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Documentation / networking / policy-routing.txt




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Based on kernel version 3.16. Page generated on 2014-08-06 21:40 EST.

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
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