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#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);
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(S)s 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 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 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 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(S) 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 */
};
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_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(S),
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;It is possible to only retrieve routes with certain flags by setting gi_arg to the flag combination desired.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 ... }