RTENTRY(9) BSD Kernel Developer's Manual RTENTRY(9)NAMErtentry — 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 protocols can store
and retrieve entries from a central table of routes. Parts of this mech‐
anism are also used to interact with user-level processes by means of a
socket in the route(4) pseudo-protocol family. The <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 created. 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).
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 refer‐
ence drops.
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 destinations which this
route represents.
struct sockaddr *rt_genmask;
When the rtalloc() family of functions performs a cloning
operation as requested by the RTF_CLONING or RTF_PRCLONING
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 infor‐
mation 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 rt_metrics rt_rmx;
See below.
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 generated by
cloning. See also the RTF_WASCLONED flag.
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 unreachable. 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 reso‐
lution for this route. (Used in conjunction with
RTF_CLONING.)
RTF_LLINFO Indicates that this route represents information
being managed by a link layer's adaptation layer
(e.g., ARP).
RTF_STATIC Indicates that this route was manually 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 Like RTF_CLONING, only managed by an entire proto‐
col. (E.g., IP uses this flag to manage a per-
host cache integrated with the routing table, for
those destinations which do not have a link layer
performing this function.)
RTF_WASCLONED Indicates that this route was generated as a
result of cloning requested by the RTF_CLONING or
RTF_PRCLONING flag. When set, the rt_parent field
indicates the route from which this one was gener‐
ated.
RTF_PINNED (Reserved for future use to indicate 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, defined by struct
rt_metrics:
u_long rmx_locks;
Flag bits indicating which metrics the kernel is not per‐
mitted to dynamically modify.
u_long rmx_mtu;
MTU for this path.
u_long rmx_hopcount;
Number of intermediate systems on the path to this destina‐
tion.
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 responsibil‐
ity 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 congestion-avoidance.
u_long rmx_rtt;
The round-trip time to this destination, in units of
RTM_RTTUNIT per second.
u_long rmx_rttvar;
The average deviation of the round-type time to this desti‐
nation, in units of RTM_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 ALSOroute(4), route(8), rtalloc(9)HISTORY
The rtentry structure first appeared in 4.2BSD. The radix-tree represen‐
tation of the routing table and the rt_metrics structure first appeared
in 4.3BSD-Reno. The RTF_PRCLONING mechanism first appeared in
FreeBSD 2.0.
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 8, 1996 BSD
