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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
     Mac OS X 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 mes-sages messages
     sages 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 sub-network subnetwork
     network (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 destination host or net-work. network.
     work.  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 destina-tion. destination.
     tion.  (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
     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 synop-sis synopsis
     sis 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 sockadders (now variable length particu-larly particularly
     larly 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 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 vector.

     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 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 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 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.

     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 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 or interface scope */
         pid_t   rtm_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 */
     #define RTF_IFSCOPE   0x1000000 /* has valid interface scope */

     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