Le 31 juil. 2013 à 18:15, Alexandru Petrescu a écrit :
> Le 31/07/2013 17:38, Wes Beebee (wbeebee) a écrit :
>>> If no prefix is found by the longetst-matching on the src address, will a
>>> default be used instead?
>>> Alex
>>>> There is also potentially a whole destination lookup table that¹s
>>>> entirely devoted to the ::/0 source prefix.
>>>> - Wes
>> My understanding is that there IS a default for source address (see the
>> above comment).
>
> Ok.
>
> The default for dst addresses leads to a nexthop.
>
> The default for src addresses leads also to a nexthop? Or to the dst of
> another dst prefix? Or to the nexthop of the default of dst?
>
> It may sound as a rathole discussion but it is fully part of what the
> longest-match algorithm does.
>
> Alex
>
I'm not sure about how it should be done but I can say how Linux seems to do it.
* Those affirmation came by looking at the code... That I didn't wrote. So I
cannot be 100% sure *
Linux uses a tree structure in order to perform the longest prefix lookup on
the destination address first.
The found node (which has the longest prefix for the source address), CAN have
a subtree.
This subtree has the same structure as a regular routing table, but is used
with the source address.
This means that each node in a routing table has its own source routing table.
There is no
destination lookup first and then a source lookup in a global table.
--> See fib6_lookup, and fib6_lookup_1 functions
Now, when you add a route with destination and source prefix,
the node for destination is created (or updated) and, then,
if the source routing prefix length is not null, a subtree is created (or
updated) and
a node with the provided source prefix is inserted.
--> See fib6_add
Therefore, for a given node in the main tree, if there is no source routing (no
subtree), the route
is always used. Otherwise, a longest prefix match is performed on the source
address, using the subtree.
If there is no corresponding node for the source (no matching prefix, where the
prefix length can be null),
then, the parent node in the destination routing table is used as default.
So, yes, inserting a route for a destination and source prefix will create an
entry for the destination alone
(without source routing).
So the default route, for a source lookup, is basically the last inserted
next-hop for the destination prefix...
Which could create unexpected behaviors.
Concerning the previous question, Linux actually has a RTF_GATEWAY flag. But it
doesn't seem to
be used in the routing decision process. I guess it is used by other protocols
that like to have a
concept of "default router".
Pierre
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