After all the messages recently about how to fix DNS, I was seriously tempted to title this messsage "And now, for something completely different", but impossible circuit is more descriptive.
Before you read further, I need everyone to put on their thinking WAY outside the box hats. I've heard from enough people already that I'm nuts and what I'm seeing can't happen, so it must not be happening...even though we see the results of it happening.
I've got this private line DS3. It connects cisco 7206 routers in Orlando (at our data center) and in Ocala (a colo rack in the Embarq CO).
According to the DLR, it's a real circuit, various portions of it ride varying sized OC circuits, and then it's handed off to us at each end the usual way (copper/coax) and plugged into PA-2T3 cards.
Last Tuesday, at about 2:30PM, "something bad happened." We saw a serious jump in traffic to Ocala, and in particular we noticed one customer's connection (a group of load sharing T1s) was just totally full. We quickly assumed it was a DDoS aimed at that customer, but looking at the traffic, we couldn't pinpoint anything that wasn't expected flows.
Then we noticed the really weird stuff. Pings to anything in Ocala responded with multiple dupes and ttl exceeded messages from a Level3 IP. Traceroutes to certain IPs in Ocala would get as far our Ocala router, then inexplicably hop onto Sprintlink's network, come back to us over our Level3 transit connection, get to Ocala, then hop over to Sprintlink again, repeating that loop as many times as max TTL would permit. Pings from router to router crossing just the DS3 would work, but we'd see 10 duplicate packets for every 1 expected packet. BTW, the cisco CLI hides dupes unless you turn on ip icmp debugging.
I've seen some sort of similar things (though contained within an AS) with MPLS and routing misconfigurations, but traffic jumping off our network (to a network to which we're not directly connected) was seemingly impossible. We did all sorts of things to troubleshoot it (studied our router configs in rancid, temporarily shut every interface on the Ocala side other than the DS3, changed IOS versions, changed out the hardware, opened a ticket with cisco TAC) but then it occurred to me, that if traffic was actually jumping off our network and coming back in via Level3, I could see/block at least some of that using an ACL on our interface to Level3. How do you explain it, when you ping the remote end of a DS3 interface with a single echo request packet and see 5 copies of that echo request arrive at one of your transit provider interfaces?
Here's a typical traceroute with the first few hops (from my home internet connection) removed. BTW, hop 9 is a customer router conveniently configured with no ip unreachables.
7 andc-br-3-f2-0.atlantic.net (184.108.40.206) 47.951 ms 56.096 ms 56.154 ms
8 ocalflxa-br-1-s1-0.atlantic.net (220.127.116.11) 56.199 ms 56.320 ms 56.196 ms
9 * * *
10 sl-bb20-dc-6-0-0.sprintlink.net (18.104.22.168) 80.774 ms 81.030 ms 81.821 ms
11 sl-st20-ash-10-0.sprintlink.net (22.214.171.124) 75.731 ms 75.902 ms 77.128 ms
12 te-10-1-0.edge2.Washington4.level3.net (126.96.36.199) 46.548 ms 53.200 ms 45.736 ms
13 vlan69.csw1.Washington1.Level3.net (188.8.131.52) 42.918 ms vlan79.csw2.Washington1.Level3.net (184.108.40.206) 55.438 ms vlan69.csw1.Washington1.Level3.net (220.127.116.11) 42.693 ms
14 ae-81-81.ebr1.Washington1.Level3.net (18.104.22.168) 48.935 ms ae-61-61.ebr1.Washington1.Level3.net (22.214.171.124) 49.317 ms ae-91-91.ebr1.Washington1.Level3.net (126.96.36.199) 48.865 ms
15 ae-2.ebr3.Atlanta2.Level3.net (188.8.131.52) 59.642 ms 56.278 ms 56.671 ms
16 ae-61-60.ebr1.Atlanta2.Level3.net (184.108.40.206) 47.401 ms 62.980 ms 62.640 ms
17 ae-1-8.bar1.Orlando1.Level3.net (220.127.116.11) 40.300 ms 40.101 ms 42.690 ms
18 ae-6-6.car1.Orlando1.Level3.net (18.104.22.168) 40.959 ms 40.963 ms 41.016 ms
19 unknown.Level3.net (22.214.171.124) 246.744 ms 240.826 ms 239.758 ms
20 andc-br-3-f2-0.atlantic.net (126.96.36.199) 39.725 ms 37.751 ms 42.262 ms
21 ocalflxa-br-1-s1-0.atlantic.net (188.8.131.52) 43.524 ms 45.844 ms 43.392 ms
22 * * *
23 sl-bb20-dc-6-0-0.sprintlink.net (184.108.40.206) 63.752 ms 61.648 ms 60.839 ms
24 sl-st20-ash-10-0.sprintlink.net (220.127.116.11) 66.923 ms 65.258 ms 70.609 ms
25 te-10-1-0.edge2.Washington4.level3.net (18.104.22.168) 67.106 ms 93.415 ms 73.932 ms
26 vlan99.csw4.Washington1.Level3.net (22.214.171.124) 88.919 ms 75.306 ms vlan79.csw2.Washington1.Level3.net (126.96.36.199) 75.048 ms
27 ae-61-61.ebr1.Washington1.Level3.net (188.8.131.52) 69.508 ms 68.401 ms ae-71-71.ebr1.Washington1.Level3.net (184.108.40.206) 79.128 ms
28 ae-2.ebr3.Atlanta2.Level3.net (220.127.116.11) 64.048 ms 67.764 ms 67.704 ms
29 ae-71-70.ebr1.Atlanta2.Level3.net (18.104.22.168) 68.372 ms 67.025 ms 68.162 ms
30 ae-1-8.bar1.Orlando1.Level3.net (22.214.171.124) 65.112 ms 65.584 ms 65.525 ms
Our circuit provider's support people have basically just maintained that this behavior isn't possible and so there's nothing they can do about it. i.e. that the problem has to be something other than the circuit.
I got tired of talking to their brick wall, so I contacted Sprint and was able to confirm with them that the traffic in question really was inexplicably appearing on their network...and not terribly close geographically to the Orlando/Ocala areas.
So, I have a circuit that's bleeding duplicate packets onto an unrelated IP network, a circuit provider who's got their head in the sand and keeps telling me "this can't happen, we can't help you", and customers who were getting tired of receiving all their packets in triplicate (or more) saturating their connections and confusing their applications. After a while, I had to give up on finding the problem and focus on just making it stop. After trying a couple of things, the solution I found was to change the encapsulation we use at each end of the DS3. I haven't gotten confirmation of this from Sprint, but I assume they're now seeing massive input errors one the one or more circuits where our packets were/are appearing. The important thing (for me) is that this makes the packets invalid to Sprint's routers and so it keeps them from forwarding the packets to us. Cisco TAC finally got back to us the day after I "fixed" the circuit...but since it was obviously not a problem with our cisco gear, I haven't pursued it with them.
The only things I can think of that might be the cause are misconfiguration in a DACS/mux somewhere along the circuit path or perhaps a mishandled lawful intercept. I don't have enough experience with either or enough access to the systems that provide the circuit to do any more than speculate. Has anyone else ever seen anything like this?
If someone from Level3 transport can wrap their head around this, I'd love to know what's really going on...but at least it's no longer an urgent problem for me.