How to know if a network interface is tap, tun, bridge or physical?

Question

As far as i know, there are 4 main types of network interfaces in linux: tun, tap, bridge and physical.

When i'm doing sys admin on machines running KVM, i usually come accross tap, bridge and physical interfaces on the same machine, without being able to tell them apart. I can't see any significant differences in ifconfig results, as in ip results.

How can i know if an interface is tun/tap/bridge/physical ?

note: I don't claim there is no other types of network interfaces in linux, but i know only this 4 ones

Answer 1

I don't think there's an easy way to distinguish them. Poking around in /sys/class/net I found the following distinctions:

• Physical devices have a /sys/class/net/eth0/device symlink
• Bridges have a /sys/class/net/br0/bridge directory
• TUN and TAP devices have a /sys/class/net/tap0/tun_flags file
• Bridges and loopback interfaces have 00:00:00:00:00:00 in /sys/class/net/lo/address

Answer 2

Regarding TUN and TAP devices: it is not enough to make the check above. The reason is that there may be cases when we create a tun device and (by error) call it tap10; or create a tap device and name it tun10. How can I know if it is a tun device or a tap device ? both of course will have "tun_flags" entry. The answer is to run "ethtool -i tunOrTapDeviceName".

In case of a tap device we will get: "bus-info: tap". In case of a tun device we will get: "bus-info: tun".