ROUTING(4P) Protocols ROUTING(4P)

NAME

routing - system support for packet network routing

DESCRIPTION

The network facilities provide general packet routing. The routing
interface described here can be used to maintain the system's IPv4
routing table. It has been maintained for compatibility with older
applications. The recommended interface for maintaining the system's
routing tables is the routing socket, described at route(4P). The
routing socket can be used to manipulate both the IPv4 and IPv6
routing tables of the system. Routing table maintenance may be
implemented in applications processes.


A simple set of data structures compose a "routing table" used in
selecting the appropriate network interface when transmitting
packets. This table contains a single entry for each route to a
specific network or host. The routing table was designed to support
routing for the Internet Protocol (IP), but its implementation is
protocol independent and thus it may serve other protocols as well.
User programs may manipulate this data base with the aid of two
ioctl(2) commands, SIOCADDRT and SIOCDELRT. These commands allow the
addition and deletion of a single routing table entry, respectively.
Routing table manipulations may only be carried out by privileged
user.


A routing table entry has the following form, as defined in
/usr/include/net/route.h:

struct rtentry {
unit_t rt_hash; /* to speed lookups */
struct sockaddr rt_dst; /* key */
struct sockaddr rt_gateway; /* value */
short rt_flags; /* up/down?, host/net */
short rt_refcnt; /* # held references */
unit_t rt_use; /* raw # packets forwarded */
/*
* The kernel does not use this field, and without it the structure is
* datamodel independent.
*/
#if !defined(_KERNEL)
struct ifnet *rt_ifp; /* the answer: interface to use */
#endif /* !defined(_KERNEL) */
};


with rt_flags defined from:

#define RTF_UP 0x1 /* route usable */
#define RTF_GATEWAY 0x2 /* destination is a gateway */
#define RTF_HOST 0x4 /* host entry (net otherwise) */


There are three types of routing table entries: those for a specific
host, those for all hosts on a specific network, and those for any
destination not matched by entries of the first two types, called a
wildcard route. Each network interface installs a routing table entry
when it is initialized. Normally the interface specifies if the
route through it is a "direct" connection to the destination host or
network. If the route is direct, the transport layer of a protocol
family usually requests the packet be sent to the same host specified
in the packet. Otherwise, the interface may be requested to address
the packet to an entity different from the eventual recipient;
essentially, the packet is forwarded.


Routing table entries installed by a user process may not specify the
hash, reference count, use, or interface fields; these are filled in
by the routing routines. If a route is in use when it is deleted,
meaning its rt_refcnt is non-zero, the resources associated with it
will not be reclaimed until all references to it are removed.


User processes read the routing tables through the /dev/ip device.


The rt_use field contains the number of packets sent along the route.
This value is used to select among multiple routes to the same
destination. When multiple routes to the same destination exist, the
least used route is selected.


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.

ERRORS

EEXIST
A request was made to duplicate an existing entry.


ESRCH
A request was made to delete a non-existent entry.


ENOBUFS
Insufficient resources were available to install a new
route.


ENOMEM
Insufficient resources were available to install a new
route.


ENETUNREACH
The gateway is not directly reachable. For example,
it does not match the destination/subnet on any of the
network interfaces.

FILES

/dev/ip
IP device driver

SEE ALSO

ioctl(2), route(4P), route(8)

November 9, 1999 ROUTING(4P)