subject came up, like setting up a net.ifnames=0 kernel parameter, and/or masking some systemd config file? If so, this may result in confusing symptoms when you try to go over to the new system. Check your administrative logbooks. What do you mean, you don't keep logs?

* USB DEVICES

since they might get plugged into a different socket each time, these use ID_NET_NAME_MAC - automated via /lib/udev/rules.d/73-usb-net-by-mac.rules.

* VIRTUAL MACHINES

on virtual machines (according to the udev README) you will need to remove the files /etc/systemd/network/99-default.link and (if using virtio network devices) /etc/systemd/network/50-virtio-kernel-names.link, then rebuild the initrd.

* UNPREDICTABILITY
it turns out even after all this there are still reported cases of interfaces changing their name on a reboot. All that needs to happen is that some buggy BIOS, or some new, less buggy version of a driver module, changes its mind about whether or not your hardware counts as the kind that should have an ONBOARD or SLOT name.
* ANTIQUE SYSTEMS

on Debian 9 "Stretch" or newer, merely booting without an ifnames=0 override should be enough to let you to test the new scheme, but on Debian 8 "Jessie" or older you'll need to actively set it to ifnames=1.


ALTERNATIVE APPROACHES

You can also use .link files to set up naming schemes to suit yourself - so for instance if you have two PCs each of which has only a single wireless card, but one calls it wlp0s1 and the other wlp1s0, you can arrange for them both to use the name wifi0 to simplify sharing firewall configurations. For details see systemd.link(5).

It's also possible to reorganize the naming policy by overriding the NamePolicy defined in /lib/systemd/network/99-default.link (so that for instance all network cards are named by MAC address).


EXTERNAL REFERENCES

Stretch and Buster Release Notes

There's some information in /usr/share/doc/udev/README.Debian.gz, and more in https://www.freedesktop.org/wiki/Software/systemd/PredictableNetworkInterfaceNames/. The big problem with the latter is that it delegates all its technical details to a link pointing at the sourcecode: https://github.com/systemd/systemd/blob/master/src/udev/udev-builtin-net_id.c ...but unfortunately most of the useful comments that used to be at the top of that file have been thrown out, so you need to find your way back through the git tree to a previous version such as https://github.com/systemd/systemd/blob/eefe36e64c1a583bb9470884ed92115e0ce4647e/src/udev/udev-builtin-net_id.c

A guide that mentions ID_NET_NAME_FROM_DATABASE: https://major.io/2015/08/21/understanding-systemds-predictable-network-device-names/

General guides to overriding systemd configuration: https://askubuntu.com/questions/659267/how-do-i-override-or-configure-systemd-services https://wiki.archlinux.org/index.php/systemd


CategoryNetwork