1.\" Copyright (c) 1990, 1991 The Regents of the University of California. 2.\" All rights reserved. 3.\" 4.\" Redistribution and use in source and binary forms, with or without 5.\" modification, are permitted provided that the following conditions 6.\" are met: 7.\" 1. Redistributions of source code must retain the above copyright 8.\" notice, this list of conditions and the following disclaimer. 9.\" 2. Redistributions in binary form must reproduce the above copyright 10.\" notice, this list of conditions and the following disclaimer in the 11.\" documentation and/or other materials provided with the distribution. 12.\" 3. All advertising materials mentioning features or use of this software 13.\" must display the following acknowledgement: 14.\" This product includes software developed by the University of 15.\" California, Berkeley and its contributors. 16.\" 4. Neither the name of the University nor the names of its contributors 17.\" may be used to endorse or promote products derived from this software 18.\" without specific prior written permission. 19.\" 20.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 21.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 22.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 23.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 24.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 25.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 26.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 27.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 28.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 29.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 30.\" SUCH DAMAGE. 31.\" 32.\" @(#)route.4 6.3 (Berkeley) 3/28/91 33.\" 34.Dd March 28, 1991 35.Dt ROUTE 4 36.Os 37.Sh NAME 38.Nm route 39.Nd Kernel Packet Forwarding Database 40.Sh SYNOPSIS 41.Fd #include <sys/socket.h> 42.Fd #include <net/if.h> 43.Fd #include <net/route.h> 44.Ft int 45.Fn socket PF_ROUTE SOCK_RAW "int family" 46.Sh DESCRIPTION 47.Tn UNIX 48provides some packet routing facilities. 49The kernel maintains a routing information database, which 50is used in selecting the appropriate network interface when 51transmitting packets. 52.Pp 53A user process (or possibly multiple co-operating processes) 54maintains this database by sending messages over a special kind 55of socket. 56This supplants fixed size 57.Xr ioctl 2 Ns 's 58used in earlier releases. 59Routing table changes may only be carried out by the super user. 60.Pp 61The operating system may spontaneously emit routing messages in response 62to external events, such as recipt of a re-direct, or failure to 63locate a suitable route for a request. 64The message types are described in greater detail below. 65.Pp 66Routing database entries come in two flavors: for a specific 67host, or for all hosts on a generic subnetwork (as specified 68by a bit mask and value under the mask. 69The effect of wildcard or default route may be achieved by using 70a mask of all zeros, and there may be hierarchical routes. 71.Pp 72When the system is booted and addresses are assigned 73to the network interfaces, each protocol family 74installs a routing table entry for each interface when it is ready for traffic. 75Normally the protocol specifies the route 76through each interface as a 77.Dq direct 78connection to the destination host 79or network. If the route is direct, the transport layer of 80a protocol family usually requests the packet be sent to the 81same host specified in the packet. Otherwise, the interface 82is requested to address the packet to the gateway listed in the routing entry 83(i.e. the packet is forwarded). 84.Pp 85When routing a packet, 86the kernel will first attempt to find a route to the destination host. 87Failing that, a search is made for a route to the network of the destination. 88Finally, any route to a default 89.Pq Dq wildcard 90gateway is chosen. 91If no entry is found, the destination is declared to be unreachable, 92and a routing\-miss message is generated if there are any 93listers on the routing control socket described below. 94.Pp 95A wildcard routing entry is specified with a zero 96destination address value. Wildcard routes are used 97only when the system fails to find a route to the 98destination host and network. The combination of wildcard 99routes and routing redirects can provide an economical 100mechanism for routing traffic. 101.Pp 102One opens the channel for passing routing control messasges 103by using the socket call shown in the synopsis above: 104.Pp 105The 106.Fa family 107paramter may be 108.Dv AF_UNSPEC 109which will provide 110routing information for all address families, or can be restricted 111to a specific address family by specifying which one is desired. 112There can be more than one routing socket open per system. 113.Pp 114Messages are formed by a header followed by a small 115number of sockadders (now variable length particularly 116in the 117.Tn ISO 118case), interpreted by position, and delimited 119by the new length entry in the sockaddr. 120An example of a message with four addresses might be an 121.Tn ISO 122redirect: 123Destination, Netmask, Gateway, and Author of the redirect. 