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30 .\" @(#)route.4 8.6 (Berkeley) 4/19/94
37 .Nd kernel packet forwarding database
43 .Fn socket PF_ROUTE SOCK_RAW "int family"
46 provides some packet routing facilities.
47 The kernel maintains a routing information database, which
48 is used in selecting the appropriate network interface when
51 A user process (or possibly multiple co-operating processes)
52 maintains this database by sending messages over a special kind
54 This supplants fixed size
56 used in earlier releases.
57 Routing table changes may only be carried out by the super user.
59 The operating system may spontaneously emit routing messages in response
60 to external events, such as receipt of a redirect, or failure to
61 locate a suitable route for a request.
62 The message types are described in greater detail below.
64 Routing database entries come in two flavors: for a specific
65 host, or for all hosts on a generic subnetwork (as specified
66 by a bit mask and value under the mask.
67 The effect of wildcard or default route may be achieved by using
68 a mask of all zeros, and there may be hierarchical routes.
70 When the system is booted and addresses are assigned
71 to the network interfaces, each protocol family
72 installs a routing table entry for each interface when it is ready for traffic.
73 Normally the protocol specifies the route
74 through each interface as a
76 connection to the destination host
77 or network. If the route is direct, the transport layer of
78 a protocol family usually requests the packet be sent to the
79 same host specified in the packet. Otherwise, the interface
80 is requested to address the packet to the gateway listed in the routing entry
81 (i.e. the packet is forwarded).
83 When routing a packet,
84 the kernel will attempt to find
85 the most specific route matching the destination.
86 (If there are two different mask and value-under-the-mask pairs
87 that match, the more specific is the one with more bits in the mask.
88 A route to a host is regarded as being supplied with a mask of
89 as many ones as there are bits in the destination).
90 If no entry is found, the destination is declared to be unreachable,
91 and a routing\-miss message is generated if there are any
92 listeners on the routing control socket described below.
94 A wildcard routing entry is specified with a zero
95 destination address value, and a mask of all zeroes.
96 Wildcard routes will be used
97 when the system fails to find other routes matching the
98 destination. The combination of wildcard
99 routes and routing redirects can provide an economical
100 mechanism for routing traffic.
102 One opens the channel for passing routing control messages
103 by using the socket call shown in the synopsis above:
110 routing information for all address families, or can be restricted
111 to a specific address family by specifying which one is desired.
112 There can be more than one routing socket open per system.
114 Messages are formed by a header followed by a small
115 number of sockaddrs (now variable length particularly
118 case), interpreted by position, and delimited
119 by the new length entry in the sockaddr.
120 An example of a message with four addresses might be an
123 Destination, Netmask, Gateway, and Author of the redirect.
124 The interpretation of which address are present is given by a
125 bit mask within the header, and the sequence is least significant
126 to most significant bit within the vector.
128 Any messages sent to the kernel are returned, and copies are sent
129 to all interested listeners. The kernel will provide the process
130 ID for the sender, and the sender may use an additional sequence
131 field to distinguish between outstanding messages. However,
132 message replies may be lost when kernel buffers are exhausted.
134 The kernel may reject certain messages, and will indicate this
138 The routing code returns
141 requested to duplicate an existing entry,
144 requested to delete a non-existent entry,
147 if insufficient resources were available
148 to install a new route.
149 In the current implementation, all routing processes run locally,
152 are available through the normal
154 mechanism, even if the routing reply message is lost.
156 A process may avoid the expense of reading replies to
157 its own messages by issuing a
159 call indicating that the
164 level is to be turned off.
165 A process may ignore all messages from the routing socket
168 system call for further input.
170 If a route is in use when it is deleted,
171 the routing entry will be marked down and removed from the routing table,
172 but the resources associated with it will not
173 be reclaimed until all references to it are released.
