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.\"	$Id: rtalloc.9,v 1.5 1997/02/22 13:26:25 peter Exp $
.Dd October 8, 1996
.Os
.Dt RTALLOC 9
.Sh NAME
.Nm rtalloc ,
.Nm rtalloc_ign ,
.Nm rtalloc1
.Nd look up a route in the kernel routing table
.Sh SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <sys/socket.h>
.Fd #include <net/route.h>
.Ft void
.Fn rtalloc "struct route *ro"
.Ft void
.Fn rtalloc_ign "struct route *ro" "u_long flags"
.Ft "struct rtentry *"
.Fn rtalloc1 "struct sockaddr *sa" "int report" "u_long flags"
.Sh DESCRIPTION
The kernel uses a radix tree structure to manage routes for the
networking subsystem.  The
.Fn rtalloc
family of routines is used by protocols to query this structure for a
route corresponding to a particular end-node address, and to cause
certain protocol\- and interface-specific actions to take place.
.\" XXX - -mdoc should contain a standard request for getting em and
 \" en dashes.
.Pp
When a route with the flag
.Dv RTF_CLONING
or
.Dv RTF_PRCLONING
is retrieved, and the action of those flags is not masked, the
.Nm rtalloc
facility automatically generates a new route using information in the
old route as a template, and in the case of
.Dv RTF_CLONING ,
sends an
.Dv RTM_RESOLVE
message to the appropriate interface-address route-management routine
.Pq Fn ifa->ifa_rtrequest .
.Dv RTF_PRCLONING
routes are assumed to be managed by the protocol family and no
resolution requests are made, but all routes generated by the cloning
process retain a reference to the route from which they were
generated.
If the
.Dv RTF_XRESOLVE
flag is set, then the
.Dv RTM_RESOLVE
message is sent instead on the
.Xr route 4
socket interface, requesting that an external program resolve the
address in question and modify the route appropriately.
.Pp
The default interface is
.Fn rtalloc .
Its only argument is
.Ar ro ,
a pointer to a
.Dq Li "struct route" ,
which is defined as follows:
.Bd -literal -offset indent
struct route {
	struct sockaddr ro_dst;
	struct rtentry *ro_rt;
};
.Ed
Thus, this function can only be used for address families which are
smaller than the default
.Dq Li "struct sockaddr" .
Before calling
.Fn rtalloc 
for the first time, callers should ensure that unused bits of the 
structure are set to zero.  On subsequent calls,
.Fn rtalloc
returns without performing a lookup if
.Ar ro->ro_rt
is non-null and the
.Dv RTF_UP
flag is set in the route's
.Li rt_flags
field.
.Pp
The
.Fn rtalloc_ign
interface can be used when the default actions of
.Fn rtalloc
in the presence of the
.Dv RTF_CLONING
and
.Dv RTF_PRCLONING
flags are undesired.  The
.Ar ro
argument is the same as 
.Fn rtalloc ,
but there is additionally a
.Ar flags
argument, which lists the flags in the route which are to be
.Em ignored
(ordinarily, one or both of
.Dv RTF_CLONING
or
.Dv RTF_PRCLONING ) .
.Pp
The
.Fn rtalloc1
function is the most general form of
.Fn rtalloc
(and both of the other forms are implemented as calls to rtalloc1).
It does not use the
.Dq Li "struct route" ,
and is therefore suitable for address families which require more
space than is in a traditional
.Dq Li "struct sockaddr" .
Instead, it takes a
.Dq Li "struct sockaddr *"
directly as the
.Ar sa
argument.  The second argument,
.Ar report ,
controls whether
.Dv RTM_RESOLVE
requests are sent to the lower layers when an
.Dv RTF_CLONING
or
.Dv RTF_PRCLONING
route is cloned.  Ordinarily a value of one should be passed, except
in the processing of those lower layers which use the cloning
facility.
The third argument,
.Ar flags ,
is a set of flags to ignore, as in
.Fn rtalloc_ign .
.Sh RETURN VALUES
The
.Fn rtalloc
and
.Fn rtalloc_ign
functions do not return a value.  The
.Fn rtalloc1
function returns a pointer to a routing-table entry if it succeeds,
otherwise a null pointer.  Lack of a route should in most cases be
translated to the
.Xr errno 2
value
.Er EHOSTUNREACH .
.Sh SEE ALSO
.Xr route 4 ,
.Xr rtentry 9
.Sh HISTORY
The
.Nm rtalloc
facility first appeared in
.Bx 4.2 ,
although with much different internals.  The
.Fn rtalloc_ign
function and the
.Ar flags
argument to
.Fn rtalloc1
first appeared in
.Fx 2.0 .
.Sh AUTHOR
This manual page was written by Garrett Wollman, as were the changes
to implement
.Dv RTF_PRCLONING
and the
.Fn rtalloc_ign
function and the
.Ar flags
argument to 
.Fn rtalloc1 .