nanog mailing list archives

Re: Finding asymmetric path


From: Brielle Bruns <bruns () 2mbit com>
Date: Sat, 28 Nov 2009 10:14:27 -0700

On 11/27/09 8:43 PM, ML wrote:
I'm reasonable certain a customer of ours who is using one of our
netblocks is using a different reverse path to reach us. How might I
figure out who is allowing them to source traffic from IPs that belong
to us?




I've had two customers pull this stunt in the past with me - one, a spammer, tried to do this with an ADSL modem from me, the other (a non-spammer with a clueless 'consultant') had a T1 from me and a T1 from UUNet.

It started with the T1 customer. I believe they had a smaller block of IPs (less then /24, more like a /25 or /26), and their 'computer consultant' with his infinite wisdom decided to send all outbound traffic through the UUNet T1 rather then source routing which we highly recommended to them. Of course, we had ingress filters in place to block IP ranges we have from coming into us from the WAN links, so when they tried to contact servers on the other half of the netblock on our end, the connections mysteriously failed. After lying up and down that it was our fault, that their computer 'consultant' was regarded as best in the country, blah blah blah, we flipped on logging on the ingress filters out of sheer curiosity and discovered exactly what was going on.

The ADSL customer was a bit more tricky - we were getting spam reports about his single IP address sending spam, but we had his outbound port 25 blocked. Ended up sniffing the port off the router he sat off of, and discovering that it was all one sided, wasn't even tickling the ingress filters.

Hey, at least your customer didn't convince AT&T to allow them to announce out one of your /24s when all they had was a /29.

Your in a tricky bind, I'd approach them under the guise of ingress filtering issues.

--
Brielle Bruns
The Summit Open Source Development Group
http://www.sosdg.org    /     http://www.ahbl.org


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