RTALLOC(9) MidnightBSD 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 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 RTF_PRCLONING flag is obsolete and thus ignored by facility.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.


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.


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().

MidnightBSD 0.3 October 11, 2004 MidnightBSD 0.3