route(4)

NAME

route − kernel packet forwarding database

SYNOPSIS

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

int socket(PF_ROUTE, SOCK_RAW, AF_INET);

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 specified in routing(4). 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: entries for a specific host, or entries 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, the internet 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 attempts to find the most specific route matching 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 below). 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.

Note: a wildcard routing entry is specified with a zero destination address value, and a mask of all zeroes. Wildcard routes are 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 section above. There can be more than one routing socket open per system.

Messages are formed by a header followed by a small number of sockadders, whose length depend on the address family. Sockaddrs are interpreted by position. An example of a type of message with three addresses might be a CIDR prefix route: Destination, Netmask, and Gateway. The interpretation of which addresses 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 provides the process ID of 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 of the rt_msghdr struct (see below). The following codes may be returned:

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 processes 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(3N) 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(3N) system call for further input.

If a route is in use when it is deleted, the routing entry is marked down and removed from the routing table, but the resources associated with it are not reclaimed until all references to it are released.

Messages

User processes can obtain information about the routing entry to a specific destination by using a RTM_GET message.

Messages include:

#define RTM_ADD/∗ Add Route ∗/
#define RTM_DELETE/∗ Delete Route ∗/
#define RTM_CHANGE/∗ Change Metrics, Flags, or Gateway ∗/
#define RTM_GET/∗ Report Information ∗/
#define RTM_LOOSING/∗ Kernel Suspects Partitioning ∗/
#define RTM_REDIRECT/∗ Told to use different route ∗/
#define RTM_MISS/∗ Lookup failed on this address ∗/
#define RTM_RESOLVE/∗ request to resolve dst to LL addr ∗/
#define RTM_NEWADDR/∗ address being added to iface ∗/
#define RTM_DELADDR/∗ address being removed from iface ∗/
#define RTM_IFINFO/∗ iface going up/down etc. ∗/

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 rtm_index;	/∗ index for associated ifp ∗/
pid_t rtm_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-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 ∗/
};

Flags include the values:

#define	RTF_UP	/∗ route usable ∗/
#define	RTF_GATEWAY	/∗ destination is a gateway ∗/
#define	RTF_HOST	/∗ host entry (net otherwise) ∗/
#define	RTF_REJECT	/∗ host or net unreachable ∗/
#define	RTF_DYNAMIC	/∗ created dynamically (by redirect) ∗/
#define	RTF_MODIFIED	/∗ modified dynamically (by redirect) ∗/
#define	RTF_DONE	/∗ message confirmed ∗/
#define	RTF_MASK	/∗ subnet mask present ∗/
#define	RTF_CLONING	/∗ generate new routes on use ∗/
#define	RTF_XRESOLVE	/∗ external daemon resolves name ∗/
#define	RTF_LLINFO	/∗ generated by ARP ∗/
#define	RTF_STATIC	/∗ manually added ∗/
#define	RTF_BLACKHOLE	/∗ just discard pkts (during updates) ∗/
#define	RTF_PROTO1	/∗ protocol specific routing flag #1 ∗/
#define	RTF_PROTO2	/∗ protocol specific routing flag #2 ∗/

Specifiers for metric values in rmx_locks and rtm_inits are:

#define	RTV_MTU	/∗ init or lock _mtu ∗/
#define	RTV_HOPCOUNT	/∗ init or lock _hopcount ∗/
#define	RTV_RPIPE	/∗ init or lock _recvpipe ∗/
#define	RTV_SPIPE	/∗ init or lock _sendpipe ∗/
#define	RTV_SSTHRESH	/∗ init or lock _ssthresh ∗/
#define	RTV_RTT	/∗ init or lock _rtt ∗/
#define	RTV_RTTVAR	/∗ init or lock _rttvar ∗/

Specifiers for which addresses are present in the messages are:

#define	RTA_DST	/∗ destination sockaddr present ∗/
#define	RTA_GATEWAY	/∗ gateway sockaddr present ∗/
#define	RTA_NETMASK	/∗ netmask sockaddr present ∗/
#define	RTA_GENMASK	/∗ cloning mask sockaddr present ∗/
#define	RTA_IFP	/∗ interface name sockaddr present ∗/
#define	RTA_IFA	/∗ interface addr sockaddr present ∗/
#define	RTA_AUTHOR	/∗ sockaddr for author of redirect ∗/
#define	RTA_BRD	/∗ for NEWADDR, broadcast or p-p dest addr ∗/

NOTES

Some of the metrics may not be implemented and return zero. The implemented metrics are set in rtm_inits.

SunOS 5.6 — Last change: 7 Jan 1997

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