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28 .\" From: @(#)route.4 8.6 (Berkeley) 4/19/94
29 .\" $FreeBSD: src/share/man/man4/route.4,v 1.9.2.6 2002/03/17 09:12:44 schweikh Exp $
30 .\" $DragonFly: src/share/man/man4/route.4,v 1.3 2004/03/11 12:28:55 hmp Exp $
37 .Nd kernel packet forwarding database
45 .Fn socket PF_ROUTE SOCK_RAW "int family"
48 provides some packet routing facilities.
49 The kernel maintains a routing information database, which
50 is used in selecting the appropriate network interface when
53 A user process (or possibly multiple co-operating processes)
54 maintains this database by sending messages over a special kind
56 This supplants fixed size
58 used in earlier releases.
59 Routing table changes may only be carried out by the super user.
61 The operating system may spontaneously emit routing messages in response
62 to external events, such as receipt of a re-direct, or failure to
63 locate a suitable route for a request.
64 The message types are described in greater detail below.
66 Routing database entries come in two flavors: for a specific
67 host, or for all hosts on a generic subnetwork (as specified
68 by a bit mask and value under the mask.
69 The effect of wildcard or default route may be achieved by using
70 a mask of all zeros, and there may be hierarchical routes.
72 When the system is booted and addresses are assigned
73 to the network interfaces, each protocol family
74 installs a routing table entry for each interface when it is ready for traffic.
75 Normally the protocol specifies the route
76 through each interface as a
78 connection to the destination host
79 or network. 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. Otherwise, the interface
82 is requested to address the packet to the gateway listed in the routing entry
83 (i.e. the packet is forwarded).
85 When routing a packet,
86 the kernel will attempt to find
87 the most specific route matching the destination.
88 (If there are two different mask and value-under-the-mask pairs
89 that match, the more specific is the one with more bits in the mask.
90 A route to a host is regarded as being supplied with a mask of
91 as many ones as there are bits in the destination).
92 If no entry is found, the destination is declared to be unreachable,
93 and a routing\-miss message is generated if there are any
94 listeners on the routing control socket described below.
96 A wildcard routing entry is specified with a zero
97 destination address value, and a mask of all zeroes.
98 Wildcard routes will be used
99 when the system fails to find other routes matching the
100 destination. The combination of wildcard
101 routes and routing redirects can provide an economical
102 mechanism for routing traffic.
104 One opens the channel for passing routing control messages
105 by using the socket call shown in the synopsis above:
112 routing information for all address families, or can be restricted
113 to a specific address family by specifying which one is desired.
114 There can be more than one routing socket open per system.
116 Messages are formed by a header followed by a small
117 number of sockaddrs (now variable length particularly
120 case), interpreted by position, and delimited
121 by the new length entry in the sockaddr.
122 An example of a message with four addresses might be an
125 Destination, Netmask, Gateway, and Author of the redirect.
126 The interpretation of which address are present is given by a
127 bit mask within the header, and the sequence is least significant
128 to most significant bit within the vector.
130 Any messages sent to the kernel are returned, and copies are sent
131 to all interested listeners. The kernel will provide the process
132 ID for the sender, and the sender may use an additional sequence
133 field to distinguish between outstanding messages. However,
134 message replies may be lost when kernel buffers are exhausted.
136 The kernel may reject certain messages, and will indicate this
140 The routing code returns
143 requested to duplicate an existing entry,
146 requested to delete a non-existent entry,
149 if insufficient resources were available
150 to install a new route.
151 In the current implementation, all routing processes run locally,
154 are available through the normal
156 mechanism, even if the routing reply message is lost.
158 A process may avoid the expense of reading replies to
159 its own messages by issuing a
161 call indicating that the
166 level is to be turned off.
167 A process may ignore all messages from the routing socket
170 system call for further input.
172 If a route is in use when it is deleted,
173 the routing entry will be marked down and removed from the routing table,
174 but the resources associated with it will not
175 be reclaimed until all references to it are released.
