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RTALLOC(9)		 BSD Kernel Developer's	Manual		    RTALLOC(9)

     rtalloc, rtalloc_ign, rtalloc1, rtfree -- look up a route in the kernel
     routing table

     #include <sys/types.h>
     #include <sys/socket.h>
     #include <net/route.h>

     rtalloc(struct route *ro);

     rtalloc_ign(struct	route *ro, u_long flags);

     struct rtentry *
     rtalloc1(struct sockaddr *sa, int report, u_long flags);

     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);

     The kernel	uses a radix tree structure to manage routes for the network-
     ing subsystem.  The 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.

     RTF_PRCLONING flag	is obsolete and	thus ignored by	facility.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	re-
     solve 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 calling rtalloc() for
     the first time, callers should ensure that	unused bits of the structure
     are set to	zero.  On subsequent calls, rtalloc() returns without perform-
     ing 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 caller does not want to
     receive the returned rtentry locked.  The ro argument is the same as
     rtalloc(),	but there is additionally a flags argument, which is now only
     used to pass RTF_RNH_LOCKED indicating that the radix tree	lock is	al-
     ready held.  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 address families
     which require more	space than is in a traditional "struct sockaddr".  In-
     stead, it takes a "struct sockaddr	*" directly as the sa argument.	 The
     second argument, report, controls whether the lower layers	are notified
     when a lookup fails.  The third argument, flags, is a set of flags	to ig-
     nore, 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 al-
     located by	rtalloc1().

     The RTFREE() macro	is used	to free	unlocked route entries,	previously al-
     located 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.

     The rtalloc(), rtalloc_ign() and rtfree() functions do not	return a
     value.  The rtalloc1() function returns a pointer to a routing-table en-
     try if it succeeds, otherwise a null pointer.  Lack of a route should in
     most cases	be translated to the errno(2) value EHOSTUNREACH.

     route(4), rtentry(9)

     The rtalloc facility first	appeared in 4.2BSD, although with much differ-
     ent internals.  The rtalloc_ign() function	and the	flags argument to
     rtalloc1()	first appeared in FreeBSD 2.0.	Routing	table locking was in-
     troduced in FreeBSD 5.2.

     This manual page was written by Garrett Wollman, as were the changes to
     implement RTF_PRCLONING and the rtalloc_ign() function and	the flags ar-
     gument to rtalloc1().

BSD			       December	11, 2008			   BSD


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