Jon Lewis's Blog
11 07 2022Mon, 11 Jul 2022
impossible circuit
I'm preserving bits of this thread I started on the NANOG mailing list back
in 2008 in case the old archives ever disappear:
From jlewis@lewis.org Sun Aug 10 23:15:47 2008 -0400
Date: Sun, 10 Aug 2008 23:15:47 -0400 (EDT)
From: Jon Lewis
To: nanog@nanog.org
Subject: impossible circuit
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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_________
Re: impossible circuit
From: Paul Wall
Date: Mon Aug 18 16:48:24 2008
List-archive:
List-help:
List-id: North American Network Operators Group
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Jon,
I think we can safely conclude from the information provided that
you're looking at some sort of a misconfigured traffic mirroring or
[un]lawful intercept.
Sadly, as neither Sprint nor your loop provider will fess up, I don't
think you're going to get much further on here.
Probably best to order a new loop and cancel the existing one.
Drive Slow,
Paul
- Original message -
I just went ahead and "re-broke" the circuit ...
On 8/17/08, Jon Lewis wrote:
> On Tue, 12 Aug 2008, Jon Lewis wrote:
>
>>> What would happen if you pinged the Ocala router such that the TTL was 1
>>> when travelling over the DS3? From your traceroute it seems it travelled
>>> two IP hops that did not send ICMP error messages, but it might just be
>>> that the ICMP errors from the Ocala router are arriving first.
>>
>> Based on where the dupes are coming from, I assume pinging across the DS3
>> with TTL tuned to expire at the Ocala side would result in TTL exceeded
>> messages from both Ocala and the Sprint router where the packets are
>> injected
>> into Sprint's network. It doesn't look as if IOS gives the option to set
>> TTL
>> on ping...so I'd try this from a Linux machine in our data center.
>
> I just went ahead and "re-broke" the circuit for a bit by turning it back
> to hdlc to see if the issue is still there and to run some additional
> tests. Someone is still cross connecting our Orlando->Ocala traffic over
> to Sprint.
>
> I did your suggested ping with short TTL and the result was close to what
> I expected.
>
> $ traceroute ocalflxa-br-1
> traceroute to ocalflxa-br-1.atlantic.net (209.208.6.229), 30 hops max, 38
> byte packets
> 1 209.208.25.165 (209.208.25.165) 0.539 ms 0.426 ms 0.388 ms
> 2 69.28.72.162 (69.28.72.162) 0.246 ms 0.351 ms 0.223 ms
> 3 andc-br-3-f2-0 (209.208.9.138) 0.559 ms 0.435 ms 0.471 ms
> 4 ocalflxa-br-1-s1-0 (209.208.112.98) 2.735 ms * 2.656 ms
>
> So, I need a TTL of 4 to get there from this machine.
>
> $ ping -t4 ocalflxa-br-1
> PING ocalflxa-br-1.atlantic.net (209.208.6.229) 56(84) bytes of data.
> 64 bytes from ocalflxa-br-1.atlantic.net (209.208.6.229): icmp_seq=0 ttl=252
> time=2.68 ms
> 64 bytes from ocalflxa-br-1.atlantic.net (209.208.6.229): icmp_seq=1 ttl=252
> time=2.72 ms
> 64 bytes from ocalflxa-br-1.atlantic.net (209.208.6.229): icmp_seq=2 ttl=252
> time=2.88 ms
>
> Decrease ttl by one, and I get the expected ttl exceeded from the Orlando
> side of the circuit.
>
> $ ping -t 3 ocalflxa-br-1
> PING ocalflxa-br-1.atlantic.net (209.208.6.229) 56(84) bytes of data.
> >From andc-br-3-f2-0.atlantic.net (209.208.9.138) icmp_seq=0 Time to live
> exceeded
>
> Now, here's a mild surprise. You'll notice that in the above -t4 trace, I
> didn't hear back from Sprint.
>
> $ ping -t 5 ocalflxa-br-1
> PING ocalflxa-br-1.atlantic.net (209.208.6.229) 56(84) bytes of data.
