Impact of Configuration Errors on DNS Robustness

V. Pappas *

Z. Xu *, S. Lu *, D. Massey **, A. Terzis ***, L. Zhang *

* UCLA, ** Colorado State, *** John Hopkins

Motivation

• DNS: part of the Internet core infrastructure– Applications: web, e-mail, e164, CDNs …

• DNS: considered as a very reliable system– Works almost always

• Question: is DNS a robust system?– User-perceived robustness– System robustness

are they the same?

– Thousands or even millions of users affected– All due to a single DNS configuration error

MotivationShort Answer:

“Microsoft's websites were offline for up to 23 hours -- the most dramatic snafu to date on the Internet --because of an equipment misconfiguration”

-- Wired News, Jan 2001

Related Work

• Traffic & implementation errors studies:– Danzig et al. [SIGCOMM92]: bugs– CAIDA : traffic & bugs

• Performance studies: – Jung et al. [IMW01]: caching– Cohen et al. [SAINT01]: proactive caching – Liston et al. [IMW02]: diversity

• Server availability :– To appear [OSDI04, IMC04]

Our Work: Study DNS Robustness

• Classify DNS operational errors:– Study known errors – Identify new types of errors

• Measure their pervasiveness

• Quantify their impact on DNS – availability – performance

Outline

• DNS Overview

• Measurement Methodology

• DNS Configuration Errors– Example Cases– Measurement Results

• Discussion & Summary

net com uk ca jp

foo

buz bar

bar1 bar2 bar3

Zone:Occupies a continues subspace Served by the same nameservers

bar.foo.com. NS ns1.bar.foo.com.bar.foo.com. NS ns3.bar.foo.com.bar.foo.com. NS ns2.bar.foo.com.bar.foo.com. MX mail.bar.foo.com. www.bar.foo.com. A 10.10.10.10

bar

name servers

resource records

Background

caching server

client

bar zone

foo zone

com zone

root zone

asking for www.bar.foo.com

answer:www.bar.foo.com A 10.10.10.10

referral:com NS RRscom A RRs

referral:foo NS RRsfoo A RRs

referral:bar NS RRsbar A RRs

Infrastructure RRs

foo.com. NS ns1.foo.com.foo.com. NS ns2.foo.com.foo.com. NS ns3.foo.com.

foo.com. NS ns1.foo.com.foo.com. NS ns2.foo.com.foo.com. NS ns3.foo.com.

foo.com

comns1.foo.com. A 1.1.1.1ns2.foo.com. A 2.2.2.2ns3.foo.com. A 3.3.3.3

ns1.foo.com. A 1.1.1.1ns2.foo.com. A 2.2.2.2ns3.foo.com. A 3.3.3.3

•NS Resource Record:–Provides the names of a zone’s authoritative servers

–Stored both at the parent and at the child zone

•A Resource Record–Associated with a NS resource record

–Stored at the parent zone (glue A record)

What Affects DNS Availability

• Name Servers:– Software failures – Network failures – Scheduled maintenance tasks

• Infrastructure Resource Records:– Availability of these records– Configuration errors

focus of our work

Classification of Measured Errors

Inconsistency Dependency

LameDelegation

DelegationInconsistency

DiminishedRedundancy

CyclicDependency

The configuration of infrastructure RRs does not correspond to the actual authoritative name-servers.

More than one name-servers share a common point of failure.

What is Measured?

• Frequency of configuration errors:– System parameters: TLDs , DNS level, zone size (i.e.

the number of delegations)

• Impact on availability:– Number of servers: lost due to these errors

– Zone’s availability: probability of resolving a name

• Impact on performance: – Total time to resolve a query

• Starting from the query issuing time

• Finishing at the query final answer time

Measurement Methodology

• Error frequency and availability impact:– 3 sets of active measurements

• Random set of 50K zones

• 20K zones that allow zone transfers

• 500 popular zones

• Performance impact:– 2 sets of passive measurements:1-week DNS

packet traces

Lame Delegation

com

foo

foo.com. NS A.foo.com.foo.com. NS B.foo.com.

