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 (209.208.9.138) 47.951 ms 56.096 ms 56.154 ms
8 ocalflxa-br-1-s1-0.atlantic.net (209.208.112.98) 56.199 ms 56.320 ms
56.196 ms
9 * * *
10 sl-bb20-dc-6-0-0.sprintlink.net (144.232.8.174) 80.774 ms 81.030 ms
81.821 ms
11 sl-st20-ash-10-0.sprintlink.net (144.232.20.152) 75.731 ms 75.902 ms
77.128 ms
12 te-10-1-0.edge2.Washington4.level3.net (4.68.63.209) 46.548 ms 53.200 ms
45.736 ms
13 vlan69.csw1.Washington1.Level3.net (4.68.17.62) 42.918 ms
vlan79.csw2.Washington1.Level3.net (4.68.17.126) 55.438 ms
vlan69.csw1.Washington1.Level3.net (4.68.17.62) 42.693 ms
14 ae-81-81.ebr1.Washington1.Level3.net (4.69.134.137) 48.935 ms
ae-61-61.ebr1.Washington1.Level3.net (4.69.134.129) 49.317 ms
ae-91-91.ebr1.Washington1.Level3.net (4.69.134.141) 48.865 ms
15 ae-2.ebr3.Atlanta2.Level3.net (4.69.132.85) 59.642 ms 56.278 ms 56.671 ms
16 ae-61-60.ebr1.Atlanta2.Level3.net (4.69.138.2) 47.401 ms 62.980 ms
62.640 ms
17 ae-1-8.bar1.Orlando1.Level3.net (4.69.137.149) 40.300 ms 40.101 ms
42.690 ms
18 ae-6-6.car1.Orlando1.Level3.net (4.69.133.77) 40.959 ms 40.963 ms 41.016
ms
19 unknown.Level3.net (63.209.98.66) 246.744 ms 240.826 ms 239.758 ms
20 andc-br-3-f2-0.atlantic.net (209.208.9.138) 39.725 ms 37.751 ms 42.262 ms
21 ocalflxa-br-1-s1-0.atlantic.net (209.208.112.98) 43.524 ms 45.844 ms
43.392 ms
22 * * *
23 sl-bb20-dc-6-0-0.sprintlink.net (144.232.8.174) 63.752 ms 61.648 ms
60.839 ms
24 sl-st20-ash-10-0.sprintlink.net (144.232.20.152) 66.923 ms 65.258 ms
70.609 ms
25 te-10-1-0.edge2.Washington4.level3.net (4.68.63.209) 67.106 ms 93.415 ms
73.932 ms
26 vlan99.csw4.Washington1.Level3.net (4.68.17.254) 88.919 ms 75.306 ms
vlan79.csw2.Washington1.Level3.net (4.68.17.126) 75.048 ms
27 ae-61-61.ebr1.Washington1.Level3.net (4.69.134.129) 69.508 ms 68.401 ms
ae-71-71.ebr1.Washington1.Level3.net (4.69.134.133) 79.128 ms
28 ae-2.ebr3.Atlanta2.Level3.net (4.69.132.85) 64.048 ms 67.764 ms 67.704 ms
29 ae-71-70.ebr1.Atlanta2.Level3.net (4.69.138.18) 68.372 ms 67.025 ms
68.162 ms
30 ae-1-8.bar1.Orlando1.Level3.net (4.69.137.149) 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.
----------------------------------------------------------------------
Jon Lewis | I route
Senior Network Engineer | therefore you are
Atlantic Net |
_________ http://www.lewis.org/~jlewis/pgp for PGP public key_________