Documentation/networking/policy-routing.txt

   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