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-Classes
--------
-
- "Class" is a complete routing table in common sense.
- 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 the number of classes is limited to 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 a 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 a route is found, return it.
- If it is not found or a THROW node was matched, continue
- to scan rules.
-
-Applications
-------------
-
-1. Just ignore classes. All the routes are put into MAIN class
- (and/or into 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 maintenance, 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: As of now the size of the routing table in this
- approach is limited to 256. If someone likes 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 widely used 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 a funny, but pretty useless algorithm.
- I listed it just to show the power of new routing code.
-
-5. All the variety 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 a compilation time switch CONFIG_IP_LOCAL_RT_POLICY.
- If it is set, locally originated packets are routed
- using all the policy list. This 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