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Merge pull request #14308 from poettering/man-naming-scheme-typo 

superficial fixes to network naming docs
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Yu Watanabe 2019-12-12 17:04:06 +09:00 committed by GitHub
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docs/PREDICTABLE_INTERFACE_NAMES.md
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@ -6,20 +6,19 @@ layout: default
# Predictable Network Interface Names
Starting with v197 systemd/udev will automatically assign predictable, stable network interface names for all local Ethernet, WLAN and WWAN interfaces. This is a departure from the traditional interface naming scheme ("eth0", "eth1", "wlan0", ...), but should fix real problems.
Starting with v197 systemd/udev will automatically assign predictable, stable network interface names for all local Ethernet, WLAN and WWAN interfaces. This is a departure from the traditional interface naming scheme (`eth0`, `eth1`, `wlan0`, ...), but should fix real problems.
## Why?
The classic naming scheme for network interfaces applied by the kernel is to simply assign names beginning with "eth0", "eth1", ... to all interfaces as they are probed by the drivers. As the driver probing is generally not predictable for modern technology this means that as soon as multiple network interfaces are available the assignment of the names "eth0", "eth1" and so on is generally not fixed anymore and it might very well happen that "eth0" on one boot ends up being "eth1" on the next. This can have serious security implications, for example in firewall rules which are coded for certain naming schemes, and which are hence very sensitive to unpredictable changing names.
The classic naming scheme for network interfaces applied by the kernel is to simply assign names beginning with `eth0`, `eth1`, ... to all interfaces as they are probed by the drivers. As the driver probing is generally not predictable for modern technology this means that as soon as multiple network interfaces are available the assignment of the names `eth0`, `eth1` and so on is generally not fixed anymore and it might very well happen that `eth0` on one boot ends up being `eth1` on the next. This can have serious security implications, for example in firewall rules which are coded for certain naming schemes, and which are hence very sensitive to unpredictable changing names.
To fix this problem multiple solutions have been proposed and implemented. For a longer time udev shipped support for assigning permanent "ethX" names to certain interfaces based on their MAC addresses. This turned out to have a multitude of problems, among them: this required a writable root directory which is generally not available; the statelessness of the system is lost as booting an OS image on a system will result in changed configuration of the image; on many systems MAC addresses are not actually fixed, such as on a lot of embedded hardware and particularly on all kinds of virtualization solutions. The biggest of all however is that the userspace components trying to assign the interface name raced against the kernel assigning new names from the same "ethX" namespace, a race condition with all kinds of weird effects, among them that assignment of names sometimes failed. As a result support for this has been removed from systemd/udev a while back.
To fix this problem multiple solutions have been proposed and implemented. For a longer time udev shipped support for assigning permanent `ethX` names to certain interfaces based on their MAC addresses. This turned out to have a multitude of problems, among them: this required a writable root directory which is generally not available; the statelessness of the system is lost as booting an OS image on a system will result in changed configuration of the image; on many systems MAC addresses are not actually fixed, such as on a lot of embedded hardware and particularly on all kinds of virtualization solutions. The biggest of all however is that the userspace components trying to assign the interface name raced against the kernel assigning new names from the same `ethX` namespace, a race condition with all kinds of weird effects, among them that assignment of names sometimes failed. As a result support for this has been removed from systemd/udev a while back.
Another solution that has been implemented is "biosdevname" which tries to find fixed slot topology information in certain firmware interfaces and uses them to assign fixed names to interfaces which incorporate their physical location on the mainboard. In a way this naming scheme is similar to what is already done natively in udev for various device nodes via /dev/*/by-path/ symlinks. In many cases, biosdevname departs from the low-level kernel device identification schemes that udev generally uses for these symlinks, and instead invents its own enumeration schemes.
Another solution that has been implemented is `biosdevname` which tries to find fixed slot topology information in certain firmware interfaces and uses them to assign fixed names to interfaces which incorporate their physical location on the mainboard. In a way this naming scheme is similar to what is already done natively in udev for various device nodes via `/dev/*/by-path/` symlinks. In many cases, biosdevname departs from the low-level kernel device identification schemes that udev generally uses for these symlinks, and instead invents its own enumeration schemes.
Finally, many distributions support renaming interfaces to user-chosen names (think: "internet0", "dmz0", ...) keyed off their MAC addresses or physical locations as part of their networking scripts. This is a very good choice but does have the problem that it implies that the user is willing and capable of choosing and assigning these names.
Finally, many distributions support renaming interfaces to user-chosen names (think: `internet0`, `dmz0`, ...) keyed off their MAC addresses or physical locations as part of their networking scripts. This is a very good choice but does have the problem that it implies that the user is willing and capable of choosing and assigning these names.
We believe it is a good default choice to generalize the scheme pioneered by "biosdevname". Assigning fixed names based on firmware/topology/location information has the big advantage that the names are fully automatic, fully predictable, that they stay fixed even if hardware is added or removed (i.e. no reenumeration takes place) and that broken hardware can be replaced seamlessly. That said, they admittedly are sometimes harder to read than the "eth0" or "wlan0" everybody is used to. Example: "enp5s0"
We believe it is a good default choice to generalize the scheme pioneered by `biosdevname`. Assigning fixed names based on firmware/topology/location information has the big advantage that the names are fully automatic, fully predictable, that they stay fixed even if hardware is added or removed (i.e. no reenumeration takes place) and that broken hardware can be replaced seamlessly. That said, they admittedly are sometimes harder to read than the `eth0` or `wlan0` everybody is used to. Example: `enp5s0`
## What precisely has changed in v197?
@ -47,14 +46,14 @@ With this new scheme you now get:
* Stable interface names even if you have to replace broken ethernet cards by new ones
* The names are automatically determined without user configuration, they just work
* The interface names are fully predictable, i.e. just by looking at lspci you can figure out what the interface is going to be called
* Fully stateless operation, changing the hardware configuration will not result in changes in /etc
* Fully stateless operation, changing the hardware configuration will not result in changes in `/etc`
* Compatibility with read-only root
* The network interface naming now follows more closely the scheme used for aliasing block device nodes and other device nodes in /dev via symlinks
* The network interface naming now follows more closely the scheme used for aliasing block device nodes and other device nodes in `/dev` via symlinks
* Applicability to both x86 and non-x86 machines
* The same on all distributions that adopted systemd/udev
* It's easy to opt out of the scheme (see below)
Does this have any drawbacks? Yes, it does. Previously it was practically guaranteed that hosts equipped with a single ethernet card only had a single "eth0" interface. With this new scheme in place, an administrator now has to check first what the local interface name is before he can invoke commands on it where previously he had a good chance that "eth0" was the right name.
Does this have any drawbacks? Yes, it does. Previously it was practically guaranteed that hosts equipped with a single ethernet card only had a single `eth0` interface. With this new scheme in place, an administrator now has to check first what the local interface name is before he can invoke commands on it where previously he had a good chance that `eth0` was the right name.
## I don't like this, how do I disable this?
@ -62,9 +61,9 @@ Does this have any drawbacks? Yes, it does. Previously it was practically guaran
You basically have three options:
1. You disable the assignment of fixed names, so that the unpredictable kernel names are used again. For this, simply mask udev's .link file for the default policy: `ln -s /dev/null /etc/systemd/network/99-default.link`
1. You create your own manual naming scheme, for example by naming your interfaces "internet0", "dmz0" or "lan0". For that create your own .link files in /etc/systemd/network/, that choose an explicit name or a better naming scheme for one, some, or all of your interfaces. See [[systemd.link(5)|http://www.freedesktop.org/software/systemd/man/systemd.link.html]] for more information.
1. You pass the net.ifnames=0 on the kernel command line
1. You create your own manual naming scheme, for example by naming your interfaces `internet0`, `dmz0` or `lan0`. For that create your own `.link` files in `/etc/systemd/network/`, that choose an explicit name or a better naming scheme for one, some, or all of your interfaces. See [systemd.link(5)](http://www.freedesktop.org/software/systemd/man/systemd.link.html) for more information.
1. You pass the `net.ifnames=0` on the kernel command line
## How does the new naming scheme look like, precisely?
That's documented in detail in a comment block [[the sources of the net_id built-in|https://github.com/systemd/systemd/blob/master/src/udev/udev-builtin-net_id.c#L20]]. Please refer to this in case you are wondering how to decode the new interface names.
That's documented in detail the [systemd.net-naming-scheme(7)](https://www.freedesktop.org/software/systemd/man/systemd.net-naming-scheme.html) man page. Please refer to this in case you are wondering how to decode the new interface names.