A.foo.com

A.foo.com. A 1.1.1.1B.foo.com. A 2.2.2.2

2) DNS error code -- 1 RTT perf. penalty

3) Useless referral -- 1 RTT perf. penalty

4) Non-authoritativeanswer (cached)

1) Non-existing server -- 3 seconds perf. penalty

B.foo.com

Lame Delegation Results

Lame Delegation Results

0.06 sec

0.4 sec3 sec

50%

Lame Delegation Results

• Error Frequency:– 15% of the zones

– 8% for the 500 most popular zones

– independent of the zone’s size, varies a lot per TLD

• Impact:– 70% of the zones with errors lose half or more of the

authoritative servers

– 8% of the queries experience increased response times (up to an order of magnitude) due to lame delegation

C) Geographic location level: - belong to the same city

B) Autonomous system level: - belong to the same AS

Diminished Server Redundancy

com

foo

foo.com. NS A.foo.com.foo.com. NS B.foo.com.

A.foo.com B.foo.com

A.foo.com. A 1.1.1.1B.foo.com. A 2.2.2.2

A) Network level: - belong to the same subnet

Diminished Server Redundancy Results

• Error Frequency:– 45% of all zones have all servers in the same /24 subnet

– 75% of all zones have servers in the same AS

– large & popular zones: better AS and geo diversity

• Impact:– less than 99.9% availability: all servers in the same /24

subnet

– more than 99.99% availability: 3 servers at different ASs or different cities

Cyclic Zone Dependency (1)

com

foo

foo.com. NS A.foo.com.foo.com. NS B.foo.com.

A.foo.com B.foo.com

A.foo.com. A 1.1.1.1

B.foo.com depends on A.foo.com

The A glue RR for B.foo.com missing

B.foo.com. A 2.2.2.2

If A.foo.com is unavailable then B.foo.com is too

Cyclic Zone Dependency (2)

com

foo

foo.com. NS A.foo.com.foo.com. NS B.bar.com.

A.foo.com B.bar.com

A.foo.com. A 1.1.1.1

bar

B.foo.com A.bar.com

bar.com. NS A.bar.com.bar.com. NS B.foo.com.

A.bar.com. A 2.2.2.2

The foo.com zone seemscorrectly configured

The combination of foo.com and bar.com zones is wrongly

configured

The B serversdepend on A servers

If A.foo and A.bar are unavailable, B addr. are unresolvable

Cyclic Zone Dependency Results

• Error Frequency:– 2% of the zones– None of the 500 most popular zones

• Impact:– 90% of the zones with cyclic dependency errors

lose 25% (or even more) of their servers– 2 or 4 zones are involved in most errors

Discussion: User-Perceived != System Robustness

• User-perceived robustness:– Data replication: only one server is needed

– Data caching: temporary masks infrastructure failures

– Popular zones: fewer configuration errors

• System robustness:– Fewer available servers: due to inconsistency errors

– Fewer redundant servers: due to dependency errors

Discussion: Why so many errors?

• Superficially: are due to operators:– Unaware of these errors – Lack of coordination

• parent-child zone, secondary servers hosting

• Fundamentally: are due to protocol design:– Lack of mechanisms to handle these errors

• proactively or reactively

– Design choices that embrace some of them:• Name-servers are recognized with names • Glue NS & A records necessary to set up the DNS tree

Summary

• DNS operational errors are widespread• DNS operational errors affect availability:

– 50% of the servers lost

– less than 99.9% availability

• DNS operational errors affect performance:– 1 or even 2 orders of magnitude

• DNS system robustness lower than user perception– Due to protocol design, not just due to operator errors

Ongoing Work

• Reactive mechanisms:– DNS Troubleshooting [NetTs 04]

• Proactive mechanisms:– Enhancing DNS replication & caching

Thank You!!!