Presentations and Notes from the 2nd Global Annual Symposium on DNS Security, Stability and Resiliency

On Monday, February 1, 2010, DNS-OARC organized a few presentations at the start of the 2nd Global Annual Symposium on DNS Security, Stability and Resiliency

Presentations and Notes

13:15: Introduction to DNS-OARC
Roy Arends / DNS-OARC

13:25: Investigating anomalous DNS traffic: A proposal for an address reputation system
Sebastian Castro / .NZ Registry Services

Willingness of operators to cooperate? There's a need from the TLD operators to handle these kinds of events - to get traction, plans to address a wider audience.

Building a reputation system is one thing, but using it is another. What's the risk of impacts of false positives (in social and technical domains)?
Blacklists frequently don't use such complicated schemes - haven to considered highly strict rules in terms of how identities are added. But this will require more thought as experience grows.

Once you have a blacklist, how would you envisage its use? Blocking services? Other? Filter at the edge? Haven't thought fully about this - future study topic.

Comment: During the event, several AS's were shown. This kind of data shows that a larger-scale study might be needed. So, before we get to "how does the blacklist work?", it might be good to do a larger study.

Comment: There needs to be some back-pressure to deal with trying to get the source to pull their weight to resolve the problem (spammers, for instance). Otherwise, the blacklist's size is going to monotonically increase.
Experientially, it is harder to work the problem back to the source.

Comment: As an operator, this data (or a blacklist) by itself is not particularly interesting, however, in combination with other data -- through a kind of intelligence fusion of data -- we can see some real useful trends. In other words, it's the combining of the data that is interesting, in terms of characterizing the curent situation.

Comment: How far down the chain of "operators" do you go to collect (and analyze) this data? Different players have different ideas about what constitutes an anomaly, or what constitutes a threat. Avoiding subjective language, such as good or bad, might be helpful.

Comment: One idea put forth was that ADSL customers who are characterized as 'bad' should be blocked from using (particular country's) network resolution services, because it could lead to an attack. (if somoene can clarify this comment, please chime in.)

Comment: Seeing an anomaly might not necessarily indicate that a problem is existential in the indicated domain. Instead, it is possible that you are measuring a problem in your own infrastructure.

How much resource (time & money) did this cost .NZ to do this investigation? And how are we going to see sharing of this data? Comment: Smaller organizations are not going to have the resources to be able to focus on all this data.
Two days of work. Tools used were largely already done (and known to the principal investigator).

14:01: APNIC DNS Measurement & Perspectives on 'DNS Health'
George Michaelson / APNIC

Interesting observation that, with DNSSEC enabled, we can see average UDP packet size increasing, including a fair body of packets with a size in excess of 800 bytes, and that this could have a significant impact in the face of network designer assumptions that valid UDP traffic should nominally not include traffic with packet sizes exceeding 512 bytes.

If we divide our measurement data into two categories: one caused by human behavior and one by machines, can we explain the diurnal pattern shown in the NXDOMAIN from DSC slide?
Yes, it seems natural that there would be two different categories, and your observation is likely correct. However, there is still work to perform to characterize the collected data in a meaningful way.

Comment: Talked about DSC and that it is doing some things and should be doing others. DNS-OARC was started as an NSF grant. A problem was "how to spend $ in a meaningful, useful way". Now gathering requirements for next generation work. I want NSTAT - a place where data can be aggregated en masse. Now a framework seems achievable. (Next need is funding.)

Admin note: Break moved to 15:00 and shortened to 1/4-hour

14:41: Measurement for ascertaining health of the DNS
James Galvin / Afilias

Note-taker comment: Taking longer notes on this talk, because the slides are highly summarized.

When we collect data in this kind of context, we have to think about massive amounts of data, the size of which is going to grow continually. Are we going to collect measurements in the raw and keep that? Or are we going to create summaries and then aggregate the summaries?

The next question is, how do we collect the data in a sampling? Do we start looking outside our own infrastructure (at the entry point, for instance) or are we going to look inside our infrastructure. There are arguments for and against each possibility, largely because sampling at one point and not sampling at another will gain or lose important statistically important data. This question is deserving of additional study.

