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28 .\" From: @(#)route.4 8.6 (Berkeley) 4/19/94
36 .Nd kernel packet forwarding database
44 .Fn socket PF_ROUTE SOCK_RAW "int family"
47 provides some packet routing facilities.
48 The kernel maintains a routing information database, which
49 is used in selecting the appropriate network interface when
52 A user process (or possibly multiple co-operating processes)
53 maintains this database by sending messages over a special kind
55 This supplants fixed size
57 used in earlier releases.
58 Routing table changes may only be carried out by the super user.
60 The operating system may spontaneously emit routing messages in response
61 to external events, such as receipt of a re-direct, or failure to
62 locate a suitable route for a request.
63 The message types are described in greater detail below.
65 Routing database entries come in two flavors: for a specific
66 host, or for all hosts on a generic subnetwork (as specified
67 by a bit mask and value under the mask.
68 The effect of wildcard or default route may be achieved by using
69 a mask of all zeros, and there may be hierarchical routes.
71 When the system is booted and addresses are assigned
72 to the network interfaces, each protocol family
73 installs a routing table entry for each interface when it is ready for traffic.
74 Normally the protocol specifies the route
75 through each interface as a
77 connection to the destination host
79 If the route is direct, the transport layer of
80 a protocol family usually requests the packet be sent to the
81 same host specified in the packet.
82 Otherwise, the interface
83 is requested to address the packet to the gateway listed in the routing entry
84 (i.e., the packet is forwarded).
86 When routing a packet,
87 the kernel will attempt to find
88 the most specific route matching the destination.
89 (If there are two different mask and value-under-the-mask pairs
90 that match, the more specific is the one with more bits in the mask.
91 A route to a host is regarded as being supplied with a mask of
92 as many ones as there are bits in the destination).
93 If no entry is found, the destination is declared to be unreachable,
94 and a routing-miss message is generated if there are any
95 listeners on the routing control socket described below.
97 A wildcard routing entry is specified with a zero
98 destination address value, and a mask of all zeroes.
99 Wildcard routes will be used
100 when the system fails to find other routes matching the
102 The combination of wildcard
103 routes and routing redirects can provide an economical
104 mechanism for routing traffic.
106 One opens the channel for passing routing control messages
107 by using the socket call shown in the synopsis above:
114 routing information for all address families, or can be restricted
115 to a specific address family by specifying which one is desired.
116 There can be more than one routing socket open per system.
118 Messages are formed by a header followed by a small
119 number of sockaddrs (now variable length particularly
122 case), interpreted by position, and delimited
123 by the new length entry in the sockaddr.
124 An example of a message with four addresses might be an
127 Destination, Netmask, Gateway, and Author of the redirect.
128 The interpretation of which address are present is given by a
129 bit mask within the header, and the sequence is least significant
130 to most significant bit within the vector.
132 Any messages sent to the kernel are returned, and copies are sent
133 to all interested listeners.
134 The kernel will provide the process
135 ID for the sender, and the sender may use an additional sequence
136 field to distinguish between outstanding messages.
137 However, message replies may be lost when kernel buffers are exhausted.
139 The kernel may reject certain messages, and will indicate this
143 The routing code returns
146 requested to duplicate an existing entry,
149 requested to delete a non-existent entry,
152 if insufficient resources were available
153 to install a new route.
154 In the current implementation, all routing processes run locally,
157 are available through the normal
159 mechanism, even if the routing reply message is lost.
161 A process may avoid the expense of reading replies to
162 its own messages by issuing a
164 call indicating that the
169 level is to be turned off.
170 A process may ignore all messages from the routing socket
173 system call for further input.
175 If a route is in use when it is deleted,
176 the routing entry will be marked down and removed from the routing table,
177 but the resources associated with it will not
178 be reclaimed until all references to it are released.
