ROUTE(4)

NAME
route - kernel packet forwarding database

SYNOPSIS
#include <sys/types.h>
#include <sys/time.h>
#include <sys/socket.h>
#include <net/if.h>
#include <net/route.h>
int
socket(PF_ROUTE, SOCK_RAW, int family);

DESCRIPTION
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.

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(2)'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 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 “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 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 family parameter 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.

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.

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 available 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(2) 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.

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, or by issuing a getkerninfo(2) 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 */
        int     rtm_flags;   /* flags, incl kern & message, e.g. DONE */
        int     rtm_addrs;   /* bitmask 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” and the flag bits are as defined in rtentry(9).

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 */

SEE ALSO
route(8), rtentry(9)

HISTORY
A PF_ROUTE protocol family first appeared in 4.3BSD reno.