We need to think about things like creating a technical advisory board of people who are knowledgable about the subject of analyzing the data so that we can make sense of the information that is collected. This analysis needs to take into account ideas from a wide variety of points of view.

What will the introduction of DNSSEC do to the collection and analysis of data? Will is create a new vector to analyze? Or will it accentuate the negative aspects of existing data analysis?

w.r.t. DNSSEC widespread implementation: As the amount of data being moved increases, and as we see more signed zones being transferred, then we have to think about whether instantaneous propagation is the right model.

Last point, for discussion, is DNS views ("views" is inteded to have a generic usage, with apologies to BIND). Hypothesis is that views and filtering are going to become mainstream -- and perhaps even mandated in some jurisdictions. In sum, entire zones will not be delivered. Filtering will be required in some circumstances.

No questions.

Admin note: Break until 15:15.

15:18: Characterizing DNS Client Behavior using Hierarchical Aggregate
Keisuke Ishibashi / NTT

(Referring to slide 11, "Experimental results":) You claim that your methodology increases accuracy by 10-20%. What's your ground truth to be able to make such an assertion?
Investigator made criteria but this was rough guidance only.

Is the mathematics intended to achieve on-the-fly results? Second question, this seems simple and cheap to calculate, but it might lead to misclassification rates that are high. What is the intent of the use of the result of the calculation? Comments?
Yes, it is an easy calculation and a valid comment. (nfi)

15:49: JPRS activities on monitoring and measurement of JP DNS and the registry system
Shinta Sato, JPRS

Do you develop these criteria for yourself and then discuss it in the community? Is there a reflection of others' needs in these criteria?
We haven't asked external communities -- these are very internal thoughts, which we have not opened up to the public.

What drove those numbers (you picked 15m, 1h) -- is there some goal you're aiming at?
These values were set merely from our own thoughts, not based on some particular objective standard. These values would need to be revisited from time to time to ensure validity.

You chose 50% change in the size of the zone. Do you also cb3r3seck to see the # of changes to the zone?
No, we only check the file size of the zone. We don't account to changes in the resource records.

In Japan, when you're referring to [medical] health, there's "public health" and "private health". We are seeing that your zone files are handled exclusively within your domain? Or do you allow transfers to areas outside your own domain (where you are not fully in control of the health of the environment)?
We don't transfer our entire dataset outside.

We've heard about how to become healthy or how to stay healthy, but it didn't really address what to do when we've become sick.
When the zone file changes are too large, we keep using the zone file and alert to the operators to see what is wrong. After the detection of the existence of an unhealthy state, we have other operational procedures that exceed the scope of this talk, so I did not cover these topics here.

16:19: L-Root Update
Joe Abley / ICANN

This is an operational update on L-Root, not a talk about the signing of L-Root.

Long-Term Query Capture (LTQC) is a tool used by several other root servers. The data from it are stored at OARC. It has the distinct advantage of being targeted and the resultant datasets are small.

Beyond what's shown on the slide (#1), we also have other ongoing tasks, such as graphically displaying trends and data.

On 2010-01-27, we made a transition from the unsigned root to the DURZ.

Questions slide: (1) What else should we measure? (2) What analysis could be done on what we are measuring to identify problems?

Comment: How many half-open TCP connections should be allowed before shutting them down? How much of that (measurement) do you keep?
We are not keeping that data, and it's a good point.

Comment: You don't know how many queries went dark (since DNSSEC went live).
Other person's comment: Trying to distinguish between requests and other data.
Other person's comment: It's also impossible to know what you don't know.

If people have thoughts about what triggers should cause alarm, we would be very interested in capturing that data.

16:41: January 12 Baidu's Attack - What Happened and What Shall We Do?
Wang Zheng / CNNIC

Many efforts underway to enhance the security of DNS service, such as DNSSEC, as one instance. The January 12 attack against Baidu is just a reminder to us to keep an eye on the security of the registry system.

Prior to the January 12 attack, had only been attacked significantly on one occasionally, in December 2006.'s registry is, based in New York.

