route.4

早期freebsd实现

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.\"     @(#)route.4	8.6 (Berkeley) 4/19/94
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.Dd April 19, 1994
.Dt ROUTE 4
.Os
.Sh NAME
.Nm route 
.Nd kernel packet forwarding database
.Sh SYNOPSIS
.Fd #include <sys/socket.h>
.Fd #include <net/if.h>
.Fd #include <net/route.h>
.Ft int
.Fn socket PF_ROUTE SOCK_RAW "int family"
.Sh DESCRIPTION
.Tn UNIX
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
.Xr ioctl 2 Ns '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 receipt 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
.Dq 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 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
listers on the routing control socket described below.
.Pp
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.
.Pp
One opens the channel for passing routing control messages
by using the socket call shown in the synopsis above:
.Pp
The
.Fa family
parameter may be
.Dv 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
.Tn 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
.Tn 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
.Ar rtm_errno
field.
The routing code returns
.Dv EEXIST
if
requested to duplicate an existing entry,
.Dv ESRCH
if
requested to delete a non-existent entry,
or
.Dv ENOBUFS
if insufficient resources were available
to install a new route.
In the current implementation, all routing process run locally,
and the values for
.Ar rtm_errno
are available through the normal
.Em 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
.Xr setsockopt 2
call indicating that the
.Dv SO_USELOOPBACK
option
at the
.Dv SOL_SOCKET
level is to be turned off.
A process may ignore all messages from the routing socket
by doing a 
.Xr 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
.Dv RTM_GET
message,
or by reading the
.Pa /dev/kmem
device, or by issuing a
.Xr getkerninfo 2
system call.
.Pp
Messages include:
.Bd -literal
#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 */
.Ed
.Pp
A message header consists of:
.Bd -literal
struct rt_msghdr {
    u_short rmt_msglen;  /* to skip over non-understood messages */
    u_char  rtm_version; /* future binary compatibility */
    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 */
};
.Ed
.Pp
where
.Bd -literal
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-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 */
};
.Ed
.Pp
Flags include the values:
.Bd -literal
#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 #1 */
#define	RTF_PROTO1    0x8000    /* protocol specific routing flag #2 */
.Ed
.Pp
Specifiers for metric values in rmx_locks and rtm_inits are:
.Bd -literal
#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 */
.Ed
.Pp
Specifiers for which addresses are present in the messages are:
.Bd -literal
#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 */
.Ed