Linux – How to Find Available Network Interfaces

deviceslinuxnetworkingsystemdudev

This is in regard to linux, but if anyone knows of a general *nix method that would be good.

I booted a system yesterday with an ethernet cable plugged in. "NetworkManager" is not installed, so once it started I went to look for the name of the ethernet interface with ifconfig to start a DHCP client manually, but it did not show anything other than lo.

The NIC was listed via lspci, and the appropriate kernel driver was loaded. The system normally uses wifi, and I could remember the interface name for that was wlan0. When I tried ifconfig wlan0 up, wlan0 appeared. But the only ethernet interface names I could remember were eth[N] and em[N] — neither of which worked.

This document refers to "predictable interface names" but does not do a good job of explaining what they might be in simple terms. It does refer to a piece of source code which implies the name in this case might be deduced from the the PCI bus and slot numbers, which seems like an unnecessarily complicated hassle.

Other searching around led me to believe that this might be determined by systemd in conjunction with udev, but there are almost 100 files in /usr/lib/udev/rules.d and spending an hour trying to determine where (and if) there's a systemd config file for this also seems ridiculous.

It would also be nice to know for certain that they are available, not just how they might be named if they are, so I can rule out hardware problems, etc. Isn't there a simple way to find the names of available network interfaces on linux?

Best Answer

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

Related Solutions

Linux – Why did the interface name of the wireless card change when I added a sound card

Predictable network interface names are not supposed to change when hardware is added or removed. Isn't that the whole point of the naming scheme???

Long story short, this is nothing new; it's expected/intended. Therefore, you don't need to file a bug, unless you want to ask your PC-maker to support Linux better (BIOS) or the hardware manufacturer (drivers). Some options if you'd like to improve the situation for hot-plugging devices and/or go back to old naming scheme:

Disable new naming scheme for network devices with net.ifnames=0 kernel cmdline
Add biosdevname=1 kernel commandline to incorporate BIOS-provided index numbers to names
Create or edit udev rules for custom names or altered naming schemes
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

If you're using systemd and/or udev, the "predictable naming scheme" argument might be different than before. Based on the naming scheme of the WiFi interface, though, I am assuming that you are using a system with systemd.

You can try appending the following boot parameter to the kernel commandline to use the "old" naming convention of network devices. However, I'm not entirely certain what, if any, additional effects this may have other than retaining the naming scheme for network devices.

net.ifnames=0

Adding it to /etc/default/grub can facilitate the persistence and reuse of this parameter; again, assuming you're using grub2:

GRUB_CMDLINE_LINUX="net.ifnames=0"

If udev uses device firmware, location and other options when determining device names, then perhaps the location or something else may have changed internally, depending on how the relevant devices interact with each other. This seems not as relevant here, as the devices are a WiFi adapter and a soundcard. Nevertheless, it may be related to the underlying bus structure; which does seem relevant, as the devices are both connected to PCI slots.

Additional info from FedoraDocs

8.1. Naming Schemes Hierarchy

By default, systemd will name interfaces using the following policy to apply the supported naming schemes:

Scheme 1: Names incorporating Firmware or BIOS provided index numbers for on-board devices (example: eno1), are applied if that information from the firmware or BIOS is applicable and available, else falling back to scheme 2.
Scheme 2: Names incorporating Firmware or BIOS provided PCI Express hotplug slot index numbers (example: ens1) are applied if that information from the firmware or BIOS is applicable and available, else falling back to scheme 3.
Scheme 3: Names incorporating physical location of the connector of the hardware (example: enp2s0), are applied if applicable, else falling directly back to scheme 5 in all other cases.
Scheme 4: Names incorporating interface's MAC address (example: enx78e7d1ea46da), is not used by default, but is available if the user chooses.
Scheme 5: The traditional unpredictable kernel naming scheme, is used if all other methods fail (example: eth0).

This policy, the procedure outlined above, is the default. If the system has biosdevname enabled, it will be used. Note that enabling biosdevname requires passing biosdevname=1 as a command-line parameter except in the case of a Dell system, where biosdevname will be used by default as long as it is installed. If the user has added udev rules which change the name of the kernel devices, those rules will take precedence.

Additional Resources

Debian Networking – Fix ‘Failed to Start Raise Network Interfaces’ Error on Debian 9

Remove the /etc/network/interfaces.d/setup file then edit your /etc/network/interfaces as follows :

auto lo
iface lo inet loopback

Save and reboot

The man interfaces:

INCLUDING OTHER FILES

  Lines  beginning  with  "source" are used to include stanzas from other

   files, so configuration can be split into many files. The word "source"
   is  followed  by the path of file to be sourced. Shell wildcards can be
   used.  (See wordexp(3) for details.)

In your case you are using the /etc/network/interfaces.d/setup to configure the network instead of /etc/network/interfaces

Lines beginning with "allow-" are used to identify interfaces that should be brought up automatically by various subsytems. This may be done using a command such as "ifup --allow=hotplug eth0 eth1", which will only bring up eth0 or eth1 if it is listed in an "allow-hotplug" line. Note that "allow-auto" and "auto" are synonyms. (Interfaces marked "allow-hotplug" are brought up when udev detects them. This can either be during boot if the interface is already present, or at a later time, for example when plugging in a USB network card. Please note that this does not have anything to do with detecting a network cable being plugged in.)