Chain of events ("at first sight"):
0740 on January 12, Baidu went offlien and traffic was redirected to a website in the Netherlands

DNS records were modified, causing the redirection
It is believed that was breached, allowing access to, and the gaining of modification rights, to's records.

At 0901, dig showed baiducom's NS records pointing to yns{1,2}
At 0936, -> ns230{3,4}

Similarly, the registry information was clearly changed at various points in time during the day.

Registrar: Rollback done by at reqeust of Baidu
Direct correction of the record was declined by the registrar, due to a claimed lack of authority.

[Outline of Registry->Registrar->Registrant chain]

Points to consider: do we need special security protections? do we need enhanced communciations between registrant and registrar?

What is the status of the lawsuit against by
We have no information about this. However, it is likely that the aim of this action is to try to get a clear explanation from as to why this was so problematic. In a larger sense, this kind of issue needs to be more clearly resolved in order to enhance the stability of the entire Internet.

What TTLs were set? How long did it take for to figure out that Baidu was making a legitimate request?
Baidu asked to reset the records. However, refused to directly correct the DNS records. The only possible mechanism was a rollback. We do not have all of the operational details of what happened, but the resolution was not handled immediately, and there was a long (measured in hours) time before the records were fully corrected.

16:58: DNSCheck and DNS2db
Patrik Wallstrom / .SE

No questions.

Tuesday Morning: Symposium Keynote Address
Andrew Sullivan / Shinkuro

Submitted by admin on Fri, 2010-02-26 16:45.

L-Root now serving "DURZ" signed responses

In case you haven't heard, L.ROOT-SERVERS.NET began serving a DNSSEC-signed root zone today. DNS-OARC has been collecting data during the signed root rollout. The graph below shows how L-root's priming response size has increased during the last hour since it first began serving signed responses:

We're also watching the data below to see if there are noticeable increases in priming query rates after signing:

So far so good.


And here's something I should have thought to include initially. The following graph shows the increase in TCP queries for L-root. This is over all queries, not just priming queries:

Submitted by wessels@ on Wed, 2010-01-27 19:15.

DITL 2009 Data: Query rates to TLDs with wildcards

Last week someone asked me if the DITL 2009 data could shed any light on the amount of queries sent to TLDs with wildcards. While we do have data from a few TLD operators, it wouldn't really help to answer this question. However, I think we can get a "first-order approximation" by looking at the queries to root nameservers. Note that by looking at queries to the roots, we have no knowledge of client queries that are cache hits and those that are sent to the TLD nameservers due to cached referrals. We could perhaps turn to a passive DNS collection, such as SIE for a second opinion.

The long, tall graph below shows the query rate to each TLD seen in the DITL 2009 data. Those TLDs known to have wildcards are shown in blue. Note, however, that some TLDs (such as CN and KR) implement wildcards only for IDN names.

Submitted by wessels@ on Mon, 2009-11-30 18:34.

Another Look at Reply Size Test Data

A couple months ago I posted some data from the OARC reply size test service. Recently some folks have been wondering if the situation is getting better or staying the same. Today I created a graph that shows the monthly trend:

The data probably does not contain enough samples to ascertain any trends. The number of samples in each month is shown at the top of the bars. Also keep in mind that this data has a self-selecting bias. We're only measuring resolvers of people that choose to use this service.

Submitted by wessels@ on Tue, 2009-11-10 19:18.

Signed Root Zone Rollout and Schedule Announced

Here at the RIPE 59 meeting in Lisbon, Joe Abley from ICANN and Matt Larson from VeriSign announced a plan and schedule for signing the Root Zone. A number of interesting tidbits:

  1. The root zone will technically be signed by December 1, 2009 although ICANN and VeriSign will keep it to themselves for internal testing.
  2. Between January and July 2010, the root servers will begin serving the signed zone one "letter" (server) at a time.
  3. Also during this rollout period, actual DNSSEC keys will be replaced with "dummy" keys so that validation CANNOT occur. In other words, the public components of the signing keys will not be published, which makes it impossible to configure a trust anchor for the root zone.
  4. During the rollout period, the traffic on both signed and unsigned roots will be monitored for impacts and effects.
  5. By July 1, 2010 the KSK will be rolled and published to achieve a fully signed root zone.

