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