Converting a route between tables via pipe proto?

[Ondrej Zajicek]

On Fri, Jan 22, 2021 at 03:26:59PM -0500, David Johnson wrote:
> Hi,
> 
> I'm trying to see if bird can accomplish the following and so far I can't
> seem to find a way to do it.
> 
> Quick summary:
> I want to be able to modify a route (the actual IP/mask) between 2
> interfaces.
> 
> 
> Setup is as follows:
> 
> table A: routes learned as normal
> 
> table B: I want to advertise a specific route into table B if there is
> another specific route learned on table A.  Note that the route in table B
> is different than A but I want to link/convert them somehow.
> 
> Why would I want to do this?  This Linux setup is performing NETMAP to
> convert IPs when forwarded between 2 interfaces.  I know the exact prefixes
> so I can enter them in the config but I want to dynamically advertise those
> into B but only if they are reachable via A.
> 
> 
> My first attempt was to use a proto pipe between the two tables and simply
> change the net variable:
> 
>   protocol pipe a_b_pipe {
>     table b;
>     peer table a;
>     import filter {   # a -> b
>       if net ~ A.B.C.D/E then { net = F.G.H.I/J; accept; };
>       reject;
>     };
>     export none;  # b -> a
>   }
> 
> but that doesn't work:
> bird: /etc/bird/bird.conf:24:56 This static attribute is read-only.
> 
> 
> My second attempt was to setup filters/functions on the protos in table A
> to set some variable if the specific route is seen, then reference that
> variable via a static proto route in table B.  This seems to be a no go as
> there are global constants (define xxx;) but attributes are local and
> function variables are also local scope.
> 
> Other suggestions appreciated, or is this just not possible?

Hi

It is not possible to do this directly, but there is one ugly trick how
to do that:

You can use pipe to import route A.B.C.D/E from table a to auxiliary
table c. Then have static route F.G.H.I/J with recursive nexthop from
A.B.C.D/E put to auxiliary table c. If route A.B.C.D/E is exported to
table c, then route F.G.H.I/J would inherit its nexthop, otherwise it
would be unreachable. Last, you can have pipe from table c to table b
that accepts only network F.G.H.I/J and only when it is reachable
(dest = RTD_UNICAST). And you should also reset its gateway (using
'gw = gw;' filter command), so it is no longer recursive when propagated
to table b.

-- 
Elen sila lumenn' omentielvo

Ondrej 'Santiago' Zajicek (email: santiago at crfreenet.org)
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"To err is human -- to blame it on a computer is even more so."