VLAN information using NETLINK

Question

How do you get VLAN information like addition and deletion of VLAN sub interface from kernel to userspace using NETLINK socket in C?

I did little study in NETLINK man as suggested in comments. I added and deleted a VLAN sub interface and monitored using netlink socket program. For addition & deletion each receiving 3 messages. Addition sends 3 NEWLINK message and deletion sends 2 NEWLINK & 1 DELLINK message. Why is that so?

For addition of new VLAN interface eth1.75:

RTM_NEWLINK Link eth2.75 Down
RTM_NEWLINK Link eth2 Up
RTM_NEWLINK Link eth2.75 Up

For Deletion of VLAN interface eth2.75:

RTM_NEWLINK Link eth2 Up
RTM_NEWLINK Link eth2.75 Down
RTM_DELLINK eth2.75

Answer 1

While creating a netlink socket, it create 3 devices. This is why you receive 3 events. Here is the 3 devices it creates while add event.

• Network subsystem.
• Sending Queue subsystem.
• Receiving Queue subsystem.

net subsystem is the upper level control device. Other two are queues for handing data. You can verify it as below.

When I do a udevadm monitor --env and create a vlan I get the following events from kernel:

UDEV  [305215.045416] add      /devices/virtual/net/vpn0 (net)
ACTION=add
DEVPATH=/devices/virtual/net/vpn0
ID_MM_CANDIDATE=1
IFINDEX=10
INTERFACE=vpn0
SEQNUM=3665
SUBSYSTEM=net
USEC_INITIALIZED=5215023319

UDEV  [305215.046658] add      /devices/virtual/net/vpn0/queues/rx-0 (queues)
ACTION=add
DEVPATH=/devices/virtual/net/vpn0/queues/rx-0
SEQNUM=3666
SUBSYSTEM=queues
USEC_INITIALIZED=15044665

UDEV  [305215.047628] add      /devices/virtual/net/vpn0/queues/tx-0 (queues)
ACTION=add
DEVPATH=/devices/virtual/net/vpn0/queues/tx-0
SEQNUM=3667
SUBSYSTEM=queues
USEC_INITIALIZED=5215044729