route - kernel packet forwarding database
#include <sys/socket.h>
#include <net/if.h>
#include <net/route.h>
int
socket(PF_ROUTE, SOCK_RAW, family);
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 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
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 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
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 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 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_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_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_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
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_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
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, bcast or p-p
dest addr */
#define RTA_LABEL 0x400 /* route label present */
netstat(1), socket(2), sysctl(3), mygate(5), route(8), routed(8)
A PF_ROUTE protocol family first appeared in 4.3BSD-Reno.
OpenBSD 3.6 April 19, 1994
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