routing(6)                      TCP/IP 5.4.2                      routing(6)


NAME
       routing - network packet routing

SYNOPSIS
       #include <net/route.h>

DESCRIPTION
       The network facilities provide limited packet routing.  A simple set
       of data structures comprise 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.
       A user program, route(1M), allows the operator to configure this
       database with the aid of two socket specific ioctl(2), commands,
       SIOCADDRT and SIOCDELRT.  The commands allow the addition and
       deletion of a single routing table entry, respectively.  Routing
       table manipulations may be carried out only by the superuser.

       A routing table entry has the following form, as defined in
       net/route.h;

            struct rtentry {
                struct    sockaddr rtdst;
                struct    sockaddr rtgateway;
                short     rtflags;
                };

       with rtflags defined from the following:

          #define  RTF_UP        0x1   /* route useable */
          #define  RTF_GATEWAY   0x2   /* destination is a gateway */
          #define  RTF_HOST      0x4   /* host entry (net otherwise) */
          #define  RTF_DYNAMIC   0x10  /* created dynamically (by redirect) */
          #define  RTF_MODIFIED  0x20  /* modified dynamically (by redirect) */

       Routing table entries come in three flavors: for a specific host, for
       all hosts on a specific network, and for any other destination not
       matched by entries of the first two types (a wildcard route).  When
       an interface is started, it installs a routing table entry.
       Normally, the interface specifies that the route coming 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 that the packet be sent to the same host specified in the
       packet.  Otherwise, the interface may be requested to address the
       packet to an entity different from the eventual recipient (that is,
       the packet is forwarded).

       The routing code returns EEXIST if requested to duplicate an existing
       entry, ESRCH if requested to delete a non-existent entry, ENOMEM if
       insufficient resources were available to install a new route, or
       ENETUNREACH if the gateway specified is not on the same network as
       the host system.

       The routing policy for transmitting packets is as follows:




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routing(6)                      TCP/IP 5.4.2                      routing(6)


        1)    When a single host route exists, it is used.  When multiple
              host routes exist, the first route entry found is used.

       2)     When host routes don't exist, but a network route does, then
              the network route is used.  When multiple network routes
              exist, the first route entry found is used.

       3)     When neither host nor network routes exist, but a wildcard
              route exists, then the wildcard route is used.

       4)     When host, network, and wildcard routes don't exist, the
              packet cannot be routed and is dropped.

       A wildcard routing entry is specified with a zero destination 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 traffic.

SEE ALSO
       ioctl(2), socket(2).




































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