FreeBSD responding with wrong receiving interface IP

[Paul S.]

Hi,

I've been deploying FreeBSD as customer edge routers for customers with 
sites that do not require high throughput (>1g/s).

Each site has two ISPs (Mostly Telstra + Verizon/Optus), and take full 
routes via OpenBGPd and BIRD. I use next-hop self on all received routes.

The FreeBSD boxes have static routes delegating the announced IP blocks 
to a L3 switch down the road. i.e: route add -net 10.100.1.0/24 
10.0.0.1, and then that /24 is originated via BGP to both upstreams.

Things in general work fine, but I've been receiving reports of 'weird 
traceroute results' from my customers.

Examples of this would be,

  1 some.random.isp (...) (...)
  2  gigabitethernet3-3.exi1.melbourne.telstra.net (203.50.77.49) 0.309 
ms  0.284 ms  0.227 ms
  3  bundle-ether3-100.exi-core10.melbourne.telstra.net (203.50.80.1)  
1.966 ms  1.675 ms  1.852 ms
  4  bundle-ether12.chw-core10.sydney.telstra.net (203.50.11.124) 16.707 
ms  15.917 ms  16.360 ms
  5  customer-gw.syd.ALTER.net (...) (...)

This traceroute seems to claim that the packet was received over the 
Verizon gateway, which in reality it was not -- it was received directly 
over the Telstra interface, but my outbound AS-PATH towards 
some.random.isp uses Verizon.

So FreeBSD replies back with the Verizon address. Another person having 
the same issue (mostly, but on OpenBSD) can be found at 
http://openbsd.7691.n7.nabble.com/BGP-responding-with-wrong-IP-address-td90264.html

I would love to know if there's a way to fix this, or if I've missed 
something, or if there's something wrong in the way I set it up.

Thank you for taking the time to read.