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route(7)
NAME
  route - Kernel packet forwarding database

SYNOPSIS
       #include <sys/socket.h>
       #include <net/if.h>
       #include <net/route.h>
       int family
       s = socket(PF_ROUTE, SOCK_RAW, family);

DESCRIPTION
  The UNIX operating system provides packet routing facilities. The kernel
  maintains a routing information database, which is used in selecting the
  appropriate network interface when transmitting packets.

  A user process (or possibly multiple cooperating processes) maintains this
  database by sending messages over a special kind of socket. Routing table
  changes may only be carried out by the superuser.

  The operating system may spontaneously emit routing messages in response to
  external events, such as receipt of a redirect, or failure to locate a
  suitable route for a request.

  Routing database entries are of two types: those for a specific host, and
  those for all hosts on a generic subnetwork (as specified by a bit mask and
  value under the mask). The effect of a wildcard or default route may be
  achieved by using a mask of all zeros.  There may be hierarchical routes.

  When the system is booted and addresses are assigned to the network
  interfaces, each protocol family installs a routing table entry for each
  interface when it is ready for traffic. Normally the protocol specifies the
  route through each interface as a ``direct'' connection to the destination
  host or network.  If the route is direct, the transport layer of a protocol
  family usually requests that the packet be sent to the host specified in
  the packet.  Otherwise, the interface is requested to address the packet to
  the gateway listed in the routing entry (that is, the packet is forwarded).

  When routing a packet, the kernel first attempts to find a route to the
  destination host. Failing that, a search is made for a route to the network
  of the destination. Finally, any route to a default (wildcard) gateway is
  chosen. If no entry is found, the destination is declared to be
  unreachable, and an error message is generated if there are any listeners
  on the routing control socket described later in this section.

  A wildcard routing entry is specified with a zero destination address
  value.  Wildcard routes are used only when the system fails to find a route
  to the destination host and network.	The combination of wildcard routes
  and routing redirects can provide an economical mechanism for routing
  traffic.

  To open the channel for passing routing control messages, use the socket
  call shown in the SYNOPSIS.

  The family parameter may be AF_UNSPEC which will provide routing
  information for all address families, or can be restricted to a specific
  address family by specifying which one is desired. There can be more than
  one routing socket open per system.

  Messages are formed by a header followed by a small number of sockaddrs
  (now variable length), interpreted by position, and delimited by the new
  length entry in the sockaddr. An example of a message with four addresses
  might be an ISO redirect: destination, netmask, gateway, and author of the
  redirect.  The interpretation of which addresses are present is given by a
  bit mask within the header, and the sequence is least significant to most
  significant bit within the vector.

  Any messages sent to the kernel are returned, and copies are sent to all
  interested listeners.	 The kernel will provide the process ID for the
  sender, and the sender may use an additional sequence field to distinguish
  between outstanding messages.	 However, message replies may be lost when
  kernel buffers are exhausted.

  The kernel may reject certain messages, and will indicate this by filling
  in the rtm_errno field. In the current implementation, all routing process
  run locally, and the values for rtm_errno are available through the normal
  errno mechanism, even if the routing reply message is lost.

  A process may avoid the expense of reading replies to its own messages by
  issuing a setsockopt() call indicating that the SO_USELOOPBACK option at
  the SOL_SOCKET level is to be turned off. A process may ignore all messages
  from the routing socket by shutting down further input with the shutdown()
  function.

  If a route is in use when it is deleted, the routing entry will be marked
  down and removed from the routing table, but the resources associated with
  it will not be reclaimed until all references to it are released. User
  processes can obtain information about the routing entry to a specific
  destination by using a RTM_GET message, or by reading the /dev/kmem device.

ERRORS
  If messages are rejected, rtm_errno may be set to one of the following
  values:

  [EEXIST]
      The entry to be created already exists.

  [ESRCH]
      The entry to be deleted does not exist.

  [ENOBUFS]
      Insufficient resources were available to install a new route.

Index for Section 7

Alphabetical listing for R

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