The RIPE presentation contains additional details such as key sizes, algorithms and rolling intervals.

Submitted by wessels@ on Wed, 2009-10-07 11:17.

Report on the Impacts of Changes to the DNS Root Zone

Earlier this year, ICANN contracted with DNS-OARC to study the impacts of potential changes facing the DNS root zone. These changes include: (1) a significant increase in the number of gTLDs, (2) signing the zone with DNSSEC, and (3) continued increase in IPv6 glue. In our study we explore how these changes affect:

  • The size of the zone (e.g., on disk and in memory)
  • How much time is required to load, or reload, the zone
  • Latency and performance of serving the zone
  • Time and bandwidth necessary for zone transfers
  • Truncated responses and retries over TCP

Executive Summary

This excerpt of the executive summary is taken from the full report:

Our analysis of zone size focuses on memory usage. As expected, we find that memory requirements increase linearly with zone size. We also find that, for a given number of TLDs, signing the zone increases the memory requirement by a factor of 1.5–2. Additionally, we find that 32 GB of memory is insufficient for serving a very large root zone (e.g., a signed zone with 10 million TLDs), particularly when using NSD.

The response latency measurements find negligible increases (typically less than one millisecond) with NSD. For BIND (9.6.0-P1), however, we find some response time degradation with a large signed root zone (e.g., greater than 100,000 TLDs). With a 100,000 TLD signed zone, BIND drops nearly 30% of all queries sent at a rate of 5000 queries per second. With a one million TLD signed zone, BIND drops over 80%. NSD also begins to show some signs of stress with a very large (4.5 million TLD) zone where over 40% of queries are dropped.

The reload and restart times measurements are relatively straightforward and contain no real surprises. Loading and reloading times are generally proportional to zone size. Loading a 1 million TLD signed zone takes 190 seconds with BIND and 227 seconds with NSD.

To measure inter-nameserver bandwidth we performed a number of zone transfers between master and slave nameservers. We tested both standard (AXFR) and incremental (IXFR) zone transfer mechanisms. One interesting result of the AXFR test is that an NSD master utilizes 20–30% less bandwidth than a BIND master to send a given zone. To assess the duration of a zone transfer under wide-area network conditions, we introduced simulated packet loss and delays. A zone transfer experiencing 1% packet loss takes more than 2.5 times longer than with no packet loss for any given tested latency.

To explore increased TCP at root servers, we replayed real query 1streams to servers with signed zones. We found that between 0.3% and 0.8% of responses to UDP queries would be truncated, likely causing most these clients to fall back to TCP. This means that root servers can expect to see at least an order of magnitude increase (e.g., from 5 to 50 per second) in queries over TCP when the root zone is signed. Additionally, we found that a large (e.g., one million TLD) signed root zone will likely result in a slightly higher proportion of TCP queries than a signed version of the current one. Finally, we examined data for the .org TLD from before and after DNSSEC was deployed and found evidence suggesting that the actual increase in TCP-based queries could be significantly higher than can be forecast by evaluating current DNS traffic patterns.

Submitted by admin on Thu, 2009-09-17 18:51.

Statistics on UDP checksums in 2009 DNS DITL data

I was recently asked if OARC had any data on the percentage of DNS queries with bad or disabled UDP checksums. After a few days of crunching through the DITL 2009 data, I have the following results:

Data Provider Matched Mismatch Disabled
afilias 99.02 0.85 0.12
apnic 99.89 0.01 0.10
arin 99.92 0.02 0.07
arl 99.28 0.62 0.10
as112-gf 99.91 0.00 0.09
cogent 52.30 47.66 0.04
cznic 73.50 26.40 0.10
icann 99.52 0.36 0.12
iis 97.58 2.36 0.06
isc 96.73 3.14 0.13
lacnic 99.79 0.01 0.19
namex 100.00 0.00 0.00
nasa 99.34 0.57 0.09
nethelp 99.90 0.06 0.05
niccl 53.94 46.01 0.04
nixcz 99.82 0.18 0.00
nominet 99.80 0.04 0.15
pktpush 69.29 26.04 4.67
regbr 98.30 1.48 0.22
ripe 98.81 1.07 0.12
switch 99.91 0.01 0.08
tix-or-tz 100.00 0.00 0.00
uninett 99.91 0.01 0.08
verisign 99.03 0.92 0.05
wide 99.58 0.36 0.06

Obviously, its interesting that most of the traces show 99% matching checksums, but a few have close to 25% or 50% with mismatches. I'm likely to suspect some kind of "middle boxes" (load balancers?) at play here, but have not yet investigated further.


