|
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 superuser.
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, that is, 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.
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 sockaddrs, 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 protocol parameter specifies which messages
an application listening on the routing socket is interested in seeing,
based on the the address family of the sockaddrs present.
Currently, you can specify AF_INET and AF_INET6 to filter the messages seen by the listener, or alternatively,
you can specify AF_UNSPEC to indicate that the listener
is interested in all routing messages.
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(3SOCKET) 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(3SOCKET) 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 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_LOCK 0x8 /* fix specified metrics */
#define RTM_OLDADD 0x9 /* caused by SIOCADDRT */
#define RTM_OLDDEL 0xa /* caused by SIOCDELRT */
#define RTM_RESOLVE 0xb /* request to resolve dst to LL addr */
#define RTM_NEWADDR 0xc /* address being added to iface */
#define RTM_DELADDR 0xd /* address being removed from iface */
#define RTM_IFINFO 0xe /* 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 */
uint_t rtm_inits; /* which values we are initializing */
struct rt_metrics rtm_rmx; /* metrics themselves */
};
|
where
| |
struct rt_metrics {
uint32_t rmx_locks; /* Kernel must leave these values alone */
uint32_t rmx_mtu; /* MTU for this path */
uint32_t rmx_hopcount; /* max hops expected */
uint32_t rmx_expire; /* lifetime for route, e.g., redirect */
uint32_t rmx_recvpipe; /* inbound delay-bandwidth product */
uint32_t rmx_sendpipe; /* outbound delay-bandwidth product */
uint32_t rmx_ssthresh; /* outbound gateway buffer limit */
uint32_t rmx_rtt; /* estimated round trip time */
uint32_t rmx_rttvar; /* estimated rtt variance */
uint32_t rmx_pksent; /* packets sent using this route */
};
/* 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 /* generated by ARP */
#define RTF_STATIC 0x800 /* manually added */
#define RTF_BLACKHOLE 0x1000 /* just discard pkts (during updates) */
#define RTF_PRIVATE 0x2000 /* do not advertise this route */
#define RTF_PROTO2 0x4000 /* protocol specific routing flag #2 */
#define RTF_PROTO1 0x8000 /* protocol specific routing flag #1 */
/* 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 _expire */
#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 */
/* 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 */
|
|
|
