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Diffstat (limited to 'Documentation/networking/policy-routing.txt')
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diff --git a/Documentation/networking/policy-routing.txt b/Documentation/networking/policy-routing.txt new file mode 100644 index 00000000000..36f6936d7f2 --- /dev/null +++ b/Documentation/networking/policy-routing.txt @@ -0,0 +1,150 @@ +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 |