Where can I find the definition of "eth0" in my Ubuntu 14.04 OS?

Question

I cannot find the definition of eth0 in either /etc/network/interfaces or /etc/NetworkManager/NetworkManager.conf, but eth0 works perfectly.

So who can tell me in which configuration file I can find the definition of eth0?

Answer 1

The assignments for network devices should occur via what are called the persistent network rules scripts.

You may be able to find this information in /etc/udev/rules.d/70-persistent-net.rules, being sure to look for device names specified via the "NAME=" parameter, along with MAC address, device type, etc.

Answer 2

There is more than one possibility. On my Ubuntu 14.04 machine, the information appears in /etc/iftab:

# This file assigns persistent names to network interfaces.
# See iftab(5) for syntax.

eth0 mac 00:0c:29:3d:c7:72 arp 1

The corresponding manual page may be useful. Although this was a new install, an old (2008) blog hints that udev is the "newer" way:

Apparently, the delightfully simple /etc/iftab is no longer used, replaced with the ugly and fiercely undelightful /etc/udev/rules.d/70-persistent-net.rules. See, you can even tell from the name of the file that you’re not going to like it.

Answer 3

It’s very likely NetworkManager simply uses DHCP by default for interfaces not otherwise configured. I’ll verify this claim later.

Update: After installing Ubuntu, no files were around and DHCP was used. After modifying some settings (set “DHCP client ID” on IPv4 tab), a file appeared in /etc/NetworkManager/system-connections.

Apparently, connection profiles can also appear in ~/.gconf/system/networking/connections, but I don’t know when. Even when I deselect the option to allow others to use a profile, it’s still saved to system-connections.