124The interpretation of which address are present is given by a 125bit mask within the header, and the sequence is least significant 126to most significant bit within the vector. 127.Pp 128Any messages sent to the kernel are returned, and copies are sent 129to all interested listeners. The kernel will provide the process 130id. for the sender, and the sender may use an additional sequence 131field to distinguish between outstanding messages. However, 132message replies may be lost when kernel buffers are exhausted. 133.Pp 134The kernel may reject certain messages, and will indicate this 135by filling in the 136.Ar rtm_errno 137field. 138The routing code returns 139.Dv EEXIST 140if 141requested to duplicate an existing entry, 142.Dv ESRCH 143if 144requested to delete a non-existent entry, 145or 146.Dv ENOBUFS 147if insufficient resources were available 148to install a new route. 149In the current implementation, all routing process run locally, 150and the values for 151.Ar rtm_errno 152are available through the normal 153.Em errno 154mechanism, even if the routing reply message is lost. 155.Pp 156A process may avoid the expense of reading replies to 157its own messages by issuing a 158.Xr setsockopt 2 159call indicating that the 160.Dv SO_USELOOPBACK 161option 162at the 163.Dv SOL_SOCKET 164level is to be turned off. 165A process may ignore all messages from the routing socket 166by doing a 167.Xr shutdown 2 168system call for further input. 169.Pp 170If a route is in use when it is deleted, 171the routing entry will be marked down and removed from the routing table, 172but the resources associated with it will not 173be reclaimed until all references to it are released. 174User processes can obtain information about the routing 175entry to a specific destination by using a 176.Dv RTM_GET 177message, 178or by reading the 179.Pa /dev/kmem 180device, or by issuing a 181.Xr getkerninfo 2 182system call. 183.Pp 184Messages include: 185.Bd -literal 186#define RTM_ADD 0x1 /* Add Route */ 187#define RTM_DELETE 0x2 /* Delete Route */ 188#define RTM_CHANGE 0x3 /* Change Metrics, Flags, or Gateway */ 189#define RTM_GET 0x4 /* Report Information */ 190#define RTM_LOOSING 0x5 /* Kernel Suspects Partitioning */ 191#define RTM_REDIRECT 0x6 /* Told to use different route */ 192#define RTM_MISS 0x7 /* Lookup failed on this address */ 193#define RTM_RESOLVE 0xb /* request to resolve dst to LL addr */ 194.Ed 195.Pp 196A message header consists of: 197.Bd -literal 198struct rt_msghdr { 199 u_short rmt_msglen; /* to skip over non-understood messages */ 200 u_char rtm_version; /* future binary compatability */ 201 u_char rtm_type; /* message type */ 202 u_short rmt_index; /* index for associated ifp */ 203 pid_t rmt_pid; /* identify sender */ 204 int rtm_addrs; /* bitmask identifying sockaddrs in msg */ 205 int rtm_seq; /* for sender to identify action */ 206 int rtm_errno; /* why failed */ 207 int rtm_flags; /* flags, incl kern & message, e.g. DONE */ 208 int rtm_use; /* from rtentry */ 209 u_long rtm_inits; /* which values we are initializing */ 210 struct rt_metrics rtm_rmx; /* metrics themselves */ 211}; 212.Ed 213.Pp 214where 215.Bd -literal 216struct rt_metrics { 217 u_long rmx_locks; /* Kernel must leave these values alone */ 218 u_long rmx_mtu; /* MTU for this path */ 219 u_long rmx_hopcount; /* max hops expected */ 220 u_long rmx_expire; /* lifetime for route, e.g. redirect */ 221 u_long rmx_recvpipe; /* inbound delay-bandwith product */ 222 u_long rmx_sendpipe; /* outbound delay-bandwith product */ 223 u_long rmx_ssthresh; /* outbound gateway buffer limit */ 224 u_long rmx_rtt; /* estimated round trip time */ 225 u_long rmx_rttvar; /* estimated rtt variance */ 226}; 227.Ed 228.Pp 229Flags include the values: 230.Bd -literal 231#define RTF_UP 0x1 /* route useable */ 232#define RTF_GATEWAY 0x2 /* destination is a gateway */ 233#define RTF_HOST 0x4 /* host entry (net otherwise) */ 234#define RTF_NORMAL 0x8 /* subnet mask is cannonical */ 235#define RTF_DYNAMIC 0x10 /* created dynamically (by redirect) */ 236#define RTF_MODIFIED 0x20 /* modified dynamically (by redirect) */ 237#define RTF_DONE 0x40 /* message confirmed */ 238#define RTF_MASK 0x80 /* subnet mask present */ 239.Ed 240.Pp 241Specfiers for metric values in rmx_locks and rtm_inits are: 242.Bd -literal 243#define RTV_SSTHRESH 0x1 /* init or lock _ssthresh */ 244#define RTV_RPIPE 0x2 /* init or lock _recvpipe */ 245#define RTV_SPIPE 0x4 /* init or lock _sendpipe */ 246#define RTV_HOPCOUNT 0x8 /* init or lock _hopcount */ 247#define RTV_RTT 0x10 /* init or lock _rtt */ 248#define RTV_RTTVAR 0x20 /* init or lock _rttvar */ 249#define RTV_MTU 0x40 /* init or lock _mtu */ 250.Ed 251.Pp 252Specifiers for which addresses are present in the messages are: 253.Bd -literal 254#define RTA_DST 0x1 /* destination sockaddr present */ 255#define RTA_GATEWAY 0x2 /* gateway sockaddr present */ 256#define RTA_NETMASK 0x4 /* netmask sockaddr present */ 257#define RTA_GENMASK 0x8 /* cloning mask sockaddr present */ 258#define RTA_IFP 0x10 /* interface name sockaddr present */ 259#define RTA_IFA 0x20 /* interface addr sockaddr present */ 260#define RTA_AUTHOR 0x40 /* sockaddr for author of redirect */ 261.Ed 262.Sh HISTORY 263The 264.Nm 265forwarding database 266.Ud 267