I am not sure about the loop, but wouldn't a static or default static route specifying an outbound interface rather than a next hop router ip address on a multi-access network like Ethernet, frame-relay or ATM with connections to 5 or more routers cause one router to output a stream of packets to a subnet and all five or more receiving routers receiving the packets possibly forwarding them separately causing the duplicate packets? Just a thought, Matt Rice Seattle ----- Original Message ----- From: <nanog-request@nanog.org> To: <nanog@nanog.org> Sent: Sunday, August 10, 2008 8:21 PM Subject: NANOG Digest, Vol 7, Issue 26
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Today's Topics:
1. Re: maybe a dumb idea on how to fix the dns problems i don't know.... (Chris Paul) 2. Re: maybe a dumb idea on how to fix the dns problems i don't know.... (Chris Paul) 3. Re: maybe a dumb idea on how to fix the dns problems i don't know.... (Cat Okita) 4. Re: maybe a dumb idea on how to fix the dns problems i don't know.... (Joe Greco) 5. impossible circuit (Jon Lewis) 6. Re: maybe a dumb idea on how to fix the dns problems i don't know.... (William Herrin)
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Message: 1 Date: Sun, 10 Aug 2008 18:27:29 -0700 From: Chris Paul <chris.paul@rexconsulting.net> Subject: Re: maybe a dumb idea on how to fix the dns problems i don't know.... To: nanog@nanog.org Message-ID: <489F9581.4010801@rexconsulting.net> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Joe Greco wrote:
Pretending for a moment that it was even possible to make such large scale changes and get them pushed into a large enough number of clients to matter, you're talking about meltdown at the recurser level, because it isn't just one connection per _computer_, but one connection per _resolver stub_ per _computer_ (which, on a UNIX machine, would tend to gravitate towards one connection per process), and this just turns into an insane number of sockets you have to manage.
Couldn't the resolver libraries be changed to not use multiple connections?
I think that the text I wrote clearly assumes that there IS only one connection per resolver instance. The problem is that hostname to IP lookup is pervasive in a modern UNIX system, and is probably pretty common on other platforms, too, so you have potentially hundreds or thousands of processes, each eating up additional system file descriptors for this purpose.
Well how I read what you first wrote implied that the resolvers are now going to DOS servers with millions of connections due to each resolver stub making a TCP connection... I say this is something that if true, can and should be changed.
Now you say that file descriptors on the client are going to run out Isn't that changing the topic? And is that even really a problem?
So each process that needs to do a lookup opens a file descriptor for a TCP connection, right? Whereas with UDP we don't have to do this. Is this what I'm hearing you say? That I understand. (Hmm don't udp connections take sockets too? Not sarcastic here.. just asking...)
And it is a good point but is this client file descriptor an insurmountable problem? Also, what about the millions of connections to the server? Is that really necessary for a dns resolver on one system to open more than one TCP connection to its caching dns server?
I'm not saying that caching dns servers should keep open TCP connections to authoritative name servers! OK? But how much latency do you increase e on that uncached recursive lookup by changing to TCP?
CP
-- Chris Paul Rex Consulting, Inc email: chris.paul@rexconsulting.net phone, direct: +1, 831.706.4211 phone, toll-free: +1, 888.403.8996
The information transmitted is intended only for the person or entity to which it is addressed and may contain confidential and/or privileged material. Any review, retransmission, dissemination or other use of, or taking of any action in reliance upon, this information by persons or entities other than the intended recipient is prohibited. Rex Consulting, Inc. is a California Corporation.
P Please don't print this e-mail, unless you really need to.
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Message: 2 Date: Sun, 10 Aug 2008 18:29:00 -0700 From: Chris Paul <chris.paul@rexconsulting.net> Subject: Re: maybe a dumb idea on how to fix the dns problems i don't know.... To: nanog@merit.edu Message-ID: <489F95DC.4010403@rexconsulting.net> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Victor Jerlin wrote:
Couldn't the resolver libraries be changed to not use multiple connections?
And we'll change to IPv6 tomorrow! Total apples and oranges. We all have to patch anyhow. This is just code and firewall rules. IPv6 is way more complicated, friend.
-- Chris Paul Rex Consulting, Inc email: chris.paul@rexconsulting.net phone, direct: +1, 831.706.4211 phone, toll-free: +1, 888.403.8996
The information transmitted is intended only for the person or entity to which it is addressed and may contain confidential and/or privileged material. Any review, retransmission, dissemination or other use of, or taking of any action in reliance upon, this information by persons or entities other than the intended recipient is prohibited. Rex Consulting, Inc. is a California Corporation.
