patch-2.1.16 linux/Documentation/networking/policy-routing.txt

• Lines: 151
• Date: Wed Dec 18 11:45:43 1996
• Orig file: v2.1.15/linux/Documentation/networking/policy-routing.txt
• Orig date: Thu Jan 1 02:00:00 1970

diff -u --recursive --new-file v2.1.15/linux/Documentation/networking/policy-routing.txt linux/Documentation/networking/policy-routing.txt
@@ -0,0 +1,150 @@
+Classes
+-------
+
+	"Class" is complete routing table in common sence.
+	I.e. it is tree of nodes (destination prefix, tos, metric)
+	with attached information: gateway, device etc.
+	This tree is looked up as specified in RFC1812 5.2.4.3
+	1. Basic match
+	2. Longest match
+	3. Weak TOS.
+	4. Metric. (should not be in kernel space, but they are)
+	5. Additional pruning rules. (not in kernel space).
+	
+	We have two special type of nodes:
+	REJECT - abort route lookup and return an error value.
+	THROW  - abort route lookup in this class.
+
+
+	Currently number of classes is limited by 255
+	(0 is reserved for "not specified class")
+
+	Three classes are builtin:
+
+	RT_CLASS_LOCAL=255 - local interface addresses,
+	broadcasts, nat addresses.
+
+	RT_CLASS_MAIN=254  - all normal routes are put there
+	by default.
+
+	RT_CLASS_DEFAULT=253 - if ip_fib_model==1, then
+	normal default routes are put there, if ip_fib_model==2
+	all gateway routes are put there.
+
+
+Rules
+-----
+	Rule is record of (src prefix, src interface, tos, dst prefix)
+	with attached information.
+
+	Rule types:
+	RTP_ROUTE - lookup in attached class
+	RTP_NAT   - lookup in attached class and if a match is found,
+		    translate packet source address.
+	RTP_MASQUERADE - lookup in attached class and if a match is found,
+		    masquerade packet as sourced by us.
+	RTP_DROP   - silently drop the packet.
+	RTP_REJECT - drop the packet and send ICMP NET UNREACHABLE.
+	RTP_PROHIBIT - drop the packet and send ICMP COMM. ADM. PROHIBITED.
+
+	Rule flags:
+	RTRF_LOG - log route creations.
+	RTRF_VALVE - One way route (used with masquerading)
+
+Default setup:
+
+root@amber:/pub/ip-routing # iproute -r
+Kernel routing policy rules
+Pref Source             Destination        TOS Iface   Cl
+   0 default            default            00  *       255
+ 254 default            default            00  *       254
+ 255 default            default            00  *       253
+
+
+Lookup algorithm
+----------------
+
+	We scan rules list, and if a rule is matched, apply it.
+	If route is found, return it.
+	If it is not found or THROW node was matched, continue
+	to scan rules.
+
+Applications
+------------
+
+1.	Just ignore classes. All the routes are put to MAIN class
+	(and/or to DEFAULT class).
+
+	HOWTO:  iproute add PREFIX [ tos TOS ] [ gw GW ] [ dev DEV ]
+		[ metric METRIC ] [ reject ] ... (look at iproute utility)
+
+		or use route utility from current net-tools.
+		
+2.	Opposite case. Just forget all that you know about routing
+	tables. Every rule is supplied with its own gateway, device
+	info. record. This approach is not appropriate for automated
+	route maintanance, but it is ideal for manual configuration.
+
+	HOWTO:  iproute addrule [ from PREFIX ] [ to PREFIX ] [ tos TOS ]
+		[ dev INPUTDEV] [ pref PREFERENCE ] route [ gw GATEWAY ]
+		[ dev OUTDEV ] .....
+
+	Warning: just now size of routing table in this approach is
+	limited by 256. If someone will like this model, I'll
+	relax this limitation.
+
+3.	OSPF classes (see RFC1583, RFC1812 E.3.3)
+	Very clean, stable and robust algorithm for OSPF routing
+	domains. Unfortunately, it is not used widely in the Internet.
+
+	Proposed setup:
+	255 local addresses
+	254 interface routes
+	253 ASE routes with external metric
+	252 ASE routes with internal metric
+	251 inter-area routes
+	250 intra-area routes for 1st area
+	249 intra-area routes for 2nd area
+	etc.
+	
+	Rules:
+	iproute addrule class 253
+	iproute addrule class 252
+	iproute addrule class 251
+	iproute addrule to a-prefix-for-1st-area class 250
+	iproute addrule to another-prefix-for-1st-area class 250
+	...
+	iproute addrule to a-prefix-for-2nd-area class 249
+	...
+
+	Area classes must be terminated with reject record.
+	iproute add default reject class 250
+	iproute add default reject class 249
+	...
+
+4.	The Variant Router Requirements Algorithm (RFC1812 E.3.2)
+	Create 16 classes for different TOS values.
+	It is funny, but pretty useless algorithm.
+	I listed it just to show power of new routing code.
+
+5.	All the variaty of combinations......
+
+
+GATED
+-----
+
+	Gated does not understand classes, but it will work
+	happily in MAIN+DEFAULT. All policy routes can be set
+	and maintained manually.
+
+IMPORTANT NOTE
+--------------
+	route.c has compilation time switch CONFIG_IP_LOCAL_RT_POLICY.
+	If it is set, locally originated packets are routed
+	using all the policy list. It is not very convenient and
+	pretty ambiguous, when used with NAT and masquerading.
+	I set it to FALSE by default.
+
+
+Alexey Kuznetov
+kuznet@ms2.inr.ac.ru