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

NAME
     route -- kernel packet forwarding database

SYNOPSIS
     #include <&lt;sys/socket.h>&gt;
     #include <&lt;net/if.h>&gt;
     #include <&lt;net/route.h>&gt;

     int
     socket(PF_ROUTE, SOCK_RAW, family);

DESCRIPTION
     OpenBSD 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 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 desti-
     nation 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 listeners on the rout-
     ing 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.  Routes created by redirects from wildcard
     routes and other routes will be marked cloned, until their ``parent''
     from which they were created has disappeared.

     Route labels can be attached to routes and may contain arbitrary informa-
     tion about the route.  Labels are sent over the routing socket (see
     below) as sockaddr_rtlabel structures.

   The Routing Socket
     One opens the channel for passing routing control messages by using the
     socket(2) call shown in the SYNOPSIS above.

     The family parameter may be AF_UNSPEC, which will provide routing infor-
     mation 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 sockaddr
     structures (which are variable length), interpreted by position, and
     delimited by the length entry in the sockaddr.  An example of a message
     with four addresses might be an IPv4 route addition: the destination,
     netmask, gateway, and label, since both netmasks and labels are sent over
     the routing socket as sockaddr structures.  The interpretation of which
     addresses 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 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 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 mes-
     sages from the routing socket by doing a shutdown(2) system call for fur-
     ther input.

     A process can specify an alternate routing table by using the SO_RTABLE
     setsockopt(2).

     A process can specify which route message types it's interested in by
     using ROUTE_FILTER(int type) and issuing a setsockopt call with the
     ROUTE_MSGFILTER option at the AF_ROUTE level.  For example, to only get
     interface specific messages:

           unsigned int rtfilter;

           rtfilter = ROUTE_FILTER(RTM_IFINFO) |
               ROUTE_FILTER(RTM_IFANNOUNCE);

           if (setsockopt(routefd, PF_ROUTE, ROUTE_MSGFILTER,
               &rtfilter, sizeof(rtfilter)) == -1)
                   err(1, "setsockopt(ROUTE_MSGFILTER)");

     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 via the PF_ROUTE
     sysctl(3).

     Messages include:

     #define RTM_ADD         0x1     /* Add Route */
     #define RTM_DELETE      0x2     /* Delete Route */
     #define RTM_CHANGE      0x3     /* Change Metrics or flags */
     #define RTM_GET         0x4     /* Report Metrics */
     #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_LOCK        0x8     /* fix specified metrics */
     #define RTM_RESOLVE     0xb     /* req 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_IFINFO      0xe     /* iface going up/down etc. */
     #define RTM_IFANNOUNCE  0xf     /* iface arrival/departure */
     #define RTM_DESYNC      0x10    /* route socket buffer overflow */

     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_hdrlen;     /* sizeof(rt_msghdr) to skip over the header */
             u_short rtm_index;      /* index for associated ifp */
             u_short rtm_tableid;    /* routing table id */
             u_char  rtm_priority;   /* routing priority */
             u_char  rtm_mpls;       /* MPLS additional infos */
             int     rtm_addrs;      /* bitmask identifying sockaddrs in msg */
             int     rtm_flags;      /* flags, incl. kern & message, e.g. DONE */
             int     rtm_fmask;      /* bitmask used in RTM_CHANGE message */
             pid_t   rtm_pid;        /* identify sender */
             int     rtm_seq;        /* for sender to identify action */
             int     rtm_errno;      /* why failed */
             u_int   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 */
             u_short ifm_hdrlen;     /* sizeof(if_msghdr) to skip over the header */
             u_short ifm_index;      /* index for associated ifp */
             u_short ifm_tableid;    /* routing table id */
             u_char  ifm_pad1;
             u_char  ifm_pad2;
             int     ifm_addrs;      /* like rtm_addrs */
             int     ifm_flags;      /* value of if_flags */
             int     ifm_xflags;
             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 */
             u_short ifam_hdrlen;    /* sizeof(ifa_msghdr) to skip over the header */
             u_short ifam_index;     /* index for associated ifp */
             u_short ifam_tableid;   /* routing table id */
             u_char  ifam_pad1;
             u_char  ifam_pad2;
             int     ifam_addrs;     /* like rtm_addrs */
             int     ifam_flags;     /* value of ifa_flags */
             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_hdrlen;    /* sizeof(ifa_msghdr) to skip over the header */
             u_short ifan_index;     /* index for associated ifp */
             u_short ifan_what;      /* what type of announcement */
             char    ifan_name[IFNAMSIZ];    /* if name, e.g. "en0" */
     };

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

     The metrics structure is:

     struct rt_metrics {
             u_int64_t       rmx_pksent;     /* packets sent using this route */
             int64_t         rmx_expire;     /* lifetime for route, e.g. redirect */
             u_int           rmx_locks;      /* Kernel must leave these values */
             u_int           rmx_mtu;        /* MTU for this path */
             u_int           rmx_refcnt;     /* # references hold */
             u_int           rmx_hopcount;   /* max hops expected */
             u_int           rmx_recvpipe;   /* inbound delay-bandwidth product */
             u_int           rmx_sendpipe;   /* outbound delay-bandwidth product */
             u_int           rmx_ssthresh;   /* outbound gateway buffer limit */
             u_int           rmx_rtt;        /* estimated round trip time */
             u_int           rmx_rttvar;     /* estimated rtt variance */
             u_int           rmx_pad;
     };

     Only rmx_mtu, rmx_expire, rmx_pksent, and rmx_locks are used by the ker-
     nel routing table.  All other values will be ignored when inserting them
     into the kernel and are set to zero in routing messages sent by the ker-
     nel.  They are left for compatibility reasons with other systems.

     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_PROTO3    0x2000    /* protocol specific routing flag */
     #define RTF_PROTO2    0x4000    /* protocol specific routing flag */
     #define RTF_PROTO1    0x8000    /* protocol specific routing flag */
     #define RTF_CLONED    0x10000   /* this is a cloned route */
     #define RTF_MPATH     0x40000   /* multipath route or operation */
     #define RTF_MPLS      0x100000  /* MPLS additional infos */
     #define RTF_LOCAL     0x200000  /* route to a local address */
     #define RTF_BROADCAST 0x400000  /* route associated to a bcast addr. */

     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 _hopcount */
     #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 */

     Only RTV_MTU and RTV_EXPIRE should be used; all other flags are ignored.

     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_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, bcast or p-p dest addr */
     #define RTA_SRC         0x100   /* source sockaddr present */
     #define RTA_SRCMASK     0x200   /* source netmask present */
     #define RTA_LABEL       0x400   /* route label present */

SEE ALSO
     netstat(1), socket(2), sysctl(3), mygate(5), route(8), route(9)

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

BSD                             April 30, 2017                             BSD