P Please don't print this e-mail, unless you really need to.
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Message: 3 Date: Sun, 10 Aug 2008 21:34:26 -0400 (EDT) From: Cat Okita <cat@reptiles.org> Subject: Re: maybe a dumb idea on how to fix the dns problems i don't know.... To: Chris Paul <chris.paul@rexconsulting.net> Cc: nanog@merit.edu Message-ID: <20080810213344.D15677@gecko.reptiles.org> Content-Type: TEXT/PLAIN; charset=US-ASCII; format=flowed
On Sun, 10 Aug 2008, Chris Paul wrote:
And we'll change to IPv6 tomorrow! Total apples and oranges. We all have to patch anyhow. This is just code and firewall rules. IPv6 is way more complicated, friend.
No - IPv6 is just code (and not even firewall rules).
cheers! ========================================================================== "A cat spends her life conflicted between a deep, passionate and profound desire for fish and an equally deep, passionate and profound desire to avoid getting wet. This is the defining metaphor of my life right now."
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Message: 4 Date: Sun, 10 Aug 2008 21:35:58 -0500 (CDT) From: Joe Greco <jgreco@ns.sol.net> Subject: Re: maybe a dumb idea on how to fix the dns problems i don't know.... To: chris.paul@rexconsulting.net (Chris Paul) Cc: nanog@nanog.org Message-ID: <200808110235.m7B2Zwxn028353@aurora.sol.net> Content-Type: text/plain; charset=us-ascii
I think that the text I wrote clearly assumes that there IS only one connection per resolver instance. The problem is that hostname to IP lookup is pervasive in a modern UNIX system, and is probably pretty common on other platforms, too, so you have potentially hundreds or thousands of processes, each eating up additional system file descriptors for this purpose.
Well how I read what you first wrote implied that the resolvers are now going to DOS servers with millions of connections due to each resolver stub making a TCP connection... I say this is something that if true, can and should be changed.
Sure. We can introduce a new feature, called a "local recurser," which will do unified name resolution for all lookups asked for by any process on the box.
Now, of course, the box enjoys certain benefits, such as being able to remember who "MX nanog.org" is for the second time without having to bother an external recurser. And a hypothetical ability to forward all requests via TCP to the external recurser. Except, why bother, now that you have the capability right on the box, why be dependent on anything else? Might as well just let it resolve all by itself.
Of course, the box also enjoys certain other liabilities, such as the next time that all the name servers in the world need to be upgraded, you now have just that many more recursers that are running on unattended autopilot (because heaven knows, most PC's run without a professional admin to keep things up to snuff, and this last problem wasn't exactly the sort of thing you can just "auto update," because someone actually has to verify that there aren't externalities such as NAT devices, etc!)
Sounds like a real fun time.
Now you say that file descriptors on the client are going to run out Isn't that changing the topic? And is that even really a problem?
Actually, it's quite a problem, for the server. Try, sometime, having a few thousand file descriptors all open, and then running select() on that fdset. But it's not even really that pleasant on many clients. It's a kernel consumable. You try to avoid introducing additional requirements without a good reason.
So each process that needs to do a lookup opens a file descriptor for a TCP connection, right? Whereas with UDP we don't have to do this. Is this what I'm hearing you say? That I understand. (Hmm don't udp connections take sockets too? Not sarcastic here.. just asking...)
You open and then close it for UDP. You can do that for TCP, too, at a substantial penalty.
And it is a good point but is this client file descriptor an insurmountable problem? Also, what about the millions of connections to the server? Is that really necessary for a dns resolver on one system to open more than one TCP connection to its caching dns server?
There is no "DNS resolver on one system" unless you put one there. At which point, you can safely ask the question of why would you then connect to a caching server (there are good reasons, in some cases).
The way libresolv works is that it takes those "nameserver" things listed in resolv.conf and sends requests to them. Since any program that uses the network is likely to be linked to libresolv, you can have lots of different programs on a system, each of which may want to resolve different name resources. There is no monolithic "thing" on a box to do name resolution unless you put one there.
I'm not saying that caching dns servers should keep open TCP connections to authoritative name servers! OK? But how much latency do you increase e on that uncached recursive lookup by changing to TCP?