> 64 bytes from ocalflxa-br-1.atlantic.net (209.208.6.229): icmp_seq=0 ttl=252
> time=2.89 ms
> 64 bytes from ocalflxa-br-1.atlantic.net (209.208.6.229): icmp_seq=1 ttl=252
> time=3.10 ms
> 64 bytes from ocalflxa-br-1.atlantic.net (209.208.6.229): icmp_seq=2 ttl=252
> time=2.97 ms
> hmm...still no ttl exceeded from Sprint?
>
> $ ping -t 6 ocalflxa-br-1
> PING ocalflxa-br-1.atlantic.net (209.208.6.229) 56(84) bytes of data.
> 64 bytes from ocalflxa-br-1.atlantic.net (209.208.6.229): icmp_seq=0 ttl=252
> time=2.95 ms
> >From sl-crs2-dc-0-5-3-0.sprintlink.net (144.232.19.93) icmp_seq=0 Time to
> live exceeded
> 64 bytes from ocalflxa-br-1.atlantic.net (209.208.6.229): icmp_seq=1 ttl=252
> time=2.78 ms
> >From sl-crs2-dc-0-5-3-0.sprintlink.net (144.232.19.93) icmp_seq=1 Time to
> live exceeded
>
> $ ping -t 7 ocalflxa-br-1
> PING ocalflxa-br-1.atlantic.net (209.208.6.229) 56(84) bytes of data.
> 64 bytes from ocalflxa-br-1.atlantic.net (209.208.6.229): icmp_seq=0 ttl=252
> time=2.88 ms
> >From sl-st20-ash-9-0-0.sprintlink.net (144.232.18.228) icmp_seq=0 Time to
> live exceeded
> 64 bytes from ocalflxa-br-1.atlantic.net (209.208.6.229): icmp_seq=1 ttl=252
> time=2.84 ms
> >From sl-st20-ash-9-0-0.sprintlink.net (144.232.18.228) icmp_seq=1 Time to
> live exceeded
>
> Is it just coincidence that there are 2 private IP hops in some
> traceroutes between us and Sprint? i.e. Look at this trace from cogent:
>
> Tracing the route to 209.208.33.1
>
> 1 fa0-8.na01.b005944-0.dca01.atlas.cogentco.com (66.250.56.189) 0 msec 4
> msec 4 msec
> 2 gi3-9.3507.core01.dca01.atlas.cogentco.com (66.28.67.225) 160 msec 4
> msec 8 msec
> 3 te3-1.ccr02.dca01.atlas.cogentco.com (154.54.3.158) 0 msec 0 msec 4
> msec
> 4 vl3493.mpd01.dca02.atlas.cogentco.com (154.54.7.230) 28 msec 4 msec
> te4-1.mpd01.dca02.atlas.cogentco.com (154.54.2.182) 52 msec
> 5 vl3494.mpd01.iad01.atlas.cogentco.com (154.54.5.42) 4 msec 4 msec
> vl3497.mpd01.iad01.atlas.cogentco.com (154.54.5.66) 4 msec
> 6 timewarner.iad01.atlas.cogentco.com (154.54.13.250) 4 msec
> peer-01-ge-3-1-2-13.asbn.twtelecom.net (66.192.252.217) 4 msec 12 msec
> 7 66-194-200-202.static.twtelecom.net (66.194.200.202) 28 msec 28 msec 32
> msec
> 8 66-194-200-202.static.twtelecom.net (66.194.200.202) 32 msec 32 msec 28
> msec
> 9 andc-br-3-f2-0.atlantic.net (209.208.9.138) 32 msec 32 msec 32 msec
> 10 172.22.122.1 32 msec 32 msec 32 msec
> 11 10.247.28.205 32 msec 32 msec 32 msec
> 12 sl-crs2-dc-0-5-3-0.sprintlink.net (144.232.19.93) 32 msec 32 msec 28
> msec
> 13 sl-st20-ash-9-0-0.sprintlink.net (144.232.18.228) 28 msec 32 msec 32
> msec
> 14 te-10-1-0.edge2.Washington4.level3.net (4.68.63.209) 32 msec 32 msec 28
> msec
> 15 vlan79.csw2.Washington1.Level3.net (4.68.17.126) 28 msec
