1.\" Copyright (c) 1990, 1991, 1993 2.\" The Regents of the University of California. All rights reserved. 3.\" 4.\" %sccs.include.redist.man% 5.\" 6.\" @(#)route.4 8.1 (Berkeley) 06/09/93 7.\" 8.Dd 9.Dt ROUTE 4 10.Os 11.Sh NAME 12.Nm route 13.Nd kernel packet forwarding database 14.Sh SYNOPSIS 15.Fd #include <sys/socket.h> 16.Fd #include <net/if.h> 17.Fd #include <net/route.h> 18.Ft int 19.Fn socket PF_ROUTE SOCK_RAW "int family" 20.Sh DESCRIPTION 21.Tn UNIX 22provides some packet routing facilities. 23The kernel maintains a routing information database, which 24is used in selecting the appropriate network interface when 25transmitting packets. 26.Pp 27A user process (or possibly multiple co-operating processes) 28maintains this database by sending messages over a special kind 29of socket. 30This supplants fixed size 31.Xr ioctl 2 Ns 's 32used in earlier releases. 33Routing table changes may only be carried out by the super user. 34.Pp 35The operating system may spontaneously emit routing messages in response 36to external events, such as recipt of a re-direct, or failure to 37locate a suitable route for a request. 38The message types are described in greater detail below. 39.Pp 40Routing database entries come in two flavors: for a specific 41host, or for all hosts on a generic subnetwork (as specified 42by a bit mask and value under the mask. 43The effect of wildcard or default route may be achieved by using 44a mask of all zeros, and there may be hierarchical routes. 45.Pp 46When the system is booted and addresses are assigned 47to the network interfaces, each protocol family 48installs a routing table entry for each interface when it is ready for traffic. 49Normally the protocol specifies the route 50through each interface as a 51.Dq direct 52connection to the destination host 53or network. If the route is direct, the transport layer of 54a protocol family usually requests the packet be sent to the 55same host specified in the packet. Otherwise, the interface 56is requested to address the packet to the gateway listed in the routing entry 57(i.e. the packet is forwarded). 58.Pp 59When routing a packet, 60the kernel will first attempt to find a route to the destination host. 61Failing that, a search is made for a route to the network of the destination. 62Finally, any route to a default 63.Pq Dq wildcard 64gateway is chosen. 65If no entry is found, the destination is declared to be unreachable, 66and a routing\-miss message is generated if there are any 67listers on the routing control socket described below. 68.Pp 69A wildcard routing entry is specified with a zero 70destination address value. Wildcard routes are used 71only when the system fails to find a route to the 72destination host and network. The combination of wildcard 73routes and routing redirects can provide an economical 74mechanism for routing traffic. 75.Pp 76One opens the channel for passing routing control messasges 77by using the socket call shown in the synopsis above: 78.Pp 79The 80.Fa family 81paramter may be 82.Dv AF_UNSPEC 83which will provide 84routing information for all address families, or can be restricted 85to a specific address family by specifying which one is desired. 86There can be more than one routing socket open per system. 87.Pp 88Messages are formed by a header followed by a small 89number of sockadders (now variable length particularly 90in the 91.Tn ISO 92case), interpreted by position, and delimited 93by the new length entry in the sockaddr. 94An example of a message with four addresses might be an 95.Tn ISO 96redirect: 97Destination, Netmask, Gateway, and Author of the redirect. 98The interpretation of which address are present is given by a 99bit mask within the header, and the sequence is least significant 100to most significant bit within the vector. 101.Pp 102Any messages sent to the kernel are returned, and copies are sent 103to all interested listeners. The kernel will provide the process 104id. for the sender, and the sender may use an additional sequence 105field to distinguish between outstanding messages. However, 106message replies may be lost when kernel buffers are exhausted. 107.Pp 108The kernel may reject certain messages, and will indicate this 109by filling in the 110.Ar rtm_errno 111field. 112The routing code returns 113.Dv EEXIST 114if 115requested to duplicate an existing entry, 116.Dv ESRCH 117if 118requested to delete a non-existent entry, 119or 120.Dv ENOBUFS 121if insufficient resources were available 122to install a new route. 