Since latency would not be extremely high on my list of concerns with this plan, I'll pass and say "I don't really care to speculate." There are many other ways you'll have lit your hair on fire before latency is a big concern.
... JG -- Joe Greco - sol.net Network Services - Milwaukee, WI - http://www.sol.net "We call it the 'one bite at the apple' rule. Give me one chance [and] then I won't contact you again." - Direct Marketing Ass'n position on e-mail spam(CNN) With 24 million small businesses in the US alone, that's way too many apples.
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Message: 5 Date: Sun, 10 Aug 2008 23:15:47 -0400 (EDT) From: Jon Lewis <jlewis@lewis.org> Subject: impossible circuit To: nanog@nanog.org Message-ID: <Pine.LNX.4.61.0808102146400.5503@soloth.lewis.org> Content-Type: TEXT/PLAIN; charset=US-ASCII; format=flowed
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_________
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Message: 6 Date: Sun, 10 Aug 2008 23:21:01 -0400 From: "William Herrin" <herrin-nanog@dirtside.com> Subject: Re: maybe a dumb idea on how to fix the dns problems i don't know.... To: nanog@merit.edu Message-ID: <3c3e3fca0808102021q5f64733em4cde0b8e00ffb697@mail.gmail.com> Content-Type: text/plain; charset=ISO-8859-1
On Sat, Aug 9, 2008 at 5:18 PM, Chris Paul <chris.paul@rexconsulting.net> wrote:
Sorry if this is real stupid for some reason because I don't think about DNS all day (I'm the ldap dude) but since we have faster networks and faster cpus today, what would be the harm in switching to use TCP for DNS clients? The latency on the web isn't dns anymore ever it seems to me.....
Latency on in-addr lookups where you typically traverse multiple forward trees to find the NS servers would seriously suck. At best, a TCP-based lookup performs at about a third of the speed of a UDP lookup. Worse unless your implementation is carefully optimized and you make sure that the OS isn't adding options to the front of the handshake. You have at least the whole syn/synack/ack handshake before you can even ask the question.
Then there's the server cost associated with keeping that much state...
On Sat, Aug 9, 2008 at 6:10 PM, Matt F <matt@credibleinstitution.org> wrote:
Why not just require TCP for a lookup if a response with an incorrect TXID is received? You could require TCP for just the one lookup or for some configured interval, say 1 hour. That should slow attackers down substantially.
Because the attacker is using a sequence of lookups in order to hit one that lets him poison the cache. That is, he looks up a.google.com, then he looks up b.google.com, then c.google.com, etc. until he gets one where the server accepts his fake DNS server record for google.com.
To be an effective defense, you'd have to do TCP lookups for the whole scope ({anything}.google.com) for some period of time following the bad ID. That in turn would open up a potential DOS where an attacker could force the DNS server to fall back on TCP for essentially everything, overwhelming it.
On Sat, Aug 9, 2008 at 8:25 PM, brett watson <brett@the-watsons.org> wrote:
On Aug 9, 2008, at 3:48 PM, Chris Paul wrote:
Hey authority DNS server operators. Can you make a change to your servers to always allow TCP client connections? Would this be difficult? What would be the harm?
SYN flooding?
SYN flooding is a solved problem.
On Sat, Aug 9, 2008 at 6:04 PM, Joe Abley <jabley@ca.afilias.info> wrote:
TCP works pretty well with anycast too, if you're careful. It's helpful if your transactions are short-lived.
Define "careful." It's always possible for someone to find themselves with an equal cost path to two different servers in the anycast set. Add per-packet load balancing at the fork (which is outside the control of the server operator) and what happens is the request times out and the resovler fails over to the other NS record that isn't anycasted.
Though the protocol is simple enough that it might be possible to fake it. Build yourself a DNS-only stateless TCP stack for the anycasted address. Have the server send the syn-ack without creating any state. The request will almost certainly be entirely contained in one packet, so when it arrives reply to it without creating any state. Ship off as many packets as you need to reply followed by a Fin. Blindly ack any packet that looks like it needs it. If any packets are lost, there won't be any retransmit (you haven't really established a TCP connection) so the query will time out and retry.
Regards, Bill Herrin
-- William D. Herrin ................ herrin@dirtside.com bill@herrin.us 3005 Crane Dr. ...................... Web: <http://bill.herrin.us/> Falls Church, VA 22042-3004
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End of NANOG Digest, Vol 7, Issue 26 ************************************