1 21. Introduction 3 4Linux distinguishes between administrative and operational state of an 5interface. Administrative state is the result of "ip link set dev 6<dev> up or down" and reflects whether the administrator wants to use 7the device for traffic. 8 9However, an interface is not usable just because the admin enabled it 10- ethernet requires to be plugged into the switch and, depending on 11a site's networking policy and configuration, an 802.1X authentication 12to be performed before user data can be transferred. Operational state 13shows the ability of an interface to transmit this user data. 14 15Thanks to 802.1X, userspace must be granted the possibility to 16influence operational state. To accommodate this, operational state is 17split into two parts: Two flags that can be set by the driver only, and 18a RFC2863 compatible state that is derived from these flags, a policy, 19and changeable from userspace under certain rules. 20 21 222. Querying from userspace 23 24Both admin and operational state can be queried via the netlink 25operation RTM_GETLINK. It is also possible to subscribe to RTMGRP_LINK 26to be notified of updates. This is important for setting from userspace. 27 28These values contain interface state: 29 30ifinfomsg::if_flags & IFF_UP: 31 Interface is admin up 32ifinfomsg::if_flags & IFF_RUNNING: 33 Interface is in RFC2863 operational state UP or UNKNOWN. This is for 34 backward compatibility, routing daemons, dhcp clients can use this 35 flag to determine whether they should use the interface. 36ifinfomsg::if_flags & IFF_LOWER_UP: 37 Driver has signaled netif_carrier_on() 38ifinfomsg::if_flags & IFF_DORMANT: 39 Driver has signaled netif_dormant_on() 40 41These interface flags can also be queried without netlink using the 42SIOCGIFFLAGS ioctl. 43 44TLV IFLA_OPERSTATE 45 46contains RFC2863 state of the interface in numeric representation: 47 48IF_OPER_UNKNOWN (0): 49 Interface is in unknown state, neither driver nor userspace has set 50 operational state. Interface must be considered for user data as 51 setting operational state has not been implemented in every driver. 52IF_OPER_NOTPRESENT (1): 53 Unused in current kernel (notpresent interfaces normally disappear), 54 just a numerical placeholder. 55IF_OPER_DOWN (2): 56 Interface is unable to transfer data on L1, f.e. ethernet is not 57 plugged or interface is ADMIN down. 58IF_OPER_LOWERLAYERDOWN (3): 59 Interfaces stacked on an interface that is IF_OPER_DOWN show this 60 state (f.e. VLAN). 61IF_OPER_TESTING (4): 62 Unused in current kernel. 63IF_OPER_DORMANT (5): 64 Interface is L1 up, but waiting for an external event, f.e. for a 65 protocol to establish. (802.1X) 66IF_OPER_UP (6): 67 Interface is operational up and can be used. 68 69This TLV can also be queried via sysfs. 70 71TLV IFLA_LINKMODE 72 73contains link policy. This is needed for userspace interaction 74described below. 75 76This TLV can also be queried via sysfs. 77 78 793. Kernel driver API 80 81Kernel drivers have access to two flags that map to IFF_LOWER_UP and 82IFF_DORMANT. These flags can be set from everywhere, even from 83interrupts. It is guaranteed that only the driver has write access, 84however, if different layers of the driver manipulate the same flag, 85the driver has to provide the synchronisation needed. 86 87__LINK_STATE_NOCARRIER, maps to !IFF_LOWER_UP: 88 89The driver uses netif_carrier_on() to clear and netif_carrier_off() to 90set this flag. On netif_carrier_off(), the scheduler stops sending 91packets. The name 'carrier' and the inversion are historical, think of 92it as lower layer. 93 94netif_carrier_ok() can be used to query that bit. 95 96__LINK_STATE_DORMANT, maps to IFF_DORMANT: 97 98Set by the driver to express that the device cannot yet be used 99because some driver controlled protocol establishment has to 100complete. Corresponding functions are netif_dormant_on() to set the 101flag, netif_dormant_off() to clear it and netif_dormant() to query. 102 103On device allocation, networking core sets the flags equivalent to 104netif_carrier_ok() and !netif_dormant(). 105 106 107Whenever the driver CHANGES one of these flags, a workqueue event is 108scheduled to translate the flag combination to IFLA_OPERSTATE as 109follows: 110 111!netif_carrier_ok(): 112 IF_OPER_LOWERLAYERDOWN if the interface is stacked, IF_OPER_DOWN 113 otherwise. Kernel can recognise stacked interfaces because their 114 ifindex != iflink. 115 116netif_carrier_ok() && netif_dormant(): 117 IF_OPER_DORMANT 118 119netif_carrier_ok() && !netif_dormant(): 120 IF_OPER_UP if userspace interaction is disabled. Otherwise 121 IF_OPER_DORMANT with the possibility for userspace to initiate the 122 IF_OPER_UP transition afterwards. 123 124 1254. Setting from userspace 126 127Applications have to use the netlink interface to influence the 128RFC2863 operational state of an interface. Setting IFLA_LINKMODE to 1 129via RTM_SETLINK instructs the kernel that an interface should go to 130IF_OPER_DORMANT instead of IF_OPER_UP when the combination 131netif_carrier_ok() && !netif_dormant() is set by the 132driver. Afterwards, the userspace application can set IFLA_OPERSTATE 133to IF_OPER_DORMANT or IF_OPER_UP as long as the driver does not set 134netif_carrier_off() or netif_dormant_on(). Changes made by userspace 135are multicasted on the netlink group RTMGRP_LINK. 136 137So basically a 802.1X supplicant interacts with the kernel like this: 138 139-subscribe to RTMGRP_LINK 140-set IFLA_LINKMODE to 1 via RTM_SETLINK 141-query RTM_GETLINK once to get initial state 142-if initial flags are not (IFF_LOWER_UP && !IFF_DORMANT), wait until 143 netlink multicast signals this state 144-do 802.1X, eventually abort if flags go down again 145-send RTM_SETLINK to set operstate to IF_OPER_UP if authentication 146 succeeds, IF_OPER_DORMANT otherwise 147-see how operstate and IFF_RUNNING is echoed via netlink multicast 148-set interface back to IF_OPER_DORMANT if 802.1X reauthentication 149 fails 150-restart if kernel changes IFF_LOWER_UP or IFF_DORMANT flag 151 152if supplicant goes down, bring back IFLA_LINKMODE to 0 and 153IFLA_OPERSTATE to a sane value. 154 155A routing daemon or dhcp client just needs to care for IFF_RUNNING or 156waiting for operstate to go IF_OPER_UP/IF_OPER_UNKNOWN before 157considering the interface / querying a DHCP address. 158 159 160For technical questions and/or comments please e-mail to Stefan Rompf 161(stefan at loplof.de). 162