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man/systemd.net-naming-scheme.xml
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@ -262,7 +262,7 @@
<varlistentry>
<term><constant>v238</constant></term>
<listitem><para>This is the naming naming that was implemented in systemd 238.</para></listitem>
<listitem><para>This is the naming scheme that was implemented in systemd 238.</para></listitem>
</varlistentry>
<varlistentry>
@ -428,8 +428,7 @@ ID_NET_NAME_PATH=encf5f0</programlisting>
<para>
<citerefentry><refentrytitle>udev</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
<citerefentry><refentrytitle>udevadm</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
<ulink url="https://www.freedesktop.org/wiki/Software/systemd/PredictableNetworkInterfaceNames">the
original page describing stable interface names</ulink>
<ulink url="https://systemd.io/PREDICTABLE_INTERFACE_NAMES">Predictable Network Interface Names</ulink>
</para>
</refsect1>

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src/udev/udev-builtin-net_id.c
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@ -7,7 +7,7 @@
* - physical/geographical location of the hardware
* - the interface's MAC address
*
* http://www.freedesktop.org/wiki/Software/systemd/PredictableNetworkInterfaceNames
* https://systemd.io/PREDICTABLE_INTERFACE_NAMES
*
* When the code here is changed, man/systemd.net-naming-scheme.xml must be updated too.
*/