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

oskar at oskar at
Thu Nov 13 23:06:25 CET 2014


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 

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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