routing - system support for packet network routing


     The network facilities provide general packet  routing.  The
     routing interface described here can be used to maintain the
     system's IPv4 routing table. It has been maintained for com-
     patibility  with  older applications. The recommended inter-
     face for maintaining the  system's  routing  tables  is  the
     routing  socket, described at route(7P).  The routing socket
     can be used to manipulate both the  IPv4  and  IPv6  routing
     tables  of  the  system.  Routing  table  maintenance may be
     implemented in applications processes.

     A simple set of data structures compose  a  "routing  table"
     used  in  selecting  the  appropriate network interface when
     transmitting packets.  This table contains  a  single  entry
     for  each  route  to a specific network or host. The routing
     table was designed to support routing for the Internet  Pro-
     tocol  (IP),  but its implementation is protocol independent
     and thus it may serve other protocols as well. User programs
     may  manipulate  this data base with the aid of two ioctl(2)
     commands, SIOCADDRT and SIOCDELRT. These commands allow  the
     addition  and  deletion  of  a  single  routing table entry,
     respectively. Routing table manipulations may only  be  car-
     ried out by privileged user.

     A routing table entry has the following form, as defined  in

     struct rtentry {
             unit_t   rt_hash;               /* to speed lookups */
             struct  sockaddr rt_dst;        /* key */
             struct  sockaddr rt_gateway;    /* value */
             short   rt_flags;               /* up/down?, host/net */
             short   rt_refcnt;              /* # held references */
             unit_t   rt_use;                /* raw # packets forwarded */
      * The kernel does not use this field, and without it the structure is
      * datamodel independent.
     #if !defined(_KERNEL)
             struct  ifnet *rt_ifp;          /* the answer: interface to use */
     #endif                                  /* !defined(_KERNEL) */

     with rt_flags defined from:

     #define RTF_UP 0x1         /* route usable */
     #define RTF_GATEWAY 0x2    /* destination is a gateway */
     #define RTF_HOST 0x4       /* host entry (net otherwise) */

     There are three types of routing table entries: those for  a
     specific  host,  those  for all hosts on a specific network,
     and those for any  destination not matched by entries of the
     first two types, called a  wildcard route.
      Each network interface installs a routing table entry  when
     it  is initialized.  Normally the interface specifies if the
     route through it is a "direct" connection to the destination
     host or network. If the route is direct, the transport layer
     of a protocol family usually requests the packet be sent  to
     the same host specified in the packet. Otherwise, the inter-
     face may be requested to address the  packet  to  an  entity
     different  from  the  eventual  recipient;  essentially, the
     packet is forwarded.

     Routing table entries installed by a user  process  may  not
     specify the hash, reference count, use, or interface fields;
     these are filled in by the routing routines. If a  route  is
     in  use  when  it  is deleted, meaning its rt_refcnt is non-
     zero, the resources associated with it will not be reclaimed
     until all references to it are removed.

     User processes read the routing tables through  the  /dev/ip

     The rt_use field contains the number of packets  sent  along
     the  route.  This  value  is  used  to select among multiple
     routes to the same destination. When multiple routes to  the
     same destination exist, the least used route is selected.

     A wildcard routing entry is specified with a  zero  destina-
     tion  address  value. Wildcard routes are used only when the
     system fails to find a route to  the  destination  host  and
     network.  The  combination  of  wildcard  routes and routing
     redirects can provide an economical  mechanism  for  routing


           A request was made to duplicate an existing entry.

     ESRCH A request was made to delete a non-existent entry.

           Insufficient resources were available to install a new

           Insufficient resources were available to install a new

           The gateway is not directly reachable.   For  example,
           it does not match the destination/subnet on any of the
           network interfaces.


           IP device driver


     route(1M), ioctl(2), route(7P)

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