> vlan89.csw3.Washington1.Level3.net (4.68.17.190) 32 msec
> vlan79.csw2.Washington1.Level3.net (4.68.17.126) 40 msec
> 16 ae-81-81.ebr1.Washington1.Level3.net (4.69.134.137) 28 msec
> ae-61-61.ebr1.Washington1.Level3.net (4.69.134.129) 28 msec
> ae-71-71.ebr1.Washington1.Level3.net (4.69.134.133) 32 msec
> 17 ae-2.ebr3.Atlanta2.Level3.net (4.69.132.85) 48 msec 48 msec 56 msec
> 18 ae-61-60.ebr1.Atlanta2.Level3.net (4.69.138.2) 44 msec 48 msec
> ae-71-70.ebr1.Atlanta2.Level3.net (4.69.138.18) 52 msec
> 19 ae-1-8.bar1.Orlando1.Level3.net (4.69.137.149) 56 msec 104 msec 56 msec
> 20 ae-6-6.car1.Orlando1.Level3.net (4.69.133.77) 52 msec 52 msec 56 msec
> 21 unknown.Level3.net (63.209.98.66) 52 msec 52 msec 56 msec
> 22 andc-br-3-f2-0.atlantic.net (209.208.9.138) 52 msec 52 msec 56 msec
> 23 172.22.122.1 52 msec 56 msec 52 msec
> 24 10.247.28.205 52 msec 52 msec 56 msec
> 25 sl-crs2-dc-0-5-3-0.sprintlink.net (144.232.19.93) 52 msec 56 msec 52
> msec
> 26 sl-st20-ash-9-0-0.sprintlink.net (144.232.18.228) 56 msec 56 msec 56
> msec
> 27 te-10-1-0.edge2.Washington4.level3.net (4.68.63.209) 52 msec 52 msec 52
> msec
> 28 vlan99.csw4.Washington1.Level3.net (4.68.17.254) 52 msec
> vlan69.csw1.Washington1.Level3.net (4.68.17.62) 56 msec
> vlan89.csw3.Washington1.Level3.net (4.68.17.190) 56 msec
> 29 ae-71-71.ebr1.Washington1.Level3.net (4.69.134.133) 64 msec
> ae-61-61.ebr1.Washington1.Level3.net (4.69.134.129) 52 msec 56 msec
> 30 ae-2.ebr3.Atlanta2.Level3.net (4.69.132.85) 76 msec 72 msec 72 msec
>
> I've seen the 172.22.122.1 & 10.247.28.205 hops before. They occasionally
> show up in traces when the traffic is jumping over to Sprint. Sometimes
> they don't show up though. i.e. Tracing from my house:
>
> traceroute to 209.208.33.1 (209.208.33.1), 30 hops max, 40 byte packets
> 1 172.31.0.1 (172.31.0.1) 0.336 ms 0.272 ms 0.268 ms
> 2 10.210.160.1 (10.210.160.1) 10.109 ms 11.719 ms 14.265 ms
> 3 gig7-0-4-101.orldflaabv-rtr1.cfl.rr.com (24.95.232.100) 15.302 ms
> 15.324 ms 16.687 ms
> 4 198.228.95.24.cfl.res.rr.com (24.95.228.198) 16.688 ms 18.812 ms
> 18.816 ms
> 5 te-3-3.car1.Orlando1.Level3.net (4.79.116.145) 20.084 ms 19.946 ms
> te-3-1.car1.Orlando1.Level3.net (4.79.116.137) 21.328 ms
> 6 unknown.Level3.net (63.209.98.66) 19.900 ms 14.714 ms 14.689 ms
> 7 andc-br-3-f2-0.atlantic.net (209.208.9.138) 104.058 ms 11.932 ms
> 13.584 ms
> 8 ocalflxa-br-1-s1-0.atlantic.net (209.208.112.98) 15.872 ms 15.886 ms
> 17.238 ms
> 9 * * *
> 10 sl-bb20-dc-6-0-0.sprintlink.net (144.232.8.174) 41.277 ms 41.964 ms
> 41.955 ms
> 11 sl-st20-ash-10-0.sprintlink.net (144.232.20.152) 43.360 ms 44.578 ms
> 35.635 ms
> 12 te-10-1-0.edge2.Washington4.level3.net (4.68.63.209) 37.035 ms 37.062
> ms 33.185 ms
