mirror of
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
synced 2024-08-26 02:39:48 +00:00
![Jouke Witteveen](/assets/img/avatar_default.png)
Netlink has moved from bitmasks to group numbers long ago. Signed-off-by: Jouke Witteveen <j.witteveen@gmail.com> Signed-off-by: David S. Miller <davem@davemloft.net>
164 lines
6.1 KiB
Text
164 lines
6.1 KiB
Text
|
|
1. Introduction
|
|
|
|
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 netif_carrier_on()
|
|
ifinfomsg::if_flags & IFF_DORMANT:
|
|
Driver has signaled netif_dormant_on()
|
|
|
|
TLV IFLA_OPERSTATE
|
|
|
|
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):
|
|
Unused in current kernel.
|
|
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.
|
|
|
|
TLV IFLA_LINKMODE
|
|
|
|
contains link policy. This is needed for userspace interaction
|
|
described below.
|
|
|
|
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 netif_carrier_off() to
|
|
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 TVL 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
|
|
flag, netif_dormant_off() to clear it and netif_dormant() to query.
|
|
|
|
On device allocation, both flags __LINK_STATE_NOCARRIER and
|
|
__LINK_STATE_DORMANT are cleared, so the effective state is equivalent
|
|
to netif_carrier_ok() and !netif_dormant().
|
|
|
|
|
|
Whenever the driver CHANGES one of these flags, a workqueue event is
|
|
scheduled to translate the flag combination to IFLA_OPERSTATE as
|
|
follows:
|
|
|
|
!netif_carrier_ok():
|
|
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():
|
|
IF_OPER_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_carrier_off() or 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).
|