rtentry(9)

NAME

rtentry - structure of an entry in the kernel routing table

SYNOPSIS

#include <sys/types.h>
#include <sys/socket.h>
#include <net/route.h>

DESCRIPTION

The kernel provides a common mechanism by which all proto

cols can store
and retrieve entries from a central table of routes. Parts

of this mechanism are also used to interact with user-level processes by

means of a
socket in the route(4) pseudo-protocol family. The #include <net/route.h>
header file defines the structures and manifest constants

used in this
facility.

The basic structure of a route is defined by struct rtentry,

which
includes the following fields:

struct radix_node rt_nodes[2];

Glue used by the radix-tree routines. These

members also
include in their substructure the key (i.e.,

destination
address) and mask used when the route was cre

ated. The
rt_key(rt) and rt_mask(rt) macros can be used

to extract
this information (in the form of a struct

sockaddr *) given
a struct rtentry *.

struct sockaddr *rt_gateway;

The ``target'' of the route, which can either

represent a
destination in its own right (some protocols

will put a
link-layer address here), or some intermediate

stop on the
way to that destination (if the RTF_GATEWAY

flag is set).

u_long rt_flags;

See below.

struct ifnet *rt_ifp;

struct ifaddr *rt_ifa;

These two fields represent the ``answer'', as

it were, to
the question posed by a route lookup; that is,

they name
the interface and interface address to be used

in sending a
packet to the destination or set of destina

tions which this
route represents.

struct rt_metrics_lite rt_rmx;

See below.

long rt_refcnt;

Route entries are reference-counted; this

field indicates
the number of external (to the radix tree)

references. If
the RTF_UP flag is not present, the rtfree()

function will
delete the route from the radix tree when the

last reference drops.

struct sockaddr *rt_genmask;

When the rtalloc() family of functions per

forms a cloning
operation as requested by the RTF_CLONING

flag, this field
is used as the mask for the new route which is

inserted
into the table. If this field is a null

pointer, then a
host route is generated.

caddr_t rt_llinfo;

When the RTF_LLINFO flag is set, this field

contains information specific to the link layer represented

by the named
interface address. (It is normally managed by

the
rt_ifa->ifa_rtrequest routine.) Protocols

such as arp(4)
use this field to reference per-destination

state internal
to that protocol.

struct rtentry *rt_gwroute;

This member is a reference to a route whose

destination is
rt_gateway. It is only used for RTF_GATEWAY

routes.

struct rtentry *rt_parent;

A reference to the route from which this route

was cloned,
or a null pointer if this route was not gener

ated by
cloning. See also the RTF_WASCLONED flag.

struct mtx rt_mtx;

Mutex to lock this routing entry.

The following flag bits are defined:

RTF_UP The route is not deleted.
RTF_GATEWAY The route points to an intermediate

destination

and not the ultimate recipient; the

rt_gateway
and rt_gwroute fields name that

destination.

RTF_HOST This is a host route.
RTF_REJECT The destination is presently un

reachable. This

should result in an EHOSTUNREACH

error from output routines.

RTF_DYNAMIC This route was created dynamically

by

rtredirect().

RTF_MODIFIED This route was modified by

rtredirect().
RTF_DONE Used only in the route(4) protocol,

indicating

that the request was executed.

RTF_CLONING When this route is returned as a

result of a

lookup, automatically create a new

route using
this one as a template and

rt_genmask (if present) as a mask.

RTF_XRESOLVE When this route is returned as a

result of a

lookup, send a report on the

route(4) interface
requesting that an external process

perform resolution for this route. (Used in

conjunction
with RTF_CLONING.)

RTF_LLINFO Indicates that this route repre

sents information

being managed by a link layer's

adaptation layer
(e.g., ARP).

RTF_STATIC Indicates that this route was manu

ally added by

means of the route(8) command.

RTF_BLACKHOLE Requests that output sent via this

route be dis

carded.

RTF_PROTO1
RTF_PROTO2
RTF_PROTO3 Protocol-specific.
RTF_PRCLONING This flag is obsolete and simply

ignored by

facility.

RTF_WASCLONED Indicates that this route was gen

erated as a

result of cloning requested by the

RTF_CLONING
flag. When set, the rt_parent

field indicates
the route from which this one was

generated.

RTF_PINNED (Reserved for future use to indi

cate routes

which are not to be modified by a

routing protocol.)

RTF_LOCAL Indicates that the destination of

this route is

an address configured as belonging

to this system.

RTF_BROADCAST Indicates that the destination is a

broadcast

address.

RTF_MULTICAST Indicates that the destination is a

multicast

address.

Every route has associated with it a set of metrics, stored

in struct
rt_metrics_lite. Metrics are supplied in struct rt_metrics

passed with
routing control messages via route(4) API. Currently only

rmx_mtu,
rmx_expire, and rmx_pksent metrics are used in struct

rt_metrics_lite.
All others are ignored.

The following metrics are defined by struct rt_metrics:

u_long rmx_locks;

Flag bits indicating which metrics the kernel

is not permitted to dynamically modify.

u_long rmx_mtu;

MTU for this path.

u_long rmx_hopcount;

Number of intermediate systems on the path to

this destination.

u_long rmx_expire;

The time (a la time(3)) at which this route

should expire,
or zero if it should never expire. It is the

responsibility of individual protocol suites to ensure

that routes are
actually deleted once they expire.

u_long rmx_recvpipe;

Nominally, the bandwidth-delay product for the

path from
the destination to this system. In practice,

this value is
used to set the size of the receive buffer

(and thus the
window in sliding-window protocols like TCP).

u_long rmx_sendpipe;

As before, but in the opposite direction.

u_long rmx_ssthresh;

The slow-start threshold used in TCP conges

tion-avoidance.

u_long rmx_rtt;

The round-trip time to this destination, in

units of
RMX_RTTUNIT per second.

u_long rmx_rttvar;

The average deviation of the round-type time

to this destination, in units of RMX_RTTUNIT per second.

u_long rmx_pksent;

A count of packets successfully sent via this

route.

u_long rmx_filler[4];

Empty space available for protocol-specific

information.

SEE ALSO

route(4), route(8), rtalloc(9)

HISTORY

The rtentry structure first appeared in 4.2BSD. The radix

tree representation of the routing table and the rt_metrics structure

first appeared
in 4.3BSD-Reno.

AUTHORS

This manual page was written by Garrett Wollman.

BUGS

There are a number of historical relics remaining in this

interface. The
rt_gateway and rmx_filler fields could be named better.

There is some disagreement over whether it is legitimate for

RTF_LLINFO
to be set by any process other than rt_ifa->ifa_rtrequest.

BSD October 7, 2004

BSD