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NAME

     rtentry - structure of an entry in the kernel routing table

SYNOPSIS

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

DESCRIPTION

     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 #include
     <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).

           u_long rt_flags;
                   See below.

           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.

           long rt_refcnt;
                   Route entries are reference-counted; this field indicates
                   the number of external (to the radix tree) references.  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 sockaddr *rt_genmask;
                   When the rtalloc() family of functions performs a cloning
                   operation as requested by the RTF_CLONING flag, this field
                   is used as the mask for the new route which is inserted
                   into the table.  If this field is a null pointer, then a
                   host route is generated.

           caddr_t rt_llinfo;
                   When the RTF_LLINFO flag is set, this field contains
                   information specific to the link layer represented by the
                   named interface address.  (It is normally managed by the
                   rt_ifa->ifa_rtrequest routine.)  Protocols such as arp(4)
                   use this field to reference per-destination state internal
                   to that protocol.

           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 rtentry *rt_parent;
                   A reference to the route from which this route was cloned,
                   or a null pointer if this route was not generated by
                   cloning.  See also the RTF_WASCLONED flag.

           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 executed.
           RTF_CLONING    When this route is returned as a result of a lookup,
                          automatically create a new route using this one as a
                          template and rt_genmask (if present) as a mask.
           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.  (Used in conjunction with RTF_CLONING.)
           RTF_LLINFO     Indicates that this route represents information
                          being managed by a link layer’s adaptation layer
                          (e.g., ARP).
           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
                          discarded.
           RTF_PROTO1
           RTF_PROTO2
           RTF_PROTO3     Protocol-specific.
           RTF_PRCLONING  This flag is obsolete and simply ignored by
                          facility.
           RTF_WASCLONED  Indicates that this route was generated as a result
                          of cloning requested by the RTF_CLONING flag.  When
                          set, the rt_parent field indicates the route from
                          which this one was generated.
           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.

     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
                   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.

SEE ALSO

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

HISTORY

     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.

AUTHORS

     This manual page was written by Garrett Wollman.

BUGS

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

     There is some disagreement over whether it is legitimate for RTF_LLINFO
     to be set by any process other than rt_ifa->ifa_rtrequest.