rtalloc(9)

NAME

rtalloc, rtalloc_ign, rtalloc1, rtfree - look up a route in

the kernel
routing table

SYNOPSIS

#include <sys/types.h>
#include <sys/socket.h>
#include <net/route.h>
void
rtalloc(struct route *ro);
void
rtalloc_ign(struct route *ro, u_long flags);
struct rtentry *
rtalloc1(struct sockaddr *sa, int report, u_long flags);
void
rtfree(struct rt_entry *rt);
RTFREE(struct rt_entry *rt);
RT_LOCK(struct rt_entry *rt);
RT_UNLOCK(struct rt_entry *rt);
RT_ADDREF(struct rt_entry *rt);
RT_REMREF(struct rt_entry *rt);

DESCRIPTION

The kernel uses a radix tree structure to manage routes for

the networking subsystem. The rtalloc() family of routines is used by

protocols to
query this structure for a route corresponding to a particu

lar end-node
address, and to cause certain protocol- and interface-spe

cific actions to
take place.

When a route with the flag RTF_CLONING is retrieved, and the

action of
this flag is not masked, the rtalloc facility automatically

generates a
new route using information in the old route as a template,

and sends an
RTM_RESOLVE message to the appropriate interface-address

route-management
routine (ifa->ifa_rtrequest()). This generated route is

called cloned,
and has RTF_WASCLONED flag set. RTF_PRCLONING flag is obso

lete and thus
ignored by facility. If the RTF_XRESOLVE flag is set, then

the
RTM_RESOLVE message is sent instead on the route(4) socket

interface,
requesting that an external program resolve the address in

question and
modify the route appropriately.

The default interface is rtalloc(). Its only argument is

ro, a pointer
to a ``struct route'', which is defined as follows:

struct route {

struct sockaddr ro_dst;
struct rtentry *ro_rt;

};

Thus, this function can only be used for address families

which are
smaller than the default ``struct sockaddr''. Before call

ing rtalloc()
for the first time, callers should ensure that unused bits

of the structure are set to zero. On subsequent calls, rtalloc() re

turns without
performing a lookup if ro->ro_rt is non-null and the RTF_UP

flag is set
in the route's rt_flags field.

The rtalloc_ign() interface can be used when the default ac

tions of
rtalloc() in the presence of the RTF_CLONING flag is unde

sired. The ro
argument is the same as rtalloc(), but there is additionally

a flags
argument, which lists the flags in the route which are to be

ignored (in
most cases this is RTF_CLONING flag). Both rtalloc() and

rtalloc_ign()
functions return a pointer to an unlocked struct rtentry.

The rtalloc1() function is the most general form of

rtalloc() (and both
of the other forms are implemented as calls to rtalloc1).

It does not
use the ``struct route'', and is therefore suitable for ad

dress families
which require more space than is in a traditional ``struct

sockaddr''.
Instead, it takes a ``struct sockaddr *'' directly as the sa

argument.
The second argument, report, controls whether RTM_RESOLVE

requests are
sent to the lower layers when an RTF_CLONING or

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 facili

ty. The third
argument, flags, is a set of flags to ignore, as in

rtalloc_ign(). The
rtalloc1() function returns a pointer to a locked struct

rtentry.

The rtfree() function frees a locked route entry, e.g., a

previously
allocated by rtalloc1().

The RTFREE() macro is used to free unlocked route entries,

previously
allocated by rtalloc() or rtalloc_ign(). The RTFREE() macro

decrements
the reference count on the routing table entry (see below),

and frees it
if the reference count has reached zero.

The preferred usage is allocating a route using rtalloc() or rtalloc_ign() and freeing using RTFREE().

The RT_LOCK() macro is used to lock a routing table entry.

The
RT_UNLOCK() macro is used to unlock a routing table entry.

The RT_ADDREF() macro increments the reference count on a

previously
locked route entry. The RT_REMREF() macro decrements the

reference count
on a previously locked route entry.

RETURN VALUES

The rtalloc(), rtalloc_ign() and rtfree() functions do not

return a
value. The rtalloc1() function returns a pointer to a rout

ing-table
entry if it succeeds, otherwise a null pointer. Lack of a

route should
in most cases be translated to the errno(2) value EHOSTUN

REACH.

SEE ALSO

route(4), rtentry(9)

HISTORY

The rtalloc facility first appeared in 4.2BSD, although with

much different internals. The rtalloc_ign() function and the flags ar

gument to
rtalloc1() first appeared in FreeBSD 2.0. Routing table

locking was
introduced in FreeBSD 5.2.

AUTHORS

This manual page was written by Garrett Wollman, as were the

changes to
implement RTF_PRCLONING and the rtalloc_ign() function and

the flags
argument to rtalloc1().

BSD October 11, 2004

BSD