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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

           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

           int rt_flags;
                   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.

           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.

           u_long rt_mtu;
                   See description of rmx_mtu below.

           u_long rt_weight;
                   See description of rmx_weight below.

           u_long rt_expire;
                   See description of rmx_expire below.

           counter64_t rt_pksent;
                   See description of rmx_pksent below.

           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 routines.
           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
           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 route.
           RTF_STATIC     Indicates that this route was manually added by means of the route(8)
           RTF_BLACKHOLE  Requests that output sent via this route be discarded.
           RTF_PROTO3     Protocol-specific.
           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 address.
           RTF_MULTICAST  Indicates that the destination is a multicast address.

     Several 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 supplied.  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

           u_long rmx_mtu;
                   MTU for this path.

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

           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.