Source address selection for transit-router with "blackholed" linknet

[Baptiste Jonglez]

Yes, it's a common pitfall, look for "krt_prefsrc" in the doc.

Or have a look at

  https://dn42.net/howto/bird#Source-address-selection

(sorry, certificate seems to have expired)

On Thu, Nov 13, 2014 at 11:06:25PM +0100, oskar at cetex.se wrote:
> Hi!
> 
> I'm working on getting a few linux-routers with bird up and running and I'm
> wondering: Is there any way to influence source-address selection for the
> routes installed by bird? (how do other bird-users handle this?, I guess I'm
> not alone?)
> 
> We have a transit from a provider that doesn't announce the linknet, as a
> lot of providers do. (and i shouldn't source traffic from the linknet, i
> should use own addresses)
> When i source traffic from the server/router it seems linux default-action
> is to select the interface closest to the destination which is the
> peering-linknet and it happily sends tcp / udp / icmp requests to the world.
> The replies never make it back though since the ip isn't reachable from
> anywhere else.
> 
> With ip route there seems to be this flag "src <addr>", so you can specify
> "ip route add x.x.x.x/y via z.z.z.z src a.a.a.a", this influences the kernel
> to select src a.a.a.a when sending traffic to x.x.x.x/y, but i haven't found
> such an option in the kernel-table in bird.
> 
> If i have a router where only the "inside"-ip is reachable from the
> internet, and all routes point to the outside, how do i make it source all
> locally generated traffic from the inside-ip?
> 
> Do you setup multiple routing-tables and set the default-table to direct
> traffic to the inside, then on the inside-interface have the full routing
> table (would this work?), or how do you handle this?
> 
> Best regards
> Oskar Stenman
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