176 User processes can obtain information about the routing
177 entry to a specific destination by using a
179 message, or by calling
184 #define RTM_ADD 0x1 /* Add Route */
185 #define RTM_DELETE 0x2 /* Delete Route */
186 #define RTM_CHANGE 0x3 /* Change Metrics, Flags, or Gateway */
187 #define RTM_GET 0x4 /* Report Information */
188 #define RTM_LOSING 0x5 /* Kernel Suspects Partitioning */
189 #define RTM_REDIRECT 0x6 /* Told to use different route */
190 #define RTM_MISS 0x7 /* Lookup failed on this address */
191 #define RTM_LOCK 0x8 /* fix specified metrics */
192 #define RTM_RESOLVE 0xb /* request to resolve dst to LL addr */
193 #define RTM_NEWADDR 0xc /* address being added to iface */
194 #define RTM_DELADDR 0xd /* address being removed from iface */
195 #define RTM_IFINFO 0xe /* iface going up/down etc. */
196 #define RTM_NEWMADDR 0xf /* mcast group membership being added to if */
197 #define RTM_DELMADDR 0x10 /* mcast group membership being deleted */
198 #define RTM_IFANNOUNCE 0x11 /* iface arrival/departure */
201 A message header consists of one of the following:
204 u_short rtm_msglen; /* to skip over non-understood messages */
205 u_char rtm_version; /* future binary compatibility */
206 u_char rtm_type; /* message type */
207 u_short rtm_index; /* index for associated ifp */
208 int rtm_flags; /* flags, incl. kern & message, e.g. DONE */
209 int rtm_addrs; /* bitmask identifying sockaddrs in msg */
210 pid_t rtm_pid; /* identify sender */
211 int rtm_seq; /* for sender to identify action */
212 int rtm_errno; /* why failed */
213 int rtm_use; /* from rtentry */
214 u_long rtm_inits; /* which metrics we are initializing */
215 struct rt_metrics rtm_rmx; /* metrics themselves */
219 u_short ifm_msglen; /* to skip over non-understood messages */
220 u_char ifm_version; /* future binary compatibility */
221 u_char ifm_type; /* message type */
222 int ifm_addrs; /* like rtm_addrs */
223 int ifm_flags; /* value of if_flags */
224 u_short ifm_index; /* index for associated ifp */
225 struct if_data ifm_data; /* statistics and other data about if */
229 u_short ifam_msglen; /* to skip over non-understood messages */
230 u_char ifam_version; /* future binary compatibility */
231 u_char ifam_type; /* message type */
232 int ifam_addrs; /* like rtm_addrs */
233 int ifam_flags; /* value of ifa_flags */
234 u_short ifam_index; /* index for associated ifp */
235 int ifam_metric; /* value of ifa_metric */
239 u_short ifmam_msglen; /* to skip over non-understood messages */
240 u_char ifmam_version; /* future binary compatibility */
241 u_char ifmam_type; /* message type */
242 int ifmam_addrs; /* like rtm_addrs */
243 int ifmam_flags; /* value of ifa_flags */
244 u_short ifmam_index; /* index for associated ifp */
247 struct if_announcemsghdr {
248 u_short ifan_msglen; /* to skip over non-understood messages */
249 u_char ifan_version; /* future binary compatibility */
250 u_char ifan_type; /* message type */
251 u_short ifan_index; /* index for associated ifp */
252 char ifan_name[IFNAMSIZ]; /* if name, e.g. "en0" */
253 u_short ifan_what; /* what type of announcement */
276 .Vt if_announcemsghdr
278 and all other messages use the
283 .Dq Li "struct rt_metrics"
284 and the flag bits are as defined in
287 Specifiers for metric values in rmx_locks and rtm_inits are:
289 #define RTV_MTU 0x1 /* init or lock _mtu */
290 #define RTV_HOPCOUNT 0x2 /* init or lock _hopcount */
291 #define RTV_EXPIRE 0x4 /* init or lock _expire */
292 #define RTV_RPIPE 0x8 /* init or lock _recvpipe */
293 #define RTV_SPIPE 0x10 /* init or lock _sendpipe */
294 #define RTV_SSTHRESH 0x20 /* init or lock _ssthresh */
295 #define RTV_RTT 0x40 /* init or lock _rtt */
296 #define RTV_RTTVAR 0x80 /* init or lock _rttvar */
299 Specifiers for which addresses are present in the messages are:
301 #define RTA_DST 0x1 /* destination sockaddr present */
302 #define RTA_GATEWAY 0x2 /* gateway sockaddr present */
303 #define RTA_NETMASK 0x4 /* netmask sockaddr present */
304 #define RTA_GENMASK 0x8 /* cloning mask sockaddr present */
305 #define RTA_IFP 0x10 /* interface name sockaddr present */
306 #define RTA_IFA 0x20 /* interface addr sockaddr present */
307 #define RTA_AUTHOR 0x40 /* sockaddr for author of redirect */
308 #define RTA_BRD 0x80 /* for NEWADDR, broadcast or p-p dest addr */
317 protocol family first appeared in