NAME | SYNOPSIS | DESCRIPTION | SEE ALSO | NOTES
#include <sys/types.h> #include <sys/socket.h> #include <net/if.h> #include <net/route.h>int socket(PF_ROUTE, SOCK_RAW, AF_INET);
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(7P). 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:
If requested to duplicate an existing entry
If requested to delete a non-existent entry
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.
#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 { | |
ushort_t rtm_msglen; | /* to skip over non-understood messages */ |
uchar_t rtm_version; | /* future binary compatibility */ |
uchar_t rtm_type; | /* message type */ |
ushort_t 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 */ |
ulong_t rtm_inits; | /* which values we are initializing */ |
struct rt_metrics rtm_rmx; | /* metrics themselves */ |
}; |
where,
struct rt_metrics { | |
ulong_t rmx_locks; | /* Kernel must leave these values alone */ |
ulong_t rmx_mtu; | /* MTU for this path */ |
ulong_t rmx_hopcount; | /* max hops expected */ |
ulong_t rmx_expire; | /* lifetime for route, e.g., redirect */ |
ulong_t rmx_recvpipe; | /* inbound delay-bandwidth product */ |
ulong_t rmx_sendpipe; | /* outbound delay-bandwidth product */ |
ulong_t rmx_ssthresh; | /* outbound gateway buffer limit */ |
ulong_t rmx_rtt; | /* estimated round trip time */ |
ulong_t rmx_rttvar; | /* estimated rtt variance */ |
ulong_t 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 */ |
Some of the metrics may not be implemented and return zero. The implemented metrics are set in rtm_inits.
NAME | SYNOPSIS | DESCRIPTION | SEE ALSO | NOTES