On Sat, May 23, 2009 at 3:25 PM, Stuart Henderson <s...@spacehopper.org>wrote:
> On 2009/05/23 13:57, Justin Credible wrote: > > On Sat, May 23, 2009 at 4:08 AM, Stuart Henderson <s...@spacehopper.org > >wrote: > > > > > On 2009-05-22, Justin Credible <mista.justin.credi...@gmail.com> > wrote: > > > > I am running OpenBGPd on an OpenBSD 4.4 router. > > > > > > > > Some times when traffic goes over one peer and finally gets to our > > > router, > > > > the last hop will respond as a different peer. For example: > > > > > > > > Level3 IP 10.0.0.1 > > > > Global Crossing IP 192.168.0.1 > > > > > > > > Traffic traverses Global crossing all the way, last hop, 10.0.0.1 > > > responds > > > > and vice versa. > > > > > > > > I noticed If i set one default gateway, this is always the IP to > respond > > > at > > > > the last hop. If i set multi-path default gateways then random IPs > > > respond > > > > at the last hop. > > > > > > > > Is there a common way that I am missing to get around this? Am i > > > overlooking > > > > something on the man pages? > > > > > > > > Thanks for your help! > > > > > > > > Regards, > > > > > > > > Ken > > > > > > > > > > > > > > If you're doing a traceroute to an address configured on the router > > > itself, it should reply from that address. Are you seeing something > > > different there? You definitely shouldn't. (ip_icmp.c:656) > > > > > > If you traceroute _through the router to another host_ (ip_icmp.c:668) > > > it will do a route lookup for the source, and use that as the source > > > address of the ICMP message (which is what shows in traceroute). > > > > > > What routes do you carry besides the default? No matter where default > > > points, if you have a specific route for the source of the traceroute > > > packets then it shouldn't be using the default. i.e. if you carry full > > > tables, you shouldn't see this. > > > > > > I point default at "localhost -reject" and take full tables and never > > > saw any strange behaviour with source address selection. > > > > > > > > Thanks for the response Stuart! > > > > I should have phrased that differently, sorry about that! What I mean by > > "the last hop" is "the last hop before the destination" so the > second-last > > hop - My bad. > > > > So here would be another example. > > > > A traceroute should be: > > > > Traceroute 172.16.1.1 > > 1.192.168.253.211 AS3549 > > 2.192.168.24.5 AS3549 > > 3. 192.168.0.1 AS3549 > > 4. 172.16.1.1 MYASN > > > > But instead it would look something like this: > > > > Traceroute 172.16.1.1 > > 1.192.168.253.211 AS3549 > > 2.192.168.24.5 AS3549 > > 3. 10.0.0.1 AS3356 > > 4. 172.16.1.1 MYASN > > > > So the IP address which i use to peer with Level3 responds at the second > > last hop, rather than the Global Crossing IP since it traversed the > entire > > way through Global Crossing. Both of the IPs which respond at the second > > last hop are on my router so the problem is on my end. It doesn't appear > to > > be a BGP problem as much as a default route problem. > > So, this part of my mail applies: > > "If you traceroute _through the router to another host_ (ip_icmp.c:668) > it will do a route lookup for the source, and use that as the source > address of the ICMP message (which is what shows in traceroute). > > What routes do you carry besides the default? No matter where default > points, if you have a specific route for the source of the traceroute > packets then it shouldn't be using the default. i.e. if you carry full > tables, you shouldn't see this." > > Do you carry full tables? Yes sir > > > > I tried adding "reply-to" rules in my pf.conf so that traffic that comes > in > > on one interface will go out the same interface but that doesn't seem to > > work either, since the reply from the wrong address happens before or > during > > the state that stateful connections are being established. > > PF isn't involved in this address selection, it's a message from the > router's IP stack because the TTL was exceeded, the lookup is entirely > done in the stack, reply-to isn't used. > > Ok I think I understand that... So what should be my next move?