179 User processes can obtain information about the routing
180 entry to a specific destination by using a
182 message, or by calling
187 #define RTM_ADD 0x1 /* Add Route */
188 #define RTM_DELETE 0x2 /* Delete Route */
189 #define RTM_CHANGE 0x3 /* Change Metrics, Flags, or Gateway */
190 #define RTM_GET 0x4 /* Report Information */
191 #define RTM_LOSING 0x5 /* Kernel Suspects Partitioning */
192 #define RTM_REDIRECT 0x6 /* Told to use different route */
193 #define RTM_MISS 0x7 /* Lookup failed on this address */
194 #define RTM_LOCK 0x8 /* fix specified metrics */
195 #define RTM_RESOLVE 0xb /* request to resolve dst to LL addr - unused */
196 #define RTM_NEWADDR 0xc /* address being added to iface */
197 #define RTM_DELADDR 0xd /* address being removed from iface */
198 #define RTM_IFINFO 0xe /* iface going up/down etc. */
199 #define RTM_NEWMADDR 0xf /* mcast group membership being added to if */
200 #define RTM_DELMADDR 0x10 /* mcast group membership being deleted */
201 #define RTM_IFANNOUNCE 0x11 /* iface arrival/departure */
202 #define RTM_IEEE80211 0x12 /* IEEE80211 wireless event */
205 A message header consists of one of the following:
208 u_short rtm_msglen; /* to skip over non-understood messages */
209 u_char rtm_version; /* future binary compatibility */
210 u_char rtm_type; /* message type */
211 u_short rtm_index; /* index for associated ifp */
212 int rtm_flags; /* flags, incl. kern & message, e.g. DONE */
213 int rtm_addrs; /* bitmask identifying sockaddrs in msg */
214 pid_t rtm_pid; /* identify sender */
215 int rtm_seq; /* for sender to identify action */
216 int rtm_errno; /* why failed */
217 int rtm_fmask; /* bitmask used in RTM_CHANGE message */
218 u_long rtm_inits; /* which metrics we are initializing */
219 struct rt_metrics rtm_rmx; /* metrics themselves */
223 u_short ifm_msglen; /* to skip over non-understood messages */
224 u_char ifm_version; /* future binary compatibility */
225 u_char ifm_type; /* message type */
226 int ifm_addrs; /* like rtm_addrs */
227 int ifm_flags; /* value of if_flags */
228 u_short ifm_index; /* index for associated ifp */
229 struct if_data ifm_data; /* statistics and other data about if */
233 u_short ifam_msglen; /* to skip over non-understood messages */
234 u_char ifam_version; /* future binary compatibility */
235 u_char ifam_type; /* message type */
236 int ifam_addrs; /* like rtm_addrs */
237 int ifam_flags; /* value of ifa_flags */
238 u_short ifam_index; /* index for associated ifp */
239 int ifam_metric; /* value of ifa_metric */
243 u_short ifmam_msglen; /* to skip over non-understood messages */
244 u_char ifmam_version; /* future binary compatibility */
245 u_char ifmam_type; /* message type */
246 int ifmam_addrs; /* like rtm_addrs */
247 int ifmam_flags; /* value of ifa_flags */
248 u_short ifmam_index; /* index for associated ifp */
251 struct if_announcemsghdr {
252 u_short ifan_msglen; /* to skip over non-understood messages */
253 u_char ifan_version; /* future binary compatibility */
254 u_char ifan_type; /* message type */
255 u_short ifan_index; /* index for associated ifp */
256 char ifan_name[IFNAMSIZ]; /* if name, e.g. "en0" */
257 u_short ifan_what; /* what type of announcement */
280 .Vt if_announcemsghdr
282 and all other messages use the
287 .Dq Li "struct rt_metrics"
288 and the flag bits are as defined in
291 Specifiers for metric values in rmx_locks and rtm_inits are:
293 #define RTV_MTU 0x1 /* init or lock _mtu */
294 #define RTV_HOPCOUNT 0x2 /* init or lock _hopcount */
295 #define RTV_EXPIRE 0x4 /* init or lock _expire */
296 #define RTV_RPIPE 0x8 /* init or lock _recvpipe */
297 #define RTV_SPIPE 0x10 /* init or lock _sendpipe */
298 #define RTV_SSTHRESH 0x20 /* init or lock _ssthresh */
299 #define RTV_RTT 0x40 /* init or lock _rtt */
300 #define RTV_RTTVAR 0x80 /* init or lock _rttvar */
301 #define RTV_WEIGHT 0x100 /* init or lock _weight */
304 Specifiers for which addresses are present in the messages are:
306 #define RTA_DST 0x1 /* destination sockaddr present */
307 #define RTA_GATEWAY 0x2 /* gateway sockaddr present */
308 #define RTA_NETMASK 0x4 /* netmask sockaddr present */
309 #define RTA_GENMASK 0x8 /* cloning mask sockaddr present - unused */
310 #define RTA_IFP 0x10 /* interface name sockaddr present */
311 #define RTA_IFA 0x20 /* interface addr sockaddr present */
312 #define RTA_AUTHOR 0x40 /* sockaddr for author of redirect */
313 #define RTA_BRD 0x80 /* for NEWADDR, broadcast or p-p dest addr */
320 The constants for the
322 field are documented in the manual page for the
328 protocol family first appeared in