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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
     particular end-node address, and to cause certain protocol- and
     interface-specific 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  RTF_PRCLONING flag is obsolete and thus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 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 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
     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 actions of
     rtalloc() in the presence of the RTF_CLONING flag is undesired.  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 address 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 facility.  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 routing-table
     entry if it succeeds, otherwise a null pointer.  Lack of a route should
     in most cases be translated to the errno(2) value EHOSTUNREACH.

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 argument
     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().