174 User processes can obtain information about the routing
175 entry to a specific destination by using a
180 device, or by calling
185 #define RTM_ADD 0x1 /* Add Route */
186 #define RTM_DELETE 0x2 /* Delete Route */
187 #define RTM_CHANGE 0x3 /* Change Metrics, Flags, or Gateway */
188 #define RTM_GET 0x4 /* Report Information */
189 #define RTM_LOSING 0x5 /* Kernel Suspects Partitioning */
190 #define RTM_REDIRECT 0x6 /* Told to use different route */
191 #define RTM_MISS 0x7 /* Lookup failed on this address */
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_OIFINFO 0xe /* Old (pre-1.5) RTM_IFINFO message */
196 #define RTM_IFINFO 0xf /* iface/link going up/down etc. */
197 #define RTM_IFANNOUNCE 0x10 /* iface arrival/departure */
200 A message header consists of one of the following:
203 u_short rtm_msglen; /* to skip over non-understood messages */
204 u_char rtm_version; /* future binary compatibility */
205 u_char rtm_type; /* message type */
206 u_short rtm_index; /* index for associated ifp */
207 int rtm_flags; /* flags, incl kern \*[Am] message, e.g. DONE */
208 int rtm_addrs; /* bitmask identifying sockaddrs in msg */
209 pid_t rtm_pid; /* identify sender */
210 int rtm_seq; /* for sender to identify action */
211 int rtm_errno; /* why failed */
212 int rtm_use; /* from rtentry */
213 u_long rtm_inits; /* which metrics we are initializing */
214 struct rt_metrics rtm_rmx; /* metrics themselves */
218 u_short ifm_msglen; /* to skip over non-understood messages */
219 u_char ifm_version; /* future binary compatibility */
220 u_char ifm_type; /* message type */
221 int ifm_addrs; /* like rtm_addrs */
222 int ifm_flags; /* value of if_flags */
223 u_short ifm_index; /* index for associated ifp */
224 struct if_data ifm_data; /* statistics and other data about if */
228 u_short ifam_msglen; /* to skip over non-understood messages */
229 u_char ifam_version; /* future binary compatibility */
230 u_char ifam_type; /* message type */
231 int ifam_addrs; /* like rtm_addrs */
232 int ifam_flags; /* value of ifa_flags */
233 u_short ifam_index; /* index for associated ifp */
234 int ifam_metric; /* value of ifa_metric */
237 struct if_announcemsghdr {
238 u_short ifan_msglen; /* to skip over non-understood messages */
239 u_char ifan_version; /* future binary compatibility */
240 u_char ifan_type; /* message type */
241 u_short ifan_index; /* index for associated ifp */
242 char ifan_name[IFNAMSIZ]; /* if name, e.g. "en0" */
243 u_short ifan_what; /* what type of announcement */
261 .Ar if_announcemsghdr
263 and all other messages use the
267 The metrics structure is:
270 u_long rmx_locks; /* Kernel must leave these values alone */
271 u_long rmx_mtu; /* MTU for this path */
272 u_long rmx_hopcount; /* max hops expected */
273 u_long rmx_expire; /* lifetime for route, e.g. redirect */
274 u_long rmx_recvpipe; /* inbound delay-bandwidth product */
275 u_long rmx_sendpipe; /* outbound delay-bandwidth product */
276 u_long rmx_ssthresh; /* outbound gateway buffer limit */
277 u_long rmx_rtt; /* estimated round trip time */
278 u_long rmx_rttvar; /* estimated rtt variance */
279 u_long rmx_pksent; /* packets sent using this route */
283 Flags include the values:
285 #define RTF_UP 0x1 /* route usable */
286 #define RTF_GATEWAY 0x2 /* destination is a gateway */
287 #define RTF_HOST 0x4 /* host entry (net otherwise) */
288 #define RTF_REJECT 0x8 /* host or net unreachable */
289 #define RTF_DYNAMIC 0x10 /* created dynamically (by redirect) */
290 #define RTF_MODIFIED 0x20 /* modified dynamically (by redirect) */
291 #define RTF_DONE 0x40 /* message confirmed */
292 #define RTF_MASK 0x80 /* subnet mask present */
293 #define RTF_CLONING 0x100 /* generate new routes on use */
294 #define RTF_XRESOLVE 0x200 /* external daemon resolves name */
295 #define RTF_LLINFO 0x400 /* generated by ARP or ESIS */
296 #define RTF_STATIC 0x800 /* manually added */
297 #define RTF_BLACKHOLE 0x1000 /* just discard pkts (during updates) */
298 #define RTF_CLONED 0x2000 /* this is a cloned route */
299 #define RTF_PROTO2 0x4000 /* protocol specific routing flag */
300 #define RTF_PROTO1 0x8000 /* protocol specific routing flag */
303 Specifiers for metric values in rmx_locks and rtm_inits are:
305 #define RTV_MTU 0x1 /* init or lock _mtu */
306 #define RTV_HOPCOUNT 0x2 /* init or lock _hopcount */
307 #define RTV_EXPIRE 0x4 /* init or lock _expire */
308 #define RTV_RPIPE 0x8 /* init or lock _recvpipe */
309 #define RTV_SPIPE 0x10 /* init or lock _sendpipe */
310 #define RTV_SSTHRESH 0x20 /* init or lock _ssthresh */
311 #define RTV_RTT 0x40 /* init or lock _rtt */
312 #define RTV_RTTVAR 0x80 /* init or lock _rttvar */
315 Specifiers for which addresses are present in the messages are:
317 #define RTA_DST 0x1 /* destination sockaddr present */
318 #define RTA_GATEWAY 0x2 /* gateway sockaddr present */
319 #define RTA_NETMASK 0x4 /* netmask sockaddr present */
320 #define RTA_GENMASK 0x8 /* cloning mask sockaddr present */
321 #define RTA_IFP 0x10 /* interface name sockaddr present */
322 #define RTA_IFA 0x20 /* interface addr sockaddr present */
323 #define RTA_AUTHOR 0x40 /* sockaddr for author of redirect */
324 #define RTA_BRD 0x80 /* for NEWADDR, broadcast or p-p dest addr */