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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 redirect, 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 proto-
     col 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 destination host or network.  If the route is direct, the transport layer of a proto-
     col 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 listeners 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 match-
     ing the destination.  The combination 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 information 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 sockaddrs (now variable length
     particularly in the ISO case), interpreted by position, and delimited by the new length
     entry in the sockaddr.  An example of a message with four addresses might be an ISO redi-
     rect: 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 vector.

     Any messages sent to the kernel are returned, and copies are sent to all interested listen-
     ers.  The kernel will provide the process ID for the sender, and the sender may use an addi-
     tional sequence field to distinguish 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 processes 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 messages from the routing socket by doing a
     shutdown(2) system call for further 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 specific destination by using a RTM_GET message, or by reading the /dev/kmem
     device, or by calling sysctl(3).

     The messages are:

     #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_LOSING      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 */
     #define RTM_NEWADDR     0xc    /* address being added to iface */
     #define RTM_DELADDR     0xd    /* address being removed from iface */
     #define RTM_OOIFINFO    0xe    /* Old (pre-1.5) RTM_IFINFO message */
     #define RTM_OIFINFO     0xf    /* Old (pre-6.0) RTM_IFINFO message */
     #define RTM_IFANNOUNCE  0x10   /* iface arrival/departure */
     #define RTM_IFINFO      0x14   /* iface/link going up/down etc. */
     #define RTM_CHGADDR     0x15   /* address has changed on iface */

     A message header consists of one of the following:

     struct rt_msghdr {
	 u_short rtm_msglen;	    /* to skip over non-understood messages */
	 u_char  rtm_version;	    /* future binary compatibility */
	 u_char  rtm_type;	    /* message type */
	 u_short rtm_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	 rtm_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 metrics we are initializing */
	 struct  rt_metrics rtm_rmx; /* metrics themselves */
     };

     struct if_msghdr {
	 u_short ifm_msglen;	    /* to skip over non-understood messages */
	 u_char  ifm_version;	    /* future binary compatibility */
	 u_char  ifm_type;	    /* message type */
	 int	 ifm_addrs;	    /* like rtm_addrs */
	 int	 ifm_flags;	    /* value of if_flags */
	 u_short ifm_index;	    /* index for associated ifp */
	 struct  if_data ifm_data;  /* statistics and other data about if */
     };

     struct ifa_msghdr {
	 u_short ifam_msglen;	    /* to skip over non-understood messages */
	 u_char  ifam_version;	    /* future binary compatibility */
	 u_char  ifam_type;	    /* message type */
	 int	 ifam_addrs;	    /* like rtm_addrs */
	 int	 ifam_flags;	    /* value of ifa_flags */
	 u_short ifam_index;	    /* index for associated ifp */
	 int	 ifam_metric;	    /* value of ifa_metric */
     };

     struct if_announcemsghdr {
	 u_short ifan_msglen;	    /* to skip over non-understood messages */
	 u_char  ifan_version;	    /* future binary compatibility */
	 u_char  ifan_type;	    /* message type */
	 u_short ifan_index;	    /* index for associated ifp */
	 char	 ifan_name[IFNAMSIZ]; /* if name, e.g. "en0" */
	 u_short ifan_what;	    /* what type of announcement */
     };

     The RTM_IFINFO message uses a if_msghdr header, the RTM_NEWADDR, RTM_CHGADDR, and
     RTM_DELADDR messages use a ifa_msghdr header, the RTM_IFANNOUNCE message uses a
     if_announcemsghdr header, and all other messages use the rt_msghdr header.

     The metrics structure is:

     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-bandwidth product */
	 u_long rmx_sendpipe;	    /* outbound delay-bandwidth product */
	 u_long rmx_ssthresh;	    /* outbound gateway buffer limit */
	 u_long rmx_rtt;	    /* estimated round trip time */
	 u_long rmx_rttvar;	    /* estimated rtt variance */
	 u_long rmx_pksent;	    /* packets sent using this route */
     };

     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_CLONED    0x2000   /* this is a cloned route */
     #define RTF_PROTO2    0x4000   /* protocol specific routing flag */
     #define RTF_PROTO1    0x8000   /* protocol specific routing flag */

     Specifiers for metric values in rmx_locks and rtm_inits are:

     #define RTV_MTU	   0x1	  /* init or lock _mtu */
     #define RTV_HOPCOUNT  0x2	  /* init or lock _hopcount */
     #define RTV_EXPIRE    0x4	  /* init or lock _expire */
     #define RTV_RPIPE	   0x8	  /* init or lock _recvpipe */
     #define RTV_SPIPE	   0x10   /* init or lock _sendpipe */
     #define RTV_SSTHRESH  0x20   /* init or lock _ssthresh */
     #define RTV_RTT	   0x40   /* init or lock _rtt */
     #define RTV_RTTVAR    0x80   /* init or lock _rttvar */

     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 */
     #define RTA_BRD	   0x80   /* for NEWADDR, broadcast or p-p dest addr */
     #define RTA_TAG	   0x100  /* route tag */

SEE ALSO
     socket(2), sysctl(3)

BSD					November 13, 2010				      BSD
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