ChorusOS man pages section 7P: Protocols

ROUTE(7P)

NAME | SYNOPSIS | DESCRIPTION | SEE ALSO | ATTRIBUTES

NAME

route- kernel packet forwarding database

SYNOPSIS

#include <sys/socket.h>
#include <net/if.h>
#include <net/route.h>

family

DESCRIPTION

UNIX provides a number of 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 multiple cooperating processes) maintains this database by sending messages over a special kind of socket. This replaces the fixed size ioctl(2POSIX) 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 redirect or failure to locate a suitable route for a request. The message types are described in greater detail below.

There are two types of routing database entries for a specific host, or for all hosts on a generic subnetwork (specified by a bit mask and value under the mask). A mask of all zeros defines a wildcard or default route. Hierarchical routes are supported.

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. 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 that the packet be sent to the same host as specified in the packet. Otherwise, the interface is requested to address the packet to the gateway listed in the routing entry (in other words, the packet is forwarded).

When routing a packet, the kernel will attempt to find the most specific route to 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 assumed to be supplied with a mask of as many "1"'s 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 listeners on the routing control socket (described in more details 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.

To open the channel for passing routing control messages, use the socket() call shown in the synopsis above.

The family parameter can be AF_UNSPEC, which will provide routing information for all address families. It can be restricted to a specific address family by specifying the one required. There can be more than one routing socket open per system.

Messages consist of a header followed by a number of sockadders (variable length, particularly in the ISO case) ; these are 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 addresses present is provided by a bit mask within the header. The sequence runs from the least significant bit to the most significant bit within the vector.

Any messages sent to the kernel are returned, and copies are sent to all interested listeners. The kernel provides 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 one of the following error conditions:

EEXIST: if requested to duplicate an existing entry,
ESRCH: if requested to delete a non-existent entry,
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.

A process may avoid reading replies to its own messages by issuing a setsockopt(2POSIX) 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(2POSIX) 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 RTM_GET message, or by reading the /dev/kmem device.

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  rtm_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 */
       int      rtm_flags;      /* flags, incl kern & message, e.g. DONE */
       int      rtm_addrs;      /* bit mask identifying sockaddrs in msg */
       pid_t    rmt_pid;        /* identify sender */
       int      rtm_seq;        /* for sender to identify action */
       int      rtm_errno;      /* why failed */
       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-bandwidth product */
        u_long  rmx_sendpipe;   /* outbound delay-bandwidth product */
        u_long  rmx_ssthresh;   /* outbound gateway buffer limit */
        u_long  rmx_rtt;        /* estimated round trip time */
        u_long  rmx_rttvar;     /* estimated rtt variance */
        u_long  rmx_pksent;     /* packets sent using this route */
        u_long  rmx_filler[4];  /* will be used for T/TCP later */
};

Values for rtm_flags include::

#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 link layer (e.g. ARP) */
#define RTF_STATIC      0x800   /* manually added */
#define RTF_BLACKHOLE   0x1000     /* just discard pkts (during updates) */
#define RTF_PROTO2      0x4000     /* protocol specific routing flag */
#define RTF_PROTO1      0x8000     /* protocol specific routing flag */
#define RTF_PRCLONING   0x10000    /* protocol requires cloning */
#define RTF_WASCLONED   0x20000    /* route generated through cloning */
#define RTF_PROTO3      0x40000    /* protocol specific routing flag */
#define RTF_PINNED      0x100000   /* future use */
#define RTF_LOCAL       0x200000   /* route represents a local address */
#define RTF_BROADCAST   0x400000   /* route represents a bcast address */
#define RTF_MULTICAST   0x800000   /* route represents a mcast address */

Specifiers for metric values in rmx_locks and rtm_inits are:

#define RTV_MTU         0x1     /* init or lock _mtu */
#define RTV_HOPCOUNT    0x2     /* init or lock _hopcount */
#define RTV_EXPIRE      0x4     /* init or lock _hopcount */
#define RTV_RPIPE       0x8     /* init or lock _recvpipe */
#define RTV_SPIPE       0x10    /* init or lock _sendpipe */
#define RTV_SSTHRESH    0x20    /* init or lock _ssthresh */
#define RTV_RTT         0x40    /* init or lock _rtt */
#define RTV_RTTVAR      0x80    /* init or lock _rttvar */

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 */
#define RTA_BRD         0x80    /* for NEWADDR, broadcast or p-p dest addr */

ATTRIBUTES

See attributes(5) for descriptions of the following attributes:

ATTRIBUTE TYPE	ATTRIBUTE VALUE
Interface Stability	Evolving

ChorusOS 4.0 Last Revised December 1999

NAME | SYNOPSIS | DESCRIPTION | SEE ALSO | ATTRIBUTES