.\" Copyright (c) 1990, 1991 The Regents of the University of California.
.\" %sccs.include.redist.man%
.\" @(#)route.4 8.1 (Berkeley) %G%
.Nd kernel packet forwarding database
.Fd #include <sys/socket.h>
.Fd #include <net/route.h>
.Fn socket PF_ROUTE SOCK_RAW "int family"
provides some packet routing facilities.
The kernel maintains a routing information database, which
is used in selecting the appropriate network interface when
A user process (or possibly multiple co-operating processes)
maintains this database by sending messages over a special kind
This supplants fixed size
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 recipt 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 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
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).
the kernel will first attempt to find a route to the destination host.
Failing that, a search is made for a route to the network of the destination.
Finally, any route to a default
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. Wildcard routes are used
only when the system fails to find a route to the
destination host and network. The combination of wildcard
routes and routing redirects can provide an economical
mechanism for routing traffic.
One opens the channel for passing routing control messasges
by using the socket call shown in the synopsis above:
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 particularly
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
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
requested to duplicate an existing entry,
requested to delete a non-existent entry,
if insufficient resources were available
In the current implementation, all routing process run locally,
are available through the normal
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
level is to be turned off.
A process may ignore all messages from the routing socket
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
#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:
u_short rmt_msglen; /* to skip over non-understood messages */
u_char rtm_version; /* future binary compatability */
u_char rtm_type; /* message type */
u_short rmt_index; /* index for associated ifp */
pid_t rmt_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 */
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 useable */
#define RTF_GATEWAY 0x2 /* destination is a gateway */
#define RTF_HOST 0x4 /* host entry (net otherwise) */
#define RTF_NORMAL 0x8 /* subnet mask is cannonical */
#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 */
Specfiers 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 */
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