Mauricio from NIC Chile reports that most of their bad UDP checksums are from replies they send out. They have some Dell hardware running Linux. The Linux installation doesn't support certain NIC hardware features, such as checksum calculations. Hardware checksumming can be disabled with this command:

# ethtool --offload ethXX tx off

Submitted by wessels@ on Wed, 2009-08-19 05:19.

Data from the Reply Size Test server

Over on the IETF namedroppers mailing list there is a discussion about DNSSEC and UDP fragmentation. See this thread, for example. Since the OARC Reply Size Test has been going for a couple of months now, I thought maybe it would have enough data for a decent analysis. Here's what I found:

The sample isn't quite as large as I'd like. Only about 1650 unique client addresses tested so far.

The bar plots show histograms of the number of clients (on y-axis) receiving certain maximum reply sizes (x-axis). Each plot is for a different advertised EDNS receive buffer size. Note that the 512 data also includes clients that didn't send any EDNS information.

Normally the Reply Size Test utility won't return responses larger than the advertised buffer size. However, in the 512 data you can see a few counts around 4000. This can happen for one of two reasons: first, a clever user can "trick" the utility by sending queries with certain values in the query name. Second, there is a special mode for Nominum CNS, which won't send EDNS information unless it first receives a truncated response.

The most interesting data is in the 4096-bytes plot. Most of the clients can receive 4K responses. However, about 20% are limited to 2K or less. The bar just left of 1500 on the x-axis represents clients that cannot receive fragmented DNS responses.

Submitted by wessels@ on Tue, 2009-08-18 23:34.

OARC's DNS Reply Size Test Server

Recent increases in DNSSEC deployment are exposing problems with DNS resolvers that cannot receive large responses.

The maximim reply size between a DNS server and resolver may be limited by a number of factors:

  • If a resolver does not support the Extension Mechanisms for DNS (EDNS), replies are limited to 512 bytes.
  • The resolver may be behind a firewall that blocks IP fragments.
  • Some DNS-aware firewalls block responses larger than 512 bytes.

The BIND resolver, since version 9.5.0, includes a feature to decrease its advertised EDNS receive buffer size (down to 512) when its queries time out. We've seen this lead to significant increases in TCP for DNSSEC-signed zones.

DNS-OARC built the DNS Reply Size Test Server to help users identify resolvers that cannot receive large DNS replies.

How To Use

To use the DNS Reply Size Test Server, simply use dig command line tool to issue a TXT query for the name

$ dig +short txt

You can test a specific DNS resolver by using the @server feature of dig.

The output should look something like this:
" sent EDNS buffer size 4096"
" DNS reply size limit is at least 4023 bytes"

The first three lines of the output are CNAME records in the response. The "x" numbers represent the sizes of successfully received responses at each step in the test. The final two lines are TXT records that provide information on the test results. Here we can see that the resolver advertised a receive buffer size of 4096 and the server was able to send a response of 4023 bytes.

If your test results in a reply size limit of less than about 4,000 you may want to investigate further for one of the following problems:


The following result comes from a DSL router that does not support EDNS:
"X.X.X.X DNS reply size limit is at least 486 bytes"
"X.X.X.X lacks EDNS, defaults to 512"

IP Fragments Filtered

If you're behind a firewall that filters IP fragments, you can expect to see a reply size limit slightly less than 1400 bytes:
"X.X.X.X sent EDNS buffer size 4096"
"X.X.X.X DNS reply size limit is at least 1382 bytes"

Inconsistent Results

The test may provide inconsistent results if any of the DNS reply packets are dropped or reordered. The records in the reply have a TTL of 60 seconds or less, so feel free to rerun your test again after waiting 60 seconds.

