Blame - Documentation/networking/policy-routing.txt - kernel/msm

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Linus Torvalds	1da177e	2005-04-16 15:20:36 -0700	[diff] [blame]	1	Classes
				2	-------
				3
				4	"Class" is a complete routing table in common sense.
				5	I.e. it is tree of nodes (destination prefix, tos, metric)
				6	with attached information: gateway, device etc.
				7	This tree is looked up as specified in RFC1812 5.2.4.3
				8	1. Basic match
				9	2. Longest match
				10	3. Weak TOS.
				11	4. Metric. (should not be in kernel space, but they are)
				12	5. Additional pruning rules. (not in kernel space).
				13
				14	We have two special type of nodes:
				15	REJECT - abort route lookup and return an error value.
				16	THROW - abort route lookup in this class.
				17
				18
				19	Currently the number of classes is limited to 255
				20	(0 is reserved for "not specified class")
				21
				22	Three classes are builtin:
				23
				24	RT_CLASS_LOCAL=255 - local interface addresses,
				25	broadcasts, nat addresses.
				26
				27	RT_CLASS_MAIN=254 - all normal routes are put there
				28	by default.
				29
				30	RT_CLASS_DEFAULT=253 - if ip_fib_model==1, then
				31	normal default routes are put there, if ip_fib_model==2
				32	all gateway routes are put there.
				33
				34
				35	Rules
				36	-----
				37	Rule is a record of (src prefix, src interface, tos, dst prefix)
				38	with attached information.
				39
				40	Rule types:
				41	RTP_ROUTE - lookup in attached class
				42	RTP_NAT - lookup in attached class and if a match is found,
				43	translate packet source address.
				44	RTP_MASQUERADE - lookup in attached class and if a match is found,
				45	masquerade packet as sourced by us.
				46	RTP_DROP - silently drop the packet.
				47	RTP_REJECT - drop the packet and send ICMP NET UNREACHABLE.
				48	RTP_PROHIBIT - drop the packet and send ICMP COMM. ADM. PROHIBITED.
				49
				50	Rule flags:
				51	RTRF_LOG - log route creations.
				52	RTRF_VALVE - One way route (used with masquerading)
				53
				54	Default setup:
				55
				56	root@amber:/pub/ip-routing # iproute -r
				57	Kernel routing policy rules
				58	Pref Source Destination TOS Iface Cl
				59	0 default default 00 * 255
				60	254 default default 00 * 254
				61	255 default default 00 * 253
				62
				63
				64	Lookup algorithm
				65	----------------
				66
				67	We scan rules list, and if a rule is matched, apply it.
				68	If a route is found, return it.
				69	If it is not found or a THROW node was matched, continue
				70	to scan rules.
				71
				72	Applications
				73	------------
				74
				75	1. Just ignore classes. All the routes are put into MAIN class
				76	(and/or into DEFAULT class).
				77
				78	HOWTO: iproute add PREFIX [ tos TOS ] [ gw GW ] [ dev DEV ]
				79	[ metric METRIC ] [ reject ] ... (look at iproute utility)
				80
				81	or use route utility from current net-tools.
				82
				83	2. Opposite case. Just forget all that you know about routing
				84	tables. Every rule is supplied with its own gateway, device
				85	info. record. This approach is not appropriate for automated
				86	route maintenance, but it is ideal for manual configuration.
				87
				88	HOWTO: iproute addrule [ from PREFIX ] [ to PREFIX ] [ tos TOS ]
				89	[ dev INPUTDEV] [ pref PREFERENCE ] route [ gw GATEWAY ]
				90	[ dev OUTDEV ] .....
				91
				92	Warning: As of now the size of the routing table in this
				93	approach is limited to 256. If someone likes this model, I'll
				94	relax this limitation.
				95
				96	3. OSPF classes (see RFC1583, RFC1812 E.3.3)
				97	Very clean, stable and robust algorithm for OSPF routing
				98	domains. Unfortunately, it is not widely used in the Internet.
				99
				100	Proposed setup:
				101	255 local addresses
				102	254 interface routes
				103	253 ASE routes with external metric
				104	252 ASE routes with internal metric
				105	251 inter-area routes
				106	250 intra-area routes for 1st area
				107	249 intra-area routes for 2nd area
				108	etc.
				109
				110	Rules:
				111	iproute addrule class 253
				112	iproute addrule class 252
				113	iproute addrule class 251
				114	iproute addrule to a-prefix-for-1st-area class 250
				115	iproute addrule to another-prefix-for-1st-area class 250
				116	...
				117	iproute addrule to a-prefix-for-2nd-area class 249
				118	...
				119
				120	Area classes must be terminated with reject record.
				121	iproute add default reject class 250
				122	iproute add default reject class 249
				123	...
				124
				125	4. The Variant Router Requirements Algorithm (RFC1812 E.3.2)
				126	Create 16 classes for different TOS values.
				127	It is a funny, but pretty useless algorithm.
				128	I listed it just to show the power of new routing code.
				129
				130	5. All the variety of combinations......
				131
				132
				133	GATED
				134	-----
				135
				136	Gated does not understand classes, but it will work
				137	happily in MAIN+DEFAULT. All policy routes can be set
				138	and maintained manually.
				139
				140	IMPORTANT NOTE
				141	--------------
				142	route.c has a compilation time switch CONFIG_IP_LOCAL_RT_POLICY.
				143	If it is set, locally originated packets are routed
				144	using all the policy list. This is not very convenient and
				145	pretty ambiguous when used with NAT and masquerading.
				146	I set it to FALSE by default.
				147
				148
				149	Alexey Kuznetov
				150	kuznet@ms2.inr.ac.ru