How to determine the logical type of a linux network device

Question

with logical I mean everything legal in the command ip link as in, for instance:

ip link add link dum0 name dum0.200 type vlan protocol 802.1Q id 200

where the logical type would be "vlan". All valid types are, to quote the man page:

vlan | veth | vcan | dummy | ifb | macvlan | macvtap | can | bridge | ipoib | ip6tnl | ipip | sit | vxlan |gre | gretap | ip6gre | ip6gretap | vti

Note that this clearly is not the physical device type (like ethernet, wifi, ppp etc.) as asked in this question, which does contain a gem of a reference to the physical type which led me to test for it :

find /sys/class/net ! -type d | xargs --max-args=1 realpath | 
  while read d; do 
    b=$(basename $d) ; n=$(find $d -name type) ; echo -n $b' ' ; cat $n;
 done
dum0.200 1
dum0.201 1
dum1.300 1
dum1.301 1
dummy0 1
ens36 1
ens33 1
lo 772
dum0 1
dum1 1
wlan0 1

But which apparently finds both dummy, vlan and wlan devices to be of type ARPHRD_ETHER.

Does somebody know more? Thanks in advance.

Answer 1

A simpler solution:

ip -details link show

For virtual devices, device type is shown on the third line.

Answer 2

There is a way looping over all available types, and showing all interfaces per type (using ip link show type <type>). From this, one can collect the interfaces for all types, and then parse for the interface one want's to know about. It's not elegant, but works:

Using bash:

#!/bin/bash

# Arguments: $1: Interface ('grep'-regexp).

# Static list of types (from `ip link help`). NOTE: On my machine, not all types are listed there, e.g. the type `tun`. And the list of types may change over time. So do not ultimately rely on this list here!:
TYPES=(bareudp bond bond_slave bridge can dummy erspan geneve gre gretap hsr ifb ip6erspan ip6gre ip6gretap ip6tnl ipip ipoib ipvlan ipvtap lowpan macsec macvlan macvtap netdevsim nlmon rmnet sit tap tun vcan veth vlan vrf vti vxcan vxlan xfrm)

iface="$1"

for type in "${TYPES[@]}"; do
  ip link show type "${type}" | grep -E '^[0-9]+:' | cut -d ':' -f 2 | sed 's|^[[:space:]]*||' | while read _if; do
    echo "${_if}:${type}"
  done | grep "^${iface}"
done

Save this to a file, make it executable, and run it with your interface you want to know about as argument.

For the example of dum0.200 beeing of type vlan over the link eth0 (created with ip link add link eth0 name dum0.200 type vlan protocol 802.1Q id 200), the output would be dum0.200@eth0:vlan, indicating that it is of type vlan. Note that the @eth0 comes from ip link show and could be parsed away if one wants to.

Since the argument to this script is interpreted as a grep-regexp, specifying nothing lists all which ip link show type <type> outputs, or specifiying just a prefix lists some, etc.

Note: (added 2021-05-12): "Normal" ethernet devices and also the loopback device seem to not have any type. So the script will not list them. If it is desired to also list devices without a type, it needs to be extended or re-written.