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| 1 | + |
| 2 | +1. Introduction |
| 3 | + |
| 4 | +Linux distinguishes between administrative and operational state of an |
| 5 | +interface. Admininstrative state is the result of "ip link set dev |
| 6 | +<dev> up or down" and reflects whether the administrator wants to use |
| 7 | +the device for traffic. |
| 8 | + |
| 9 | +However, an interface is not usable just because the admin enabled it |
| 10 | +- ethernet requires to be plugged into the switch and, depending on |
| 11 | +a site's networking policy and configuration, an 802.1X authentication |
| 12 | +to be performed before user data can be transferred. Operational state |
| 13 | +shows the ability of an interface to transmit this user data. |
| 14 | + |
| 15 | +Thanks to 802.1X, userspace must be granted the possibility to |
| 16 | +influence operational state. To accommodate this, operational state is |
| 17 | +split into two parts: Two flags that can be set by the driver only, and |
| 18 | +a RFC2863 compatible state that is derived from these flags, a policy, |
| 19 | +and changeable from userspace under certain rules. |
| 20 | + |
| 21 | + |
| 22 | +2. Querying from userspace |
| 23 | + |
| 24 | +Both admin and operational state can be queried via the netlink |
| 25 | +operation RTM_GETLINK. It is also possible to subscribe to RTMGRP_LINK |
| 26 | +to be notified of updates. This is important for setting from userspace. |
| 27 | + |
| 28 | +These values contain interface state: |
| 29 | + |
| 30 | +ifinfomsg::if_flags & IFF_UP: |
| 31 | + Interface is admin up |
| 32 | +ifinfomsg::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. |
| 36 | +ifinfomsg::if_flags & IFF_LOWER_UP: |
| 37 | + Driver has signaled netif_carrier_on() |
| 38 | +ifinfomsg::if_flags & IFF_DORMANT: |
| 39 | + Driver has signaled netif_dormant_on() |
| 40 | + |
| 41 | +These interface flags can also be queried without netlink using the |
| 42 | +SIOCGIFFLAGS ioctl. |
| 43 | + |
| 44 | +TLV IFLA_OPERSTATE |
| 45 | + |
| 46 | +contains RFC2863 state of the interface in numeric representation: |
| 47 | + |
| 48 | +IF_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. |
| 52 | +IF_OPER_NOTPRESENT (1): |
| 53 | + Unused in current kernel (notpresent interfaces normally disappear), |
| 54 | + just a numerical placeholder. |
| 55 | +IF_OPER_DOWN (2): |
| 56 | + Interface is unable to transfer data on L1, f.e. ethernet is not |
| 57 | + plugged or interface is ADMIN down. |
| 58 | +IF_OPER_LOWERLAYERDOWN (3): |
| 59 | + Interfaces stacked on an interface that is IF_OPER_DOWN show this |
| 60 | + state (f.e. VLAN). |
| 61 | +IF_OPER_TESTING (4): |
| 62 | + Unused in current kernel. |
| 63 | +IF_OPER_DORMANT (5): |
| 64 | + Interface is L1 up, but waiting for an external event, f.e. for a |
| 65 | + protocol to establish. (802.1X) |
| 66 | +IF_OPER_UP (6): |
| 67 | + Interface is operational up and can be used. |
| 68 | + |
| 69 | +This TLV can also be queried via sysfs. |
| 70 | + |
| 71 | +TLV IFLA_LINKMODE |
| 72 | + |
| 73 | +contains link policy. This is needed for userspace interaction |
| 74 | +described below. |
| 75 | + |
| 76 | +This TLV can also be queried via sysfs. |
| 77 | + |
| 78 | + |
| 79 | +3. Kernel driver API |
| 80 | + |
| 81 | +Kernel drivers have access to two flags that map to IFF_LOWER_UP and |
| 82 | +IFF_DORMANT. These flags can be set from everywhere, even from |
| 83 | +interrupts. It is guaranteed that only the driver has write access, |
| 84 | +however, if different layers of the driver manipulate the same flag, |
| 85 | +the driver has to provide the synchronisation needed. |
| 86 | + |
| 87 | +__LINK_STATE_NOCARRIER, maps to !IFF_LOWER_UP: |
| 88 | + |
| 89 | +The driver uses netif_carrier_on() to clear and netif_carrier_off() to |
| 90 | +set this flag. On netif_carrier_off(), the scheduler stops sending |
| 91 | +packets. The name 'carrier' and the inversion are historical, think of |
| 92 | +it as lower layer. |
| 93 | + |
| 94 | +netif_carrier_ok() can be used to query that bit. |
| 95 | + |
| 96 | +__LINK_STATE_DORMANT, maps to IFF_DORMANT: |
| 97 | + |
| 98 | +Set by the driver to express that the device cannot yet be used |
| 99 | +because some driver controlled protocol establishment has to |
| 100 | +complete. Corresponding functions are netif_dormant_on() to set the |
| 101 | +flag, netif_dormant_off() to clear it and netif_dormant() to query. |
| 102 | + |
| 103 | +On device allocation, networking core sets the flags equivalent to |
| 104 | +netif_carrier_ok() and !netif_dormant(). |
| 105 | + |
| 106 | + |
| 107 | +Whenever the driver CHANGES one of these flags, a workqueue event is |
| 108 | +scheduled to translate the flag combination to IFLA_OPERSTATE as |
| 109 | +follows: |
| 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 | + |
| 116 | +netif_carrier_ok() && netif_dormant(): |
| 117 | + IF_OPER_DORMANT |
| 118 | + |
| 119 | +netif_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 | + |
| 125 | +4. Setting from userspace |
| 126 | + |
| 127 | +Applications have to use the netlink interface to influence the |
| 128 | +RFC2863 operational state of an interface. Setting IFLA_LINKMODE to 1 |
| 129 | +via RTM_SETLINK instructs the kernel that an interface should go to |
| 130 | +IF_OPER_DORMANT instead of IF_OPER_UP when the combination |
| 131 | +netif_carrier_ok() && !netif_dormant() is set by the |
| 132 | +driver. Afterwards, the userspace application can set IFLA_OPERSTATE |
| 133 | +to IF_OPER_DORMANT or IF_OPER_UP as long as the driver does not set |
| 134 | +netif_carrier_off() or netif_dormant_on(). Changes made by userspace |
| 135 | +are multicasted on the netlink group RTMGRP_LINK. |
| 136 | + |
| 137 | +So 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 | + |
| 152 | +if supplicant goes down, bring back IFLA_LINKMODE to 0 and |
| 153 | +IFLA_OPERSTATE to a sane value. |
| 154 | + |
| 155 | +A routing daemon or dhcp client just needs to care for IFF_RUNNING or |
| 156 | +waiting for operstate to go IF_OPER_UP/IF_OPER_UNKNOWN before |
| 157 | +considering the interface / querying a DHCP address. |
| 158 | + |
| 159 | + |
| 160 | +For technical questions and/or comments please e-mail to Stefan Rompf |
| 161 | +(stefan at loplof.de). |
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