route(7)route(7)NAMEroute - 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 ker‐
nel maintains a routing information database, which is used in select‐
ing 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. Rout‐
ing 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 hier‐
archical 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 trans‐
port 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 lis‐
teners 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 infor‐
mation 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 dis‐
tinguish 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 fill‐
ing 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 asso‐
ciated 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: The entry to be created already exists. The entry to be
deleted does not exist. Insufficient resources were available to
install a new route.
route(7)
