route(4)

NAME

route - kernel packet forwarding database

SYNOPSIS

#include <sys/types.h>
#include <sys/time.h>
#include <sys/socket.h>
#include <net/if.h>
#include <net/route.h>
int
socket(PF_ROUTE, SOCK_RAW, int family);

DESCRIPTION

FreeBSD 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 re-direct, or fail
ure to locate a suitable route for a request. The message types are de
scribed 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 proto
col specifies the route through each interface as a ``direct'' connection
to the destination host or network. If the route is direct, the trans
port layer of a protocol family usually requests the packet be sent to the
same host specified in the packet. Otherwise, the interface is re
quested to address the packet to the gateway listed in the routing en
try (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 be
ing supplied with a mask of as many ones as there are bits in the desti
nation). If no entry is found, the destination is declared to be unreach
able, 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 destina
tion 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 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), inter
preted 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 interpre
tation 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 pro
cesses 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_USELOOP
BACK option at the SOL_SOCKET level is to be turned off. A process may ig
nore 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 en
try to a specific destination by using a RTM_GET message, or by calling
sysctl(3).
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_LOSING 0x5 /* Kernel Suspects Partition
ing */ #define RTM_REDIRECT 0x6 /* Told to use different
route */ #define RTM_MISS 0x7 /* Lookup failed on this ad
dress */ #define RTM_LOCK 0x8 /* fix specified metrics */ #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_IFINFO 0xe /* iface going up/down etc.
*/ #define RTM_NEWMADDR 0xf /* mcast group membership be
ing added to if */ #define RTM_DELMADDR 0x10 /* mcast group membership be
ing deleted */ #define RTM_IFANNOUNCE 0x11 /* iface arrival/departure */
A message header consists of one of the following:
struct rt_msghdr {: u_short rtm_msglen; /* to skip over non-under; stood messages */
u_char rtm_version; /* future binary compatibil; ity */
u_char rtm_type; /* message type */
u_short rtm_index; /* index for associated ifp; */
int rtm_flags; /* flags, incl. kern & mes; sage, e.g. DONE */
int rtm_addrs; /* bitmask identifying sock; addrs 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 ini; tializing */
struct rt_metrics rtm_rmx; /* metrics themselves */
};
struct if_msghdr {: u_short ifm_msglen; /* to skip over non-under; stood messages */
u_char ifm_version; /* future binary compatibil; ity */
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-under; stood messages */
u_char ifam_version; /* future binary compatibil; ity */
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 ifma_msghdr {: u_short ifmam_msglen; /* to skip over non-under; stood messages */
u_char ifmam_version; /* future binary compatibil; ity */
u_char ifmam_type; /* message type */
int ifmam_addrs; /* like rtm_addrs */
int ifmam_flags; /* value of ifa_flags */
u_short ifmam_index; /* index for associated ifp; */
};
struct if_announcemsghdr {: u_short ifan_msglen; /* to skip over non-under; stood messages */
u_char ifan_version; /* future binary compatibil; ity */
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 and
RTM_DELADDR messages use a ifa_msghdr header, the RTM_NEW
MADDR and
RTM_DELMADDR messages use a ifma_msghdr header, the
RTM_IFANNOUNCE message uses a if_announcemsghdr header, and all other messages
use the
rt_msghdr header.
The ``struct rt_metrics'' and the flag bits are as defined
in rtentry(9).
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 pre
sent */ #define RTA_IFP 0x10 /* interface name sockaddr pre
sent */ #define RTA_IFA 0x20 /* interface addr sockaddr pre
sent */ #define RTA_AUTHOR 0x40 /* sockaddr for author of redi
rect */ #define RTA_BRD 0x80 /* for NEWADDR, broadcast or p
p dest addr */

HISTORY

A PF_ROUTE protocol family first appeared in 4.3BSD-Reno.
BSD November 4, 2004

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In section

See also

route(4)

NAME

SYNOPSIS

DESCRIPTION

SEE ALSO

HISTORY