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

NAME
     route — kernel packet forwarding database

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

     int
     socket(PF_ROUTE, SOCK_RAW, int family);

DESCRIPTION
     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
     interfaces, 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
     address 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‐
     tor.

     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
     locally, 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 */
	 pid_t	 rmt_pid;     /* identify sender */
	 int	 rtm_addrs;   /* bitmask identifying sockaddrs in msg */
	 int	 rtm_seq;     /* for sender to identify action */
	 int	 rtm_errno;   /* why failed */
	 int	 rtm_flags;   /* flags, incl kern & message, e.g. DONE */
	 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 {
	 u_long rmx_locks;     /* Kernel must leave these values alone */
	 u_long rmx_mtu;       /* MTU for this path */
	 u_long rmx_hopcount;  /* max hops expected */
	 u_long rmx_expire;    /* lifetime for route, e.g. redirect */
	 u_long rmx_recvpipe;  /* inbound delay-bandwith product */
	 u_long rmx_sendpipe;  /* outbound delay-bandwith product */
	 u_long rmx_ssthresh;  /* outbound gateway buffer limit */
	 u_long rmx_rtt;       /* estimated round trip time */
	 u_long rmx_rttvar;    /* estimated rtt variance */
     };

     Flags include the values:

     #define RTF_UP	   0x1	     /* route usable */
     #define RTF_GATEWAY   0x2	     /* destination is a gateway */
     #define RTF_HOST	   0x4	     /* host entry (net otherwise) */
     #define RTF_REJECT	   0x8	     /* host or net unreachable */
     #define RTF_DYNAMIC   0x10	     /* created dynamically (by redirect) */
     #define RTF_MODIFIED  0x20	     /* modified dynamically (by redirect) */
     #define RTF_DONE	   0x40	     /* message confirmed */
     #define RTF_MASK	   0x80	     /* subnet mask present */
     #define RTF_CLONING   0x100     /* generate new routes on use */
     #define RTF_XRESOLVE  0x200     /* external daemon resolves name */
     #define RTF_LLINFO	   0x400     /* generated by ARP or ESIS */
     #define RTF_STATIC	   0x800     /* manually added */
     #define RTF_BLACKHOLE 0x1000    /* just discard pkts (during updates) */
     #define RTF_PROTO2	   0x4000    /* protocol specific routing flag #1 */
     #define RTF_PROTO1	   0x8000    /* protocol specific routing flag #2 */

     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 */

BSD				April 19, 1994				   BSD
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