RTENTRY(9) BSD Kernel Developer's Manual RTENTRY(9)
rtentry -- structure of an entry in the kernel routing table
The kernel provides a common mechanism by which all protocols 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 <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;
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).
Route entries are reference-counted; this field indicates the number of external (to the radix tree) references.
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 inter-
face and interface address to be used in sending a packet to the destination or set of destinations which this route repre-
struct rt_metrics_lite rt_rmx;
See below. If the RTF_UP flag is not present, the rtfree() function will delete the route from the radix tree when the last
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 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 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_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.
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 discarded.
RTF_PRCLONING This flag is obsolete and simply ignored by facility.
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, 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:
Flag bits indicating which metrics the kernel is not permitted to dynamically modify.
MTU for this path.
Number of intermediate systems on the path to this destination.
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.
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).
As before, but in the opposite direction.
The slow-start threshold used in TCP congestion-avoidance.
The round-trip time to this destination, in units of RMX_RTTUNIT per second.
The average deviation of the round-trip time to this destination, in units of RMX_RTTUNIT per second.
A count of packets successfully sent via this route.
Empty space available for protocol-specific information.
route(4), route(8), rtalloc(9)
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.
This manual page was written by Garrett Wollman.
There are a number of historical relics remaining in this interface. The rt_gateway and rmx_filler fields could be named better.
December 11, 2008 BSD