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


rtalloc1_fib, rtalloc_ign_fib, rtalloc_fiblook up a route in the kernel routing table


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

struct rtentry *
rtalloc1_fib(struct sockaddr *dst, int report, u_long flags, u_int fibnum);

rtalloc_fib(struct route *ro, u_int fibnum);

rtalloc_ign_fib(struct route *ro, u_long flags, u_int fibnum);

RTFREE_LOCKED(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);

RO_RTFREE(struct route *ro);

rtfree(struct rt_entry *rt);

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

rtalloc(struct route *ro);

rtalloc_ign(struct route *ro, u_long flags);



The kernel uses a radix tree structure to manage routes for the networking subsystem. If compiled with options RADIX_MPATH kernel may maintain several independent forwarding information databases (FIBs). The () family of routines is used by protocols to query these structures for a route corresponding to a particular end-node address, and to cause certain protocol- and interface-specific actions to take place.

The () function is the most general form of rtalloc(), and all of the other forms are implemented as calls to it. It takes a struct sockaddr * directly as the dst argument. The second argument, report, controls whether the routing sockets are notified when a lookup fails. The third argument, flags, is a combination of the following values:

The last argument fibnum specifies number of forwarding information database (FIB) on which the lookup should be performed. In case of success the () function returns a pointer to a locked struct rtentry with an additional reference.

The () is the most simple variant. Its main argument is ro, a pointer to a struct route, which is defined as follows:

struct route {
	struct rtentry *ro_rt;
	struct llentry *ro_lle;
	struct sockaddr ro_dst;

Thus, this function can only be used for address families which are smaller than the default struct sockaddr. Before calling () for the first time, callers should ensure that unused bits of the structure are set to zero. The second argument fibnum is FIB number. In case of success of the rtalloc_fib() the ro_rt points to a valid and unlocked rtentry(9), which has an additional reference put on it, freeing which is responsibility of the caller. On subsequent calls, rtalloc_fib() returns without performing a lookup if ro->ro_rt is non-null and the RTF_UP flag is set in the rtentry's rt_flags field.

The () function is the same as the rtalloc_fib(), but there is additional flags argument, which is same as in rtalloc1_fib().

The () macro is used to unref and possibly free a locked routing entry with one our reference, for example previously allocated by rtalloc1_fib().

The () macro is used to unref and possibly free an unlocked route entries with one our reference, for example previously allocated by rtalloc_fib() or rtalloc_ign_fib().

Both () and RTFREE() macros decrement the reference count on the routing table entry, and proceed with actual freeing if the reference count has reached zero.

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

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

The () macro increments the reference count on a previously locked route entry. It should be used whenever a reference to an rtentry(9) is going to be stored outside the routing table.

The () macro decrements the reference count on a previously locked route entry. Its usage is contrary to RT_ADDREF().

The () macro is used to free route entry that is referenced by struct route. At certain circumstances the latter may not hold a reference on rtentry, and RO_RTFREE() treats such routes correctly.

The () function does the actual free of the routing table entry, and shouldn't be called directly by facilities, that just perform routing table lookups.


Prior to introduction of multiple routing tables functions did not require the u_int fibnum argument. Legacy (), rtalloc() and () functions are kept for compatibility, and are equivalent to calling new interface with fibnum argument equal to 0, which implies default forwarding table.


The rtalloc1_fib() function returns a pointer to a locked routing-table entry if it succeeds, otherwise a null pointer. The rtalloc_fib() and rtalloc_ign_fib() functions do not return a value, but they fill in the *ro_rt member of the *ro argument with a pointer to an unlocked routing-table entry if they succeed, otherwise a null pointer. In a case of success all functions put a reference on the routing-table entry, freeing of which is responsibility of the caller. 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. Multiple routing tables were introduced in FreeBSD 8.0.


The original version of this manual page was written by Garrett Wollman. It was significantly updated by Gleb Smirnoff.

July 4, 2012 Debian