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

     rtentry — structure of an entry in the kernel routing table

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

     The kernel provides a common mechanism by which all protocols can store
     and retrieve entries from a central table of routes.  Parts of this mech‐
     anism are also used to interact with user-level processes by means of a
     socket in the route(4) pseudo-protocol family.  The <net/route.h> header
     file defines the structures and manifest constants used in this facility.

     The basic structure of a route is defined by struct rtentry, which
     includes the following fields:

	   struct radix_node rt_nodes[2];
		   Glue used by the radix-tree routines.  These members also
		   include in their substructure the key (i.e., destination
		   address) and mask used when the route was created.  The
		   rt_key(rt) and rt_mask(rt) macros can be used to extract
		   this information (in the form of a struct sockaddr *) given
		   a struct rtentry *.

	   struct sockaddr *rt_gateway;
		   The “target” of the route, which can either represent a
		   destination in its own right (some protocols will put a
		   link-layer address here), or some intermediate stop on the
		   way to that destination (if the RTF_GATEWAY flag is set).

	   int rt_flags;
		   See below.

	   int rt_refcnt;
		   Route entries are reference-counted; this field indicates
		   the number of external (to the radix tree) references.

	   struct ifnet *rt_ifp;

	   struct ifaddr *rt_ifa;
		   These two fields represent the “answer”, as it were, to the
		   question posed by a route lookup; that is, they name the
		   interface and interface address to be used in sending a
		   packet to the destination or set of destinations which this
		   route represents.

	   struct rt_metrics_lite rt_rmx;
		   See below.  If the RTF_UP flag is not present, the rtfree()
		   function will delete the route from the radix tree when the
		   last reference drops.

	   struct rtentry *rt_gwroute;
		   This member is a reference to a route whose destination is
		   rt_gateway.	It is only used for RTF_GATEWAY routes.

	   struct mtx rt_mtx;
		   Mutex to lock this routing entry.

     The following flag bits are defined:
	   RTF_UP	  The route is not deleted.
	   RTF_GATEWAY	  The route points to an intermediate destination and
			  not the ultimate recipient; the rt_gateway and
			  rt_gwroute fields name that destination.
	   RTF_HOST	  This is a host route.
	   RTF_REJECT	  The destination is presently unreachable.  This
			  should result in an EHOSTUNREACH error from output
	   RTF_DYNAMIC	  This route was created dynamically by rtredirect().
	   RTF_MODIFIED	  This route was modified by rtredirect().
	   RTF_DONE	  Used only in the route(4) protocol, indicating that
			  the request was executed.
	   RTF_XRESOLVE	  When this route is returned as a result of a lookup,
			  send a report on the route(4) interface requesting
			  that an external process perform resolution for this
	   RTF_STATIC	  Indicates that this route was manually added by
			  means of the route(8) command.
	   RTF_BLACKHOLE  Requests that output sent via this route be dis‐
	   RTF_PROTO3	  Protocol-specific.
	   RTF_PRCLONING  This flag is obsolete and simply ignored by facil‐
	   RTF_PINNED	  (Reserved for future use to indicate routes which
			  are not to be modified by a routing protocol.)
	   RTF_LOCAL	  Indicates that the destination of this route is an
			  address configured as belonging to this system.
	   RTF_BROADCAST  Indicates that the destination is a broadcast
	   RTF_MULTICAST  Indicates that the destination is a multicast

     Every route has associated with it a set of metrics, stored in struct
     rt_metrics_lite.  Metrics are supplied in struct rt_metrics passed with
     routing control messages via route(4) API.	 Currently only rmx_mtu,
     rmx_expire, and rmx_pksent metrics are used in struct rt_metrics_lite.
     All others are ignored.

     The following metrics are defined by struct rt_metrics:

	   u_long rmx_locks;
		   Flag bits indicating which metrics the kernel is not per‐
		   mitted to dynamically modify.

	   u_long rmx_mtu;
		   MTU for this path.

	   u_long rmx_hopcount;
		   Number of intermediate systems on the path to this destina‐

	   u_long rmx_expire;
		   The time (a la time(3)) at which this route should expire,
		   or zero if it should never expire.  It is the responsibil‐
		   ity of individual protocol suites to ensure that routes are
		   actually deleted once they expire.

	   u_long rmx_recvpipe;
		   Nominally, the bandwidth-delay product for the path from
		   the destination to this system.  In practice, this value is
		   used to set the size of the receive buffer (and thus the
		   window in sliding-window protocols like TCP).

	   u_long rmx_sendpipe;
		   As before, but in the opposite direction.

	   u_long rmx_ssthresh;
		   The slow-start threshold used in TCP congestion-avoidance.

	   u_long rmx_rtt;
		   The round-trip time to this destination, in units of
		   RMX_RTTUNIT per second.

	   u_long rmx_rttvar;
		   The average deviation of the round-trip time to this desti‐
		   nation, in units of RMX_RTTUNIT per second.

	   u_long rmx_pksent;
		   A count of packets successfully sent via this route.

	   u_long rmx_filler[4];
		   Empty space available for protocol-specific information.

     route(4), route(8), rtalloc(9)

     The rtentry structure first appeared in 4.2BSD.  The radix-tree represen‐
     tation of the routing table and the rt_metrics structure first appeared
     in 4.3BSD-Reno.

     This manual page was written by Garrett Wollman.

     There are a number of historical relics remaining in this interface.  The
     rt_gateway and rmx_filler fields could be named better.

BSD			       December 11, 2008			   BSD

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