ipv4: ipmr: support multiple tables

This patch adds support for multiple independant multicast routing instances,
named "tables".

Userspace multicast routing daemons can bind to a specific table instance by
issuing a setsockopt call using a new option MRT_TABLE. The table number is
stored in the raw socket data and affects all following ipmr setsockopt(),
getsockopt() and ioctl() calls. By default, a single table (RT_TABLE_DEFAULT)
is created with a default routing rule pointing to it. Newly created pimreg
devices have the table number appended ("pimregX"), with the exception of
devices created in the default table, which are named just "pimreg" for
compatibility reasons.

Packets are directed to a specific table instance using routing rules,
similar to how regular routing rules work. Currently iif, oif and mark
are supported as keys, source and destination addresses could be supported
additionally.

Example usage:

- bind pimd/xorp/... to a specific table:

uint32_t table = 123;
setsockopt(fd, IPPROTO_IP, MRT_TABLE, &table, sizeof(table));

- create routing rules directing packets to the new table:

# ip mrule add iif eth0 lookup 123
# ip mrule add oif eth0 lookup 123

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/fib_rules.h

@@ -21,6 +21,7 @@
 #define FIB_RULES_IPV4		AF_INET
 #define FIB_RULES_IPV6		AF_INET6
 #define FIB_RULES_DECNET	AF_DECnet
+#define FIB_RULES_IPMR		128
 
 struct fib_rule_hdr {
 	__u8		family;

include/linux/mroute.h

@@ -27,7 +27,8 @@
 #define MRT_DEL_MFC	(MRT_BASE+5)	/* Delete a multicast forwarding entry	*/
 #define MRT_VERSION	(MRT_BASE+6)	/* Get the kernel multicast version	*/
 #define MRT_ASSERT	(MRT_BASE+7)	/* Activate PIM assert mode		*/
-#define MRT_PIM		(MRT_BASE+8)	/* enable PIM code	*/
+#define MRT_PIM		(MRT_BASE+8)	/* enable PIM code			*/
+#define MRT_TABLE	(MRT_BASE+9)	/* Specify mroute table ID		*/
 
 #define SIOCGETVIFCNT	SIOCPROTOPRIVATE	/* IP protocol privates */
 #define SIOCGETSGCNT	(SIOCPROTOPRIVATE+1)