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32 .\" From: @(#)route.4 8.6 (Berkeley) 4/19/94
40 .Nd kernel packet forwarding database
48 .Fn socket PF_ROUTE SOCK_RAW "int family"
51 provides some packet routing facilities.
52 The kernel maintains a routing information database, which
53 is used in selecting the appropriate network interface when
56 A user process (or possibly multiple co-operating processes)
57 maintains this database by sending messages over a special kind
59 This supplants fixed size
61 used in earlier releases.
62 Routing table changes may only be carried out by the super user.
64 The operating system may spontaneously emit routing messages in response
65 to external events, such as receipt of a re-direct, or failure to
66 locate a suitable route for a request.
67 The message types are described in greater detail below.
69 Routing database entries come in two flavors: for a specific
70 host, or for all hosts on a generic subnetwork (as specified
71 by a bit mask and value under the mask.
72 The effect of wildcard or default route may be achieved by using
73 a mask of all zeros, and there may be hierarchical routes.
75 When the system is booted and addresses are assigned
76 to the network interfaces, each protocol family
77 installs a routing table entry for each interface when it is ready for traffic.
78 Normally the protocol specifies the route
79 through each interface as a
81 connection to the destination host
82 or network. If the route is direct, the transport layer of
83 a protocol family usually requests the packet be sent to the
84 same host specified in the packet. Otherwise, the interface
85 is requested to address the packet to the gateway listed in the routing entry
86 (i.e. the packet is forwarded).
88 When routing a packet,
89 the kernel will attempt to find
90 the most specific route matching the destination.
91 (If there are two different mask and value-under-the-mask pairs
92 that match, the more specific is the one with more bits in the mask.
93 A route to a host is regarded as being supplied with a mask of
94 as many ones as there are bits in the destination).
95 If no entry is found, the destination is declared to be unreachable,
96 and a routing\-miss message is generated if there are any
97 listers on the routing control socket described below.
99 A wildcard routing entry is specified with a zero
100 destination address value, and a mask of all zeroes.
101 Wildcard routes will be used
102 when the system fails to find other routes matching the
103 destination. The combination of wildcard
104 routes and routing redirects can provide an economical
105 mechanism for routing traffic.
107 One opens the channel for passing routing control messages
108 by using the socket call shown in the synopsis above:
115 routing information for all address families, or can be restricted
116 to a specific address family by specifying which one is desired.
117 There can be more than one routing socket open per system.
119 Messages are formed by a header followed by a small
120 number of sockaddrs (now variable length particularly
123 case), interpreted by position, and delimited
124 by the new length entry in the sockaddr.
125 An example of a message with four addresses might be an
128 Destination, Netmask, Gateway, and Author of the redirect.
129 The interpretation of which address are present is given by a
130 bit mask within the header, and the sequence is least significant
131 to most significant bit within the vector.
133 Any messages sent to the kernel are returned, and copies are sent
134 to all interested listeners. 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. However,
137 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 */
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 */
203 A message header consists of one of the following:
206 u_short rtm_msglen; /* to skip over non-understood messages */
207 u_char rtm_version; /* future binary compatibility */
208 u_char rtm_type; /* message type */
209 u_short rtm_index; /* index for associated ifp */
210 int rtm_flags; /* flags, incl. kern & message, e.g. DONE */
211 int rtm_addrs; /* bitmask identifying sockaddrs in msg */
212 pid_t rtm_pid; /* identify sender */
213 int rtm_seq; /* for sender to identify action */
214 int rtm_errno; /* why failed */
215 int rtm_use; /* from rtentry */
216 u_long rtm_inits; /* which metrics we are initializing */
217 struct rt_metrics rtm_rmx; /* metrics themselves */
221 u_short ifm_msglen; /* to skip over non-understood messages */
222 u_char ifm_version; /* future binary compatibility */
223 u_char ifm_type; /* message type */
224 int ifm_addrs; /* like rtm_addrs */
225 int ifm_flags; /* value of if_flags */
226 u_short ifm_index; /* index for associated ifp */
227 struct if_data ifm_data; /* statistics and other data about if */
231 u_short ifam_msglen; /* to skip over non-understood messages */
232 u_char ifam_version; /* future binary compatibility */
233 u_char ifam_type; /* message type */
234 int ifam_addrs; /* like rtm_addrs */
235 int ifam_flags; /* value of ifa_flags */
236 u_short ifam_index; /* index for associated ifp */
237 int ifam_metric; /* value of ifa_metric */
241 u_short ifmam_msglen; /* to skip over non-understood messages */
242 u_char ifmam_version; /* future binary compatibility */
243 u_char ifmam_type; /* message type */
244 int ifmam_addrs; /* like rtm_addrs */
245 int ifmam_flags; /* value of ifa_flags */
246 u_short ifmam_index; /* index for associated ifp */
266 and all other messages use the
271 .Dq Li "struct rt_metrics"
272 and the flag bits are as defined in
275 Specifiers for metric values in rmx_locks and rtm_inits are:
277 #define RTV_MTU 0x1 /* init or lock _mtu */
278 #define RTV_HOPCOUNT 0x2 /* init or lock _hopcount */
279 #define RTV_EXPIRE 0x4 /* init or lock _expire */
280 #define RTV_RPIPE 0x8 /* init or lock _recvpipe */
281 #define RTV_SPIPE 0x10 /* init or lock _sendpipe */
282 #define RTV_SSTHRESH 0x20 /* init or lock _ssthresh */
283 #define RTV_RTT 0x40 /* init or lock _rtt */
284 #define RTV_RTTVAR 0x80 /* init or lock _rttvar */
287 Specifiers for which addresses are present in the messages are:
289 #define RTA_DST 0x1 /* destination sockaddr present */
290 #define RTA_GATEWAY 0x2 /* gateway sockaddr present */
291 #define RTA_NETMASK 0x4 /* netmask sockaddr present */
292 #define RTA_GENMASK 0x8 /* cloning mask sockaddr present */
293 #define RTA_IFP 0x10 /* interface name sockaddr present */
294 #define RTA_IFA 0x20 /* interface addr sockaddr present */
295 #define RTA_AUTHOR 0x40 /* sockaddr for author of redirect */
296 #define RTA_BRD 0x80 /* for NEWADDR, broadcast or p-p dest addr */
305 protocol family first appeared in