route -- kernel packet forwarding database


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

fd = open(_PATH_ROUTE, O_RDWR);


The kernel 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 stream. This supplants fixed size ioctl(2) calls used in earlier releases. Routing table changes may only be carried out by root.

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 (that is, the packet is routed).

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 listeners on the routing control stream described below.

A wildcard routing entry is specified with a zero destination 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 messages by using the open call shown in the syntax above.

There can be more than one routing stream open per system.

Messages are formed by a header (different headers are used for different message types) followed by a small number of sockaddrs interpreted by position. An example of a message with four addresses might be a Destination, Netmask, Gateway, and Author of the redirect. 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 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 ENOSR or ENOMEM 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 RTSTR_USELOOPBACK ioctl(2) call indicating that the process does not wish to hear routing messages. A process may ignore all messages from the routing stream by opening the driver write-only.

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.

Messages are sent using the RTSTR_SEND ioctl.

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_LOSING	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 */
   #define RTM_WINNING	0xc    /* Partitioning repaired */
   #define RTM_NEWADDR	0xd    /* address being added to iface */
   #define RTM_DELADDR	0xe    /* address being removed from iface */
   #define RTM_IFINFO	0xf    /* interface going up/down etc. */
A routing message header is used for all messages except RTM_NEWADDR, RTM_DELADDR, and RTM_IFINFO. It 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_long  rtm_index;   /* index for associated ifp */
       ushort  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 */
       u_long  rtm_flags;   /* flags, incl kern & message, e.g. DONE */
       int     rtm_refcnt;  /* from rtentry */
       int     rtm_use;     /* from rtentry */
       u_long  rtm_inits;   /* which values we are initializing */
       struct  rt_metrics rtm_rmx;	/* metrics themselves */
       int     rtm_proto;   /* from rtentry: proto for this route */
       time_t  rtm_age;     /* from rtentry: age of this route */
   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_tos;       /* type of service */
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  /* link-layer information present */
   #define	RTF_STATIC    0x800  /* statically created */
   #define	RTF_PROTO2    0x4000 /* protocol-specific */
   #define	RTF_PROTO1    0x8000 /* protocol-specific */
   #define RTF_LOSING    0x10000 /* this router may be dead */
   #define RTF_PMTU      0x20000 /* perform PMTU discovery */
   #define RTF_PMTUMOD   0x40000 /* PMTU discovery modified this route */
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 */
   #define	RTV_TOS       0x100  /* init or lock _tos */
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 */
An interface message header is used with the RTM_IFINFO message. It consists of:
   struct if_msghdr {
       u_short ifm_msglen;      /* to skip over non-understood messages */
       u_char  ifm_version;     /* future binary compatibility */
       u_char  ifm_type;        /* message type */
       int ifm_addrs;           /* like rtm_addrs */
       u_long  ifm_flags;       /* value of if_flags */
       u_long  ifm_index;       /* index for associated ifp */
An interface address message header is used with the RTM_NEWADDR and RTM_DELADDR messages. It consists of:
   struct ifa_msghdr {
       u_short ifam_msglen;   /* to skip over non-understood messages */
       u_char  ifam_version;  /* future binary compatibility */
       u_char  ifam_type;     /* message type */
       int ifam_addrs;        /* like rtm_addrs */
       int ifam_flags;        /* value of ifa_flags */
       u_long  ifam_index;    /* index for associated ifp */
       int ifam_metric;       /* value of ifa_metric (currently unused) */
The entire routing table can be retrieved using the RTSTR_GETROUTE ioctl. This ioctl uses a structure, gi_arg , to inform the kernel about what is desired. Routing table retrieval is normally performed in two steps. The first operation determines the size of the routing table, and the second actually performs the retrieval. This allows a user process to allocate, using malloc(3C), a buffer of sufficient size. The following example shows a typical scenario:
   	char *buf, *next, *lim;
   	register struct rt_msghdr *rtm;
   	int fd;
   	int r;
   	struct rt_giarg gi_arg, *gp;

fd = open(_PATH_ROUTE, O_RDONLY); if (fd < 0) ... error ...

gi_arg.gi_op = KINFO_RT_DUMP; gi_arg.gi_where = (caddr_t)0; gi_arg.gi_size = 0; gi_arg.gi_arg = 0; r = ioctl(fd, RTSTR_GETROUTE, &gi_arg); if (r < 0) ... error ... /* gi_size includes sizeof(gi_arg) */ if ((buf = malloc(gi_arg.gi_size)) == 0) ... error ... gp = (struct rt_giarg *)buf; gp->gi_size = gi_arg.gi_size; gp->gi_op = KINFO_RT_DUMP; gp->gi_where = (caddr_t)buf; gp->gi_arg = 0; r = ioctl(fd, RTSTR_GETROUTE, buf); if (r < 0) .. error ... lim = buf + gp->gi_size; buf += sizeof(gi_arg); for (next = buf; next < lim; next += rtm->rtm_msglen) { rtm = (struct rt_msghdr *)next; ... process message ... }

It is possible to only retrieve routes with certain flags by setting gi_arg to the flag combination desired.


An ioctl operation on the route driver may fail with the errno set to one of the following:

when the associated file descriptor is no longer open/valid.

when an invalid argument is passed to the driver.

insufficient memory resources were available.

insufficient STREAMS resources were available.


arp(7tcp), gated(1Mtcp), ip(7tcp), route(1Mtcp), routed(1Mtcp), streamio(7)
© 2004 The SCO Group, Inc. All rights reserved.
UnixWare 7 Release 7.1.4 - 25 April 2004