Provided by: freebsd-manpages_8.2-1_all bug


     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 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.

     RTF_PRCLONING flag is obsolete and thus ignored by facility.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

     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 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 already 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”.  Instead, 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 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

     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

     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 entry if it succeeds, otherwise a null
     pointer.  Lack of a route should in most cases be translated to the errno(2) value


     route(4), rtentry(9)


     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.


     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().