Linux distinguishes between administrative and operational state of an interface. Administrative state is the result of “ip link set dev <dev> up or down” and reflects whether the administrator wants to use the device for traffic.
However, an interface is not usable just because the admin enabled it - ethernet requires to be plugged into the switch and, depending on a site’s networking policy and configuration, an 802.1X authentication to be performed before user data can be transferred. Operational state shows the ability of an interface to transmit this user data.
Thanks to 802.1X, userspace must be granted the possibility to influence operational state. To accommodate this, operational state is split into two parts: Two flags that can be set by the driver only, and a RFC2863 compatible state that is derived from these flags, a policy, and changeable from userspace under certain rules.
2. Querying from userspace¶
Both admin and operational state can be queried via the netlink operation RTM_GETLINK. It is also possible to subscribe to RTNLGRP_LINK to be notified of updates while the interface is admin up. This is important for setting from userspace.
These values contain interface state:
- ifinfomsg::if_flags & IFF_UP:
- Interface is admin up
- ifinfomsg::if_flags & IFF_RUNNING:
- Interface is in RFC2863 operational state UP or UNKNOWN. This is for backward compatibility, routing daemons, dhcp clients can use this flag to determine whether they should use the interface.
- ifinfomsg::if_flags & IFF_LOWER_UP:
- Driver has signaled
- ifinfomsg::if_flags & IFF_DORMANT:
- Driver has signaled
contains RFC2863 state of the interface in numeric representation:
- IF_OPER_UNKNOWN (0):
- Interface is in unknown state, neither driver nor userspace has set operational state. Interface must be considered for user data as setting operational state has not been implemented in every driver.
- IF_OPER_NOTPRESENT (1):
- Unused in current kernel (notpresent interfaces normally disappear), just a numerical placeholder.
- IF_OPER_DOWN (2):
- Interface is unable to transfer data on L1, f.e. ethernet is not plugged or interface is ADMIN down.
- IF_OPER_LOWERLAYERDOWN (3):
- Interfaces stacked on an interface that is IF_OPER_DOWN show this state (f.e. VLAN).
- IF_OPER_TESTING (4):
- Interface is in testing mode, for example executing driver self-tests or media (cable) test. It can’t be used for normal traffic until tests complete.
- IF_OPER_DORMANT (5):
- Interface is L1 up, but waiting for an external event, f.e. for a protocol to establish. (802.1X)
- IF_OPER_UP (6):
- Interface is operational up and can be used.
This TLV can also be queried via sysfs.
3. Kernel driver API¶
Kernel drivers have access to two flags that map to IFF_LOWER_UP and IFF_DORMANT. These flags can be set from everywhere, even from interrupts. It is guaranteed that only the driver has write access, however, if different layers of the driver manipulate the same flag, the driver has to provide the synchronisation needed.
__LINK_STATE_NOCARRIER, maps to !IFF_LOWER_UP:
The driver uses
netif_carrier_on() to clear and
set this flag. On
netif_carrier_off(), the scheduler stops sending
packets. The name ‘carrier’ and the inversion are historical, think of
it as lower layer.
Note that for certain kind of soft-devices, which are not managing any real hardware, it is possible to set this bit from userspace. One should use TLV IFLA_CARRIER to do so.
netif_carrier_ok() can be used to query that bit.
__LINK_STATE_DORMANT, maps to IFF_DORMANT:
Set by the driver to express that the device cannot yet be used
because some driver controlled protocol establishment has to
complete. Corresponding functions are
netif_dormant_on() to set the
netif_dormant_off() to clear it and
netif_dormant() to query.
Whenever the driver CHANGES one of these flags, a workqueue event is scheduled to translate the flag combination to IFLA_OPERSTATE as follows:
- IF_OPER_LOWERLAYERDOWN if the interface is stacked, IF_OPER_DOWN otherwise. Kernel can recognise stacked interfaces because their ifindex != iflink.
- netif_carrier_ok() && netif_dormant():
- netif_carrier_ok() && !netif_dormant():
- IF_OPER_UP if userspace interaction is disabled. Otherwise IF_OPER_DORMANT with the possibility for userspace to initiate the IF_OPER_UP transition afterwards.
4. Setting from userspace¶
Applications have to use the netlink interface to influence the
RFC2863 operational state of an interface. Setting IFLA_LINKMODE to 1
via RTM_SETLINK instructs the kernel that an interface should go to
IF_OPER_DORMANT instead of IF_OPER_UP when the combination
netif_carrier_ok() && !
netif_dormant() is set by the
driver. Afterwards, the userspace application can set IFLA_OPERSTATE
to IF_OPER_DORMANT or IF_OPER_UP as long as the driver does not set
netif_dormant_on(). Changes made by userspace
are multicasted on the netlink group RTNLGRP_LINK.
So basically a 802.1X supplicant interacts with the kernel like this:
- subscribe to RTNLGRP_LINK
- set IFLA_LINKMODE to 1 via RTM_SETLINK
- query RTM_GETLINK once to get initial state
- if initial flags are not (IFF_LOWER_UP && !IFF_DORMANT), wait until netlink multicast signals this state
- do 802.1X, eventually abort if flags go down again
- send RTM_SETLINK to set operstate to IF_OPER_UP if authentication succeeds, IF_OPER_DORMANT otherwise
- see how operstate and IFF_RUNNING is echoed via netlink multicast
- set interface back to IF_OPER_DORMANT if 802.1X reauthentication fails
- restart if kernel changes IFF_LOWER_UP or IFF_DORMANT flag
if supplicant goes down, bring back IFLA_LINKMODE to 0 and IFLA_OPERSTATE to a sane value.
A routing daemon or dhcp client just needs to care for IFF_RUNNING or waiting for operstate to go IF_OPER_UP/IF_OPER_UNKNOWN before considering the interface / querying a DHCP address.
For technical questions and/or comments please e-mail to Stefan Rompf (stefan at loplof.de).