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.\" Copyright (c) 1990 The Regents of the University of California.
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.\"	@(#)route.4	6.2 (Berkeley) 5/30/90
.\"
.TH ROUTE 4 "May 30, 1990"
.UC 5
.SH NAME
.UC 4
.SH NAME
ROUTE \- Kernel Packet Forwarding Database
.SH SYNOPSIS
.nf
\fB#include <sys/socket.h>\fR
\fB#include <net/if.h>\fR
\fB#include <net/route.h>\fR
.br
.B int family
.br
.B s = socket(PF_ROUTE, SOCK_RAW, family);
.SH DESCRIPTION
.PP
.UX
provides some packet routing facilities.
The kernel maintains a routing information database, which
is used in selecting the appropriate network interface when
transmitting packets.
.PP
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
.IR ioctl 's
used in earlier releases.
Routing table changes may only be carried out by the super user.
.PP
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.
.PP
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.
.PP
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).
.PP
When routing a packet,
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 (``wildcard'') gateway is chosen.
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.
.PP
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.
.PP
One opens the channel for passing routing control messasges
by using the socket call shown in the synopsis above:
.PP
The \fIfamily\fP paramter 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.
.PP
Messages are formed by a header followed by a small
number of sockadders (now variable length particularly
in the ISO 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 ISO redirect:
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.
.PP
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.
.PP
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 process run locally,
and the values for rtm_errno are available through the normal
errno mechanism, even if the routing reply message is lost.
.PP
A process may avoid the expense of reading replies to
its own messages by issuing a
.IR setsockopt ()
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 
.IR shutdown (2)
system call for further input.
.PP
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 by reading the
.I /dev/kmem 
device, or by issuing a
.I getkerninfo ()
system call.
.nf

Messages include:

.ta \w'#define  'u +\w'RTM_REDIRECT  'u +\w'0x7  'u
#define	RTM_ADD	0x1	/* \fIAdd Route\fP */
#define	RTM_DELETE	0x2	/* \fIDelete Route\fP */
#define	RTM_CHANGE	0x3	/* \fIChange Metrics, Flags, or Gateway\fP */
#define	RTM_GET	0x4	/* \fIReport Information\fP */
#define	RTM_LOOSING	0x5	/* \fIKernel Suspects Partitioning\fP */
#define	RTM_REDIRECT	0x6	/* \fITold to use different route\fP */
#define	RTM_MISS	0x7	/* \fILookup failed on this address\fP */
#define	RTM_RESOLVE	0xb	/* \fIrequest to resolve dst to LL addr\fP */

.ta \w'struct  'u +\w'u_short   'u +\w'rt_metrics rtm_rmx    'u
A message header consists of:
struct rt_msghdr {
	u_short	rmt_msglen;	/* \fIto skip over non-understood messages\fP */
	u_char	rtm_version;	/* \fIfuture binary compatability\fP */
	u_char	rtm_type;	/* \fImessage type\fP */
	u_short	rmt_index;	/* \fIindex for associated ifp \fP */
	pid_t	rmt_pid;	/* \fIidentify sender\fP */
	int	rtm_addrs;	/* \fIbitmask identifying sockaddrs in msg\fP */
	int	rtm_seq;	/* \fIfor sender to identify action\fP */
	int	rtm_errno;	/* \fIwhy failed\fP */
	int	rtm_flags;	/* \fIflags, incl. kern & message, e.g. DONE\fP */
	int	rtm_use;	/* \fIfrom rtentry\fP */
	u_long	rtm_inits;	/* \fIwhich values we are initializing\fP */
	struct	rt_metrics rtm_rmx;	/* \fImetrics themselves\fP */
};

where

.ta \w'struct  'u +\w'u_long  'u +\w'rmx_hopcount    'u
struct rt_metrics {
	u_long	rmx_locks;	/* \fIKernel must leave these values alone\fP */
	u_long	rmx_mtu;	/* \fIMTU for this path\fP */
	u_long	rmx_hopcount;	/* \fImax hops expected\fP */
	u_long	rmx_expire;	/* \fIlifetime for route, e.g. redirect\fP */
	u_long	rmx_recvpipe;	/* \fIinbound delay-bandwith product\fP */
	u_long	rmx_sendpipe;	/* \fIoutbound delay-bandwith product\fP */
	u_long	rmx_ssthresh;	/* \fIoutbound gateway buffer limit\fP */
	u_long	rmx_rtt;	/* \fIestimated round trip time\fP */
	u_long	rmx_rttvar;	/* \fIestimated rtt variance\fP */
};


Flags include the values:

.ta \w'#define  'u +\w'RTF_MODIFIED  'u +\w'0x80     'u
#define	RTF_UP	0x1	/* \fIroute useable\fP */
#define	RTF_GATEWAY	0x2	/* \fIdestination is a gateway\fP */
#define	RTF_HOST	0x4	/* \fIhost entry (net otherwise)\fP */
#define	RTF_NORMAL	0x8	/* \fIsubnet mask is cannonical\fP */
#define	RTF_DYNAMIC	0x10	/* \fIcreated dynamically (by redirect)\fP */
#define	RTF_MODIFIED	0x20	/* \fImodified dynamically (by redirect)\fP */
#define	RTF_DONE	0x40	/* \fImessage confirmed\fP */
#define	RTF_MASK	0x80	/* \fIsubnet mask present\fP */

Specfiers for metric values in rmx_locks and rtm_inits are:

.ta \w'#define  'u +\w'RTF_HOPCOUNT  'u +\w'0x80     'u
#define	RTV_SSTHRESH	0x1	/* \fIinit or lock _ssthresh\fP */
#define	RTV_RPIPE	0x2	/* \fIinit or lock _recvpipe\fP */
#define	RTV_SPIPE	0x4	/* \fIinit or lock _sendpipe\fP */
#define	RTV_HOPCOUNT	0x8	/* \fIinit or lock _hopcount\fP */
#define	RTV_RTT	0x10	/* \fIinit or lock _rtt\fP */
#define	RTV_RTTVAR	0x20	/* \fIinit or lock _rttvar\fP */
#define	RTV_MTU	0x40	/* \fIinit or lock _mtu\fP */

Specifiers for which addresses are present in the messages are:

.ta \w'#define  'u +\w'RTA_GATEWAY  'u +\w'0x80     'u
#define RTA_DST	0x1	/* \fIdestination sockaddr present\fP */
#define RTA_GATEWAY	0x2	/* \fIgateway sockaddr present\fP */
#define RTA_NETMASK	0x4	/* \fInetmask sockaddr present\fP */
#define RTA_GENMASK	0x8	/* \fIcloning mask sockaddr present\fP */
#define RTA_IFP	0x10	/* \fIinterface name sockaddr present\fP */
#define RTA_IFA	0x20	/* \fIinterface addr sockaddr present\fP */
#define RTA_AUTHOR	0x40	/* \fIsockaddr for author of redirect\fP */

.fi

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