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3a01c1ef SR |
1 | |
2 | 1. Introduction | |
3 | ||
4 | Linux distinguishes between administrative and operational state of an | |
3f6dee9b | 5 | interface. Administrative state is the result of "ip link set dev |
3a01c1ef SR |
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). |