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ROUTE(4)		 BSD Kernel Interfaces Manual		      ROUTE(4)

     route -- kernel packet forwarding database

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

     socket(PF_ROUTE, SOCK_RAW,	int family);

     UNIX provides some	packet routing facilities.  The	kernel maintains a
     routing information database, which is used in selecting the appropriate
     network interface when transmitting packets.

     A user process (or	possibly multiple co-operating processes) maintains
     this database by sending messages over a special kind of socket.  This
     supplants fixed size ioctl(2)'s used in earlier releases.	Routing	table
     changes may only be carried out by	the super user.

     The operating system may spontaneously emit routing messages in response
     to	external events, such as receipt of a re-direct, or failure to locate
     a suitable	route for a request.  The message types	are described in
     greater detail below.

     Routing database entries come in two flavors: for a specific host,	or for
     all hosts on a generic subnetwork (as specified by	a bit mask and value
     under the mask.  The effect of wildcard or	default	route may be achieved
     by	using a	mask of	all zeros, and there may be hierarchical routes.

     When the system is	booted and addresses are assigned to the network in-
     terfaces, each protocol family installs a routing table entry for each
     interface when it is ready	for traffic.  Normally the protocol specifies
     the route through each interface as a "direct" connection to the destina-
     tion 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 interface	is requested to	ad-
     dress the packet to the gateway listed in the routing entry (i.e. the
     packet is forwarded).

     When routing a packet, the	kernel will attempt to find the	most specific
     route matching the	destination.  (If there	are two	different mask and
     value-under-the-mask pairs	that match, the	more specific is the one with
     more bits in the mask.  A route to	a host is regarded as being supplied
     with a mask of as many ones as there are bits in the destination).	 If no
     entry is found, the destination is	declared to be unreachable, and	a
     routing-miss message is generated if there	are any	listers	on the routing
     control socket described below.

     A wildcard	routing	entry is specified with	a zero destination address
     value, and	a mask of all zeroes.  Wildcard	routes will be used when the
     system fails to find other	routes matching	the destination.  The combina-
     tion of wildcard routes and routing redirects can provide an economical
     mechanism for routing traffic.

     One opens the channel for passing routing control messages	by using the
     socket call shown in the synopsis above:

     The family	parameter may be AF_UNSPEC which will provide routing informa-
     tion for all address families, or can be restricted to a specific address
     family by specifying which	one is desired.	 There can be more than	one
     routing socket open per system.

     Messages are formed by a header followed by a small number	of sockadders
     (now variable length particularly in the ISO case), interpreted by	posi-
     tion, and delimited by the	new length entry in the	sockaddr.  An example
     of	a message with four addresses might be an ISO redirect:	Destination,
     Netmask, Gateway, and Author of the redirect.  The	interpretation of
     which address are present is given	by a bit mask within the header, and
     the sequence is least significant to most significant bit within the vec-

     Any messages sent to the kernel are returned, and copies are sent to all
     interested	listeners.  The	kernel will provide the	process	id. for	the
     sender, and the sender may	use an additional sequence field to distin-
     guish between outstanding messages.  However, message replies may be lost
     when kernel buffers are exhausted.

     The kernel	may reject certain messages, and will indicate this by filling
     in	the rtm_errno field.  The routing code returns EEXIST if requested to
     duplicate an existing entry, ESRCH	if requested to	delete a non-existent
     entry, or ENOBUFS if insufficient resources were available	to install a
     new route.	 In the	current	implementation,	all routing process run	lo-
     cally, and	the values for rtm_errno are available through the normal
     errno mechanism, even if the routing reply	message	is lost.

     A process may avoid the expense of	reading	replies	to its own messages by
     issuing a setsockopt(2) call indicating that the SO_USELOOPBACK option at
     the SOL_SOCKET level is to	be turned off.	A process may ignore all mes-
     sages from	the routing socket by doing a shutdown(2) system call for fur-
     ther input.

     If	a route	is in use when it is deleted, the routing entry	will be	marked
     down and removed from the routing table, but the resources	associated
     with it will not be reclaimed until all references	to it are released.
     User processes can	obtain information about the routing entry to a	spe-
     cific destination by using	a RTM_GET message, or by reading the /dev/kmem
     device, or	by issuing a getkerninfo(2) system call.

     Messages include:

     #define RTM_ADD	     0x1    /* Add Route */
     #define RTM_DELETE	     0x2    /* Delete Route */
     #define RTM_CHANGE	     0x3    /* Change Metrics, Flags, or Gateway */
     #define RTM_GET	     0x4    /* Report Information */
     #define RTM_LOOSING     0x5    /* Kernel Suspects Partitioning */
     #define RTM_REDIRECT    0x6    /* Told to use different route */
     #define RTM_MISS	     0x7    /* Lookup failed on	this address */
     #define RTM_RESOLVE     0xb    /* request to resolve dst to LL addr */

     A message header consists of:

     struct rt_msghdr {
	 u_short rmt_msglen;  /* to skip over non-understood messages */
	 u_char	 rtm_version; /* future	binary compatibility */
	 u_char	 rtm_type;    /* message type */
	 u_short rmt_index;   /* index for associated ifp */
	 int	 rtm_flags;   /* flags,	incl kern & message, e.g. DONE */
	 int	 rtm_addrs;   /* bitmask identifying sockaddrs in msg */
	 pid_t	 rmt_pid;     /* identify sender */
	 int	 rtm_seq;     /* for sender to identify	action */
	 int	 rtm_errno;   /* why failed */
	 int	 rtm_use;     /* from rtentry */
	 u_long	 rtm_inits;   /* which values we are initializing */
	 struct	 rt_metrics rtm_rmx; /*	metrics	themselves */

     where "struct rt_metrics" and the flag bits are as	defined	in rtentry(9).

     Specifiers	for metric values in rmx_locks and rtm_inits are:

     #define RTV_SSTHRESH  0x1	  /* init or lock _ssthresh */
     #define RTV_RPIPE	   0x2	  /* init or lock _recvpipe */
     #define RTV_SPIPE	   0x4	  /* init or lock _sendpipe */
     #define RTV_HOPCOUNT  0x8	  /* init or lock _hopcount */
     #define RTV_RTT	   0x10	  /* init or lock _rtt */
     #define RTV_RTTVAR	   0x20	  /* init or lock _rttvar */
     #define RTV_MTU	   0x40	  /* init or lock _mtu */

     Specifiers	for which addresses are	present	in the messages	are:

     #define RTA_DST	   0x1	  /* destination sockaddr present */
     #define RTA_GATEWAY   0x2	  /* gateway sockaddr present */
     #define RTA_NETMASK   0x4	  /* netmask sockaddr present */
     #define RTA_GENMASK   0x8	  /* cloning mask sockaddr present */
     #define RTA_IFP	   0x10	  /* interface name sockaddr present */
     #define RTA_IFA	   0x20	  /* interface addr sockaddr present */
     #define RTA_AUTHOR	   0x40	  /* sockaddr for author of redirect */

     route(8), rtentry(9)

     A PF_ROUTE	protocol family	first appeared in 4.3BSD-Reno.

BSD				October	8, 1996				   BSD


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