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.
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.
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.
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”
With systemd 197 we have added native support for a number of different naming policies into systemd/udevd proper and made a scheme similar to biosdevname’s (but generally more powerful, and closer to kernel-internal device identification schemes) the default. The following different naming schemes for network interfaces are now supported by udev natively:
By default, systemd v197 will now name interfaces following policy 1) if that information from the firmware is applicable and available, falling back to 2) if that information from the firmware is applicable and available, falling back to 3) if applicable, falling back to 5) in all other cases. Policy 4) is not used by default, but is available if the user chooses so.
This combined policy is only applied as last resort. That means, if the system has biosdevname installed, it will take precedence. If the user has added udev rules which change the name of the kernel devices these will take precedence too. Also, any distribution specific naming schemes generally take precedence.
With this new scheme you now get:
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.
You basically have three options:
ln -s /dev/null /etc/systemd/network/99-default.link
|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.|
|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.|