Understanding Linux Network Internals

Enabling and Disabling a Network Device

Once a device has been registered it is available for use, but it will not transmit and receive traffic until it is explicitly enabled by the user (or a user-space application). Requests to enable a device are taken care of by dev_open, defined in net/core/dev.c. Enabling a device consists of the following tasks:

• Call dev->open if it is defined. Not all device drivers initialize this function.

• Set the _ _LINK_STATE_START flag in dev->state to mark the device as up and running.

  

  Figure 8-6. Function netdev_wait_allrefs

• Set the IFF_UP flag in dev->flags to mark the device as up.

• Call dev_activate to initialize the egress queuing discipline used by Traffic Control, and start the watchdog timer.^[*] If there is no user configuration for Traffic Control, assign a default First In, First Out (FIFO) queue.

• Send a NETDEV_UP notification to the netdev_chain notification chain to notify interested kernel components that the device is now enabled.

While a device needs to be explicitly enabled, it can be disabled either explicitly by a user command or implicitly by other events. For example, before a device is unregistered, it is first disabled (see the section "Device Unregistration"). Network devices are disabled with dev_close. Disabling a device consists of the following tasks:

• Send a NETDEV_GOING_DOWN notification to the netdev_chain notification chain to notify ...