123In the current implementation, all routing process run locally, 124and the values for 125.Ar rtm_errno 126are available through the normal 127.Em errno 128mechanism, even if the routing reply message is lost. 129.Pp 130A process may avoid the expense of reading replies to 131its own messages by issuing a 132.Xr setsockopt 2 133call indicating that the 134.Dv SO_USELOOPBACK 135option 136at the 137.Dv SOL_SOCKET 138level is to be turned off. 139A process may ignore all messages from the routing socket 140by doing a 141.Xr shutdown 2 142system call for further input. 143.Pp 144If a route is in use when it is deleted, 145the routing entry will be marked down and removed from the routing table, 146but the resources associated with it will not 147be reclaimed until all references to it are released. 148User processes can obtain information about the routing 149entry to a specific destination by using a 150.Dv RTM_GET 151message, 152or by reading the 153.Pa /dev/kmem 154device, or by issuing a 155.Xr getkerninfo 2 156system call. 157.Pp 158Messages include: 159.Bd -literal 160#define RTM_ADD 0x1 /* Add Route */ 161#define RTM_DELETE 0x2 /* Delete Route */ 162#define RTM_CHANGE 0x3 /* Change Metrics, Flags, or Gateway */ 163#define RTM_GET 0x4 /* Report Information */ 164#define RTM_LOOSING 0x5 /* Kernel Suspects Partitioning */ 165#define RTM_REDIRECT 0x6 /* Told to use different route */ 166#define RTM_MISS 0x7 /* Lookup failed on this address */ 167#define RTM_RESOLVE 0xb /* request to resolve dst to LL addr */ 168.Ed 169.Pp 170A message header consists of: 171.Bd -literal 172struct rt_msghdr { 173 u_short rmt_msglen; /* to skip over non-understood messages */ 174 u_char rtm_version; /* future binary compatability */ 175 u_char rtm_type; /* message type */ 176 u_short rmt_index; /* index for associated ifp */ 177 pid_t rmt_pid; /* identify sender */ 178 int rtm_addrs; /* bitmask identifying sockaddrs in msg */ 179 int rtm_seq; /* for sender to identify action */ 180 int rtm_errno; /* why failed */ 181 int rtm_flags; /* flags, incl kern & message, e.g. DONE */ 182 int rtm_use; /* from rtentry */ 183 u_long rtm_inits; /* which values we are initializing */ 184 struct rt_metrics rtm_rmx; /* metrics themselves */ 185}; 186.Ed 187.Pp 188where 189.Bd -literal 190struct rt_metrics { 191 u_long rmx_locks; /* Kernel must leave these values alone */ 192 u_long rmx_mtu; /* MTU for this path */ 193 u_long rmx_hopcount; /* max hops expected */ 194 u_long rmx_expire; /* lifetime for route, e.g. redirect */ 195 u_long rmx_recvpipe; /* inbound delay-bandwith product */ 196 u_long rmx_sendpipe; /* outbound delay-bandwith product */ 197 u_long rmx_ssthresh; /* outbound gateway buffer limit */ 198 u_long rmx_rtt; /* estimated round trip time */ 199 u_long rmx_rttvar; /* estimated rtt variance */ 200}; 201.Ed 202.Pp 203Flags include the values: 204.Bd -literal 205#define RTF_UP 0x1 /* route useable */ 206#define RTF_GATEWAY 0x2 /* destination is a gateway */ 207#define RTF_HOST 0x4 /* host entry (net otherwise) */ 208#define RTF_NORMAL 0x8 /* subnet mask is cannonical */ 209#define RTF_DYNAMIC 0x10 /* created dynamically (by redirect) */ 210#define RTF_MODIFIED 0x20 /* modified dynamically (by redirect) */ 211#define RTF_DONE 0x40 /* message confirmed */ 212#define RTF_MASK 0x80 /* subnet mask present */ 213.Ed 214.Pp 215Specfiers for metric values in rmx_locks and rtm_inits are: 216.Bd -literal 217#define RTV_SSTHRESH 0x1 /* init or lock _ssthresh */ 218#define RTV_RPIPE 0x2 /* init or lock _recvpipe */ 219#define RTV_SPIPE 0x4 /* init or lock _sendpipe */ 220#define RTV_HOPCOUNT 0x8 /* init or lock _hopcount */ 221#define RTV_RTT 0x10 /* init or lock _rtt */ 222#define RTV_RTTVAR 0x20 /* init or lock _rttvar */ 223#define RTV_MTU 0x40 /* init or lock _mtu */ 224.Ed 225.Pp 226Specifiers for which addresses are present in the messages are: 227.Bd -literal 228#define RTA_DST 0x1 /* destination sockaddr present */ 229#define RTA_GATEWAY 0x2 /* gateway sockaddr present */ 230#define RTA_NETMASK 0x4 /* netmask sockaddr present */ 231#define RTA_GENMASK 0x8 /* cloning mask sockaddr present */ 232#define RTA_IFP 0x10 /* interface name sockaddr present */ 233#define RTA_IFA 0x20 /* interface addr sockaddr present */ 234#define RTA_AUTHOR 0x40 /* sockaddr for author of redirect */ 235.Ed 236