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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 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 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). 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. 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. One opens the
channel for passing routing control messages by using the socket call shown
in the synopsis above: The parameter may be 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 sockadders (now variable length particularly in the
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
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. 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 field. The routing code
returns if requested to duplicate an existing entry, if requested to
delete a non-existent entry, or if insufficient resources were available
to install a new route. In the current implementation, all routing process
run locally, and the values for 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 call indicating that
the option at the level is to be turned off. A process may ignore all
messages from the routing socket by doing a 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 message, or by reading the device, or by issuing
a system call. 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_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: 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 */
}; where 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 */
}; 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 #1 */ #define RTF_PROTO1 0x8000 /* protocol specific
routing flag #2 */ Specifiers 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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