Truncated, retrying in TCP mode

Some resolvers (e.g. BIND-9.6) send the bloated authority section back to dig. Since dig doesn't set an EDNS receive buffer size by default, the reply may be truncated. You can avoid this problem by telling dig to advertise a large receive buffer. For example:

$ dig +bufsize=1024 txt

Note that the EDNS receive buffer size is a "hop-by-hop" parameter, which means that setting it here does not affect the test between your resolver and the OARC server.

Nominum CNS

Nominum's CNS resolver is designed to utilize EDNS only after first receiving a truncated response. To use this test with a CNS resolver, issue the following query:

$ dig txt

The special name "tcf" instructs the server to set the TC bit in responses if the query doesn't have an EDNS pseudo-record. This should cause CNS to re-query with EDNS.

How It Works

For the technically inclined, here's how the DNS Reply Size Test Server works:

  • An initial DNS query leads to a chain of CNAME responses. It currently uses a CNAME chain of length three.
  • A custom DNS server sends multiple replies. The replies decrease in size. The CNAME rdata encodes the size of the reply. Assuming the UDP replies don't become reordered, the resolver will follow the CNAME of the largest reply it can receive.
  • The authority and additional sections of the CNAME reply are bloated to make the response large.
  • The server uses something akin to a binary search to refine the reply size limit. For example, if the first step indicates that the limit is between 1024 and 2048, then the next step will send a number of logarithmically-stepped responses between that range (2048, 1625, 1290, 1024).
  • At the end a TXT RR is sent indicating the measured reply size limit.

Additional Notes:

  • This tool can only test the path between itself and the resolver. I cannot test the path between the resolver and dig.
  • The test server works over both IPv4 and IPv6.
  • The custom DNS server does not yet support name compression. This has a small effect on its ability to build a message of a certain size. Currently it takes 37 NS+glue records to bloat the response to near 4K. With compression it would take significantly more than that, so I'm not sure compression should even be enabled.
  • It might be better to bloat the response with a small number of Unknown (ie binary) RR types, assuming they be passed through by forwarders.

Additional Resources

Submitted by admin on Tue, 2009-07-07 21:54.


A couple weeks ago I gave a lightning talk at NANOG46 titled DNSSEC, EDNS and TCP using data from before and after the .ORG zone became signed. Afilias and PIR have been gracious enough to share data from this event with DNS-OARC.

As we recently speculated, the .ORG data clearly shows that queries with DO=1 and a small EDNS buffer size leads to an increase in DNS queries over TCP:

Prior to publishing the signed zone, this server recieved very little TCP traffic (maybe once per 5 seconds). After the signed zone was fully published at 4PM, the TCP query rate is closer to 75 60/second.

Next I looked at what types of queries are in the TCP traffic. The following plot shows it is mostly A, MX, and AAAA:

Prior to signing, the TCP traffic is just noise and should be ignored. After signing we see about 70% A, 15% MX, and 10% AAAA. This is more-or-less normal. Upon comparison to the UDP query types, we see that TCP has a higher percentage of A queries, lower percentage of MX, and a higher percentage of AAAA.

I also looked at the Rcodes in the TCP traffic:

It shows 82% NOERROR and 17% NXDOMAIN for TCP, whereas it is 71% and 29% respectively for UDP. In other words, the TCP queries are not due a particular query type or response code.


I was asked for some more specific numbers since it can be difficult to determine values from the logarithmically scaled graphs. The following table shows hourly mean values for the data in the first plot above. All numbers are queries per second:

Hour UDP EDNS=512 TC=1 TCP
12:00:00 3799 15.4 0.00 0.23
13:00:00 4271 16.8 0.00 0.21
14:00:00 4350 17.1 3.23 1.82
15:00:00 4305 26.7 21.1 13.3
16:00:00 5002 112.6 96.3 59.9
17:00:00 4923 100.7 85.3 56.9
18:00:00 4581 99.2 84.3 57.9
Submitted by wessels@ on Thu, 2009-07-02 00:20.