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     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
     mechanism 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

     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
           RTF_PROTO3     Protocol-specific.
           RTF_PRCLONING  This flag is obsolete and simply ignored by
           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
                   permitted to dynamically modify.

           u_long rmx_mtu;
                   MTU for this path.

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

           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
                   responsibility 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
                   destination, 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
     representation 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.