> 13 vlan89.csw3.Washington1.Level3.net (4.68.17.190) 44.060 ms 44.057 ms
> vlan99.csw4.Washington1.Level3.net (4.68.17.254) 39.603 ms
> 14 ae-81-81.ebr1.Washington1.Level3.net (4.69.134.137) 38.123 ms
> ae-91-91.ebr1.Washington1.Level3.net (4.69.134.141) 39.546 ms
> ae-71-71.ebr1.Washington1.Level3.net (4.69.134.133) 38.115 ms
> 15 ae-2.ebr3.Atlanta2.Level3.net (4.69.132.85) 46.284 ms 46.275 ms
> 46.274 ms
> 16 ae-71-70.ebr1.Atlanta2.Level3.net (4.69.138.18) 52.523 ms
> ae-61-60.ebr1.Atlanta2.Level3.net (4.69.138.2) 53.338 ms
> ae-71-70.ebr1.Atlanta2.Level3.net (4.69.138.18) 53.299 ms
> 17 ae-1-8.bar1.Orlando1.Level3.net (4.69.137.149) 34.964 ms 39.582 ms
> 38.088 ms
> 18 ae-6-6.car1.Orlando1.Level3.net (4.69.133.77) 36.701 ms 38.144 ms
> 36.949 ms
> 19 unknown.Level3.net (63.209.98.66) 36.902 ms 37.750 ms 37.717 ms
> 20 andc-br-3-f2-0.atlantic.net (209.208.9.138) 37.729 ms 35.812 ms
> 35.048 ms
> 21 ocalflxa-br-1-s1-0.atlantic.net (209.208.112.98) 37.485 ms 37.601 ms
> 36.495 ms
> 22 * * *
> 23 sl-bb20-dc-6-0-0.sprintlink.net (144.232.8.174) 56.459 ms 56.449 ms
> 57.709 ms
> 24 sl-st20-ash-10-0.sprintlink.net (144.232.20.152) 57.694 ms 57.692 ms
> 60.243 ms
> 25 te-10-1-0.edge2.Washington4.level3.net (4.68.63.209) 103.257 ms
> 100.829 ms 82.571 ms
> 26 vlan99.csw4.Washington1.Level3.net (4.68.17.254) 70.401 ms
> vlan89.csw3.Washington1.Level3.net (4.68.17.190) 69.262 ms
> vlan99.csw4.Washington1.Level3.net (4.68.17.254) 82.700 ms
> 27 ae-81-81.ebr1.Washington1.Level3.net (4.69.134.137) 74.132 ms
> ae-61-61.ebr1.Washington1.Level3.net (4.69.134.129) 74.135 ms
> ae-81-81.ebr1.Washington1.Level3.net (4.69.134.137) 75.540 ms
> 28 ae-2.ebr3.Atlanta2.Level3.net (4.69.132.85) 58.656 ms 60.838 ms
> 54.346 ms
> 29 ae-71-70.ebr1.Atlanta2.Level3.net (4.69.138.18) 59.323 ms
> ae-61-60.ebr1.Atlanta2.Level3.net (4.69.138.2) 59.336 ms
> ae-71-70.ebr1.Atlanta2.Level3.net (4.69.138.18) 63.323 ms
> 30 ae-1-8.bar1.Orlando1.Level3.net (4.69.137.149) 127.652 ms 57.884 ms
> 57.851 ms
>
> >From the traces I've seen, it seems if the first Sprint hop is sl-bb20-dc,
> the private IP hops don't show up. If the first Sprint hop is sl-crs2-dc,
> then the private IP hops are there. I wonder if anyone from Sprint can
> shed some light on that?
>
> Unfortunately, the Sprint engineer I intitially made contact with who was
> helpful and seemed curious about the issue seems to have vanished and
> isn't returning my calls or emails. Anyone else from Sprintlink care to
> play?
>
> ----------------------------------------------------------------------
> Jon Lewis | I route
> Senior Network Engineer | therefore you are
> Atlantic Net |
> _________ http://www.lewis.org/~jlewis/pgp for PGP public key_________
>
>
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