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.Dd October 7, 2004
.Os
.Dt RTENTRY 9
.Sh NAME
.Nm rtentry
.Nd structure of an entry in the kernel routing table
.Sh SYNOPSIS
.In sys/types.h
.In sys/socket.h
.In net/route.h
.Sh 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
mechanism are also used to interact with user-level processes by means
of a socket in the
.Xr route 4
pseudo-protocol family.
The
.In net/route.h
header file defines the structures and manifest constants used in this
facility.
.Pp
The basic structure of a route is defined by
.Vt "struct rtentry" ,
which includes the following fields:
.Bl -tag -offset indent -width 6n
.It Vt "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
.Fn rt_key rt
and
.Fn rt_mask rt
macros can be used to extract this information (in the form of a
.Vt "struct sockaddr *" )
given a
.Vt "struct rtentry *" .
.It Vt "struct sockaddr *rt_gateway" ;
The
.Dq 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
.Dv RTF_GATEWAY
flag is set).
.It Vt "u_long rt_flags" ;
See below.
.It Vt "struct ifnet *rt_ifp" ;
.It Vt "struct ifaddr *rt_ifa" ;
These two fields represent the
.Dq 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.
.It Vt "struct rt_metrics_lite rt_rmx" ;
See below.
.It Vt "long rt_refcnt" ;
Route entries are reference-counted; this field indicates the number
of external (to the radix tree) references.
If the
.Dv RTF_UP
flag is not present, the
.Fn rtfree
function will delete the route from the radix tree when the last
reference drops.
.It Vt "struct sockaddr *rt_genmask" ;
When the
.Fn rtalloc
family of functions performs a cloning operation as requested by the
.Dv 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.
.It Vt "caddr_t rt_llinfo" ;
When the
.Dv 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
.Va rt_ifa->ifa_rtrequest
routine.)
Protocols such as
.Xr arp 4
use this field to reference per-destination state internal to that
protocol.
.It Vt "struct rtentry *rt_gwroute" ;
This member is a reference to a route whose destination is
.Va rt_gateway .
It is only used for
.Dv RTF_GATEWAY
routes.
.It Vt "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
.Dv RTF_WASCLONED
flag.
.It Vt "struct mtx rt_mtx" ;
Mutex to lock this routing entry.
.El
.Pp
The following flag bits are defined:
.Bl -tag -offset indent -width ".Dv RTF_BLACKHOLE" -compact
.It Dv RTF_UP
The route is not deleted.
.It Dv RTF_GATEWAY
The route points to an intermediate destination and not the ultimate
recipient; the
.Va rt_gateway
and
.Va rt_gwroute
fields name that destination.
.It Dv RTF_HOST
This is a host route.
.It Dv RTF_REJECT
The destination is presently unreachable.
This should result in an
.Er EHOSTUNREACH
error from output routines.
.It Dv RTF_DYNAMIC
This route was created dynamically by
.Fn rtredirect .
.It Dv RTF_MODIFIED
This route was modified by
.Fn rtredirect .
.It Dv RTF_DONE
Used only in the
.Xr route 4
protocol, indicating that the request was executed.
.It Dv 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
.Va rt_genmask
(if present) as a mask.
.It Dv RTF_XRESOLVE
When this route is returned as a result of a lookup, send a report on
the
.Xr route 4
interface requesting that an external process perform resolution for
this route.
(Used in conjunction with
.Dv RTF_CLONING . )
.It Dv RTF_LLINFO
Indicates that this route represents information being managed by a
link layer's adaptation layer (e.g.,
.Tn ARP ) .
.It Dv RTF_STATIC
Indicates that this route was manually added by means of the
.Xr route 8
command.
.It Dv RTF_BLACKHOLE
Requests that output sent via this route be discarded.
.It Dv RTF_PROTO1
.It Dv RTF_PROTO2
.It Dv RTF_PROTO3
Protocol-specific.
.It Dv RTF_PRCLONING
This flag is obsolete and simply ignored by facility.
.It Dv RTF_WASCLONED
Indicates that this route was generated as a result of cloning
requested by the
.Dv RTF_CLONING
flag.
When set, the
.Va rt_parent
field indicates the route from which this one was generated.
.It Dv RTF_PINNED
(Reserved for future use to indicate routes which are not to be
modified by a routing protocol.)
.It Dv RTF_LOCAL
Indicates that the destination of this route is an address configured
as belonging to this system.
.It Dv RTF_BROADCAST
Indicates that the destination is a broadcast address.
.It Dv RTF_MULTICAST
Indicates that the destination is a multicast address.
.El
.Pp
Every route has associated with it a set of metrics, stored in
.Vt "struct rt_metrics_lite" .
Metrics are supplied in
.Vt "struct rt_metrics"
passed with routing control messages via
.Xr route 4
API.
Currently only
.Vt rmx_mtu , rmx_expire ,
and
.Vt rmx_pksent
metrics are used in
.Vt "struct rt_metrics_lite" .  
All others are ignored.
.Pp
The following metrics are defined by
.Vt "struct rt_metrics" :
.Bl -tag -offset indent -width 6n
.It Vt "u_long rmx_locks" ;
Flag bits indicating which metrics the kernel is not permitted to
dynamically modify.
.It Vt "u_long rmx_mtu" ;
MTU for this path.
.It Vt "u_long rmx_hopcount" ;
Number of intermediate systems on the path to this destination.
.It Vt "u_long rmx_expire" ;
The time
(a la
.Xr 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.
.It Vt "u_long rmx_recvpipe" ;
Nominally, the bandwidth-delay product for the path
.Em from
the destination
.Em 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
.Tn TCP ) .
.It Vt "u_long rmx_sendpipe" ;
As before, but in the opposite direction.
.It Vt "u_long rmx_ssthresh" ;
The slow-start threshold used in
.Tn TCP
congestion-avoidance.
.It Vt "u_long rmx_rtt" ;
The round-trip time to this destination, in units of
.Dv RMX_RTTUNIT
per second.
.It Vt "u_long rmx_rttvar" ;
The average deviation of the round-type time to this destination, in
units of
.Dv RMX_RTTUNIT
per second.
.It Vt "u_long rmx_pksent" ;
A count of packets successfully sent via this route.
.It Vt "u_long rmx_filler[4]" ;
.\" XXX badly named
Empty space available for protocol-specific information.
.El
.Sh SEE ALSO
.Xr route 4 ,
.Xr route 8 ,
.Xr rtalloc 9
.Sh HISTORY
The
.Vt rtentry
structure first appeared in
.Bx 4.2 .
The radix-tree representation of the routing table and the
.Vt rt_metrics
structure first appeared in
.Bx 4.3 reno .
.Sh BUGS
There are a number of historical relics remaining in this interface.
The
.Va rt_gateway
and
.Va rmx_filler
fields could be named better.
.Pp
There is some disagreement over whether it is legitimate for
.Dv RTF_LLINFO
to be set by any process other than
.Va rt_ifa->ifa_rtrequest .
.Sh AUTHORS
This manual page was written by
.An Garrett Wollman .