ROUTE(4-SVR4) RISC/os Reference Manual ROUTE(4-SVR4)
NAME
NAME
ROUTE - Kernel Packet Forwarding Database
SYNOPSIS
#include <sys/socket.h>
#include <net/if.h>
#include <net/route.h>
int family
s = socket(PF_ROUTE, SOCK_RAW, family);
DESCRIPTION
provides some packet routing facilities. The kernel main-
tains a routing information database, which is used in
selecting the appropriate network interface when transmit-
ting 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'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 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 wild-
card 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 ``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).
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
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described below.
A wildcard routing entry is specified with a zero destina-
tion 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:
The family paramter may be AF_UNSPEC which will provide
routing information for all address families, or can be res-
tricted 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 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, Net-
mask, Gateway, and Author of the redirect. The interpreta-
tion 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
process run locally, and the values for rtm_errno are avail-
able 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() 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 shutdown(2) system call for
further input.
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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 destina-
tion by using a RTM_GET message, or by reading the /dev/kmem
device, or by issuing a getkerninfo () 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 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 */
};
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:
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#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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