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yaml --- r: 26651 b: refs/heads/master c: 2bf9d6d h: refs/heads/master i: 26649: 23c5435 26647: c11c344 v: v3
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Linus Torvalds
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| --- | ||
| refs/heads/master: d8a5a8d7cc32e4474326e0ecc1b959063490efc9 | ||
| refs/heads/master: 2bf9d6d0f2dadc2a6c13684719c67dc043b9ce67 |
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| 1. Introduction | ||
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| Linux distinguishes between administrative and operational state of an | ||
| interface. Admininstrative 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. | ||
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| 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. | ||
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| 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. | ||
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| 2. Querying from userspace | ||
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| Both admin and operational state can be queried via the netlink | ||
| operation RTM_GETLINK. It is also possible to subscribe to RTMGRP_LINK | ||
| 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: | ||
| 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() | ||
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| These interface flags can also be queried without netlink using the | ||
| SIOCGIFFLAGS ioctl. | ||
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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): | ||
| 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. | ||
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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 | ||
| 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 | ||
| 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. | ||
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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 | ||
| 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. | ||
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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 | ||
| 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. | ||
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| On device allocation, networking core sets the flags equivalent to | ||
| netif_carrier_ok() and !netif_dormant(). | ||
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| Whenever the driver CHANGES one of these flags, a workqueue event is | ||
| scheduled to translate the flag combination to IFLA_OPERSTATE as | ||
| follows: | ||
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| !netif_carrier_ok(): | ||
| IF_OPER_LOWERLAYERDOWN if the interface is stacked, IF_OPER_DOWN | ||
| otherwise. Kernel can recognise stacked interfaces because their | ||
| ifindex != iflink. | ||
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| netif_carrier_ok() && netif_dormant(): | ||
| IF_OPER_DORMANT | ||
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| 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. | ||
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| 4. Setting from userspace | ||
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| 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 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 | ||
| -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 | ||
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| if supplicant goes down, bring back IFLA_LINKMODE to 0 and | ||
| IFLA_OPERSTATE to a sane value. | ||
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| 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. | ||
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| For technical questions and/or comments please e-mail to Stefan Rompf | ||
| (stefan at loplof.de). |
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