route - Kernel packet forwarding database
#include <sys/socket.h> #include <net/if.h> #include
<net/route.h> int family s = socket(PF_ROUTE, SOCK_RAW,
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
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
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
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
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
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.
If messages are rejected, rtm_errno may be set to one of
the following values: The entry to be created already
exists. The entry to be deleted does not exist. Insufficient
resources were available to install a new route.
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