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NAME    [Toc]    [Back]

     route - kernel packet forwarding database

SYNOPSIS    [Toc]    [Back]

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

     socket(PF_ROUTE, SOCK_RAW, family);

DESCRIPTION    [Toc]    [Back]

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

     A user process (or possibly multiple co-operating processes)
     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

     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 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 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
     mechanism  for routing traffic.  Routes created by redirects
from wildcard
     routes and other routes will be marked cloned,  until  their
     from which they were created has disappeared.

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

   The Routing Socket    [Toc]    [Back]
     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 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 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
     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 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
     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

     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
     with it will not be reclaimed until all references to it are
     User  processes can obtain information about the routing entry to a specific
 destination by using a  RTM_GET  message  or  via  the

     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_OLDADD      0x9     /* caused by SIOCADDRT */
     #define RTM_OLDDEL      0xa     /* caused by SIOCDELRT */
     #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

     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, eg 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  an  if_msghdr  header,   the
     RTM_DELADDR messages use an ifa_msghdr header, the RTM_IFANNOUNCE message
     uses an if_announcemsghdr header, and all other messages use
     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 */

     Only rmx_mtu, rmx_expire, rmx_pksent, and rmx_locks are used
by the kernel
 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 kernel.
  They are left for  compatibility  reasons  with  other

     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 */
     #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 */

     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

     #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, bcast or p-p
dest addr */
     #define RTA_LABEL       0x400   /* route label present */

SEE ALSO    [Toc]    [Back]

     netstat(1), socket(2), sysctl(3), mygate(5), route(8), routed(8)

HISTORY    [Toc]    [Back]

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

OpenBSD      3.6                          April      19,     1994
[ Back ]
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