Encrypted DNS - DNS over TLS / DNS over HTTPS

Encryption is coming to mainstream DNS. This briefing discusses the history, protocols and architecture of encrypted DNS, specifically DNS over TLS and DNS over HTTPS. It also describes the impact of DoT and DoH on various operational models.

This briefing was given during DNSheads Vienna #5 at the nic.at office in Vienna on Jan 30 2018.

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Briefing „EncryptedDNS“
DNS overTLS / DNS overHTTPS
DNSheads Vienna #5 · public
2019-01-30 · Alex Mayrhofer · Head ofResearch & Development

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Background
WhyDNS encryptionwas developed

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The DNS anno circa 2012
•SensationalSuccessStory
Age 25, andpracticallyunmodified
•Today: „Nothinggoes“ withoutDNS
•Clear text. Everything
„DNS ispublicanyways?“
•99% UDP, 1% TCP „fallback“
WorstTCP supportever!
•DNSSEC? Makeseverythingsecure, doesn‘tit!!?!
Doesonly„sign“, not „encrypt“
•2013: Snowdenrevelations
NSA: „Clear textPII data… mmmmm…“
IETF: „Ohhsheesh–wedidn‘texpect*that* scale!“
PhotobySimone AcquarolionUnsplash

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„PervasiveMonitoring isan Attack“
•RFC 7258 –„PervasiveMonitoring isa technical
attackthatshouldbemitigatedin thedesign of
IETF protocols, wherepossible“
•Consequence: Review ofall importantprocotols
•DNS –there‘snot evena standardized*option*
forencryption
•Worse–contains„privacydefeating“ mechanism
UnneccessarilytransmitsfullQNAME in manycases
EDNS(0) Client Subnet
•LeakofMeta-Data & Fingerprinting
Re-identificationofindividualsacrossnetworks
But, but…
ohhhhh…
Photo byKotePuertoonUnsplash

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„Weneedencryption“
But wheretostart?

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The DNS Protocol arena
„RecursiveDNS Server“
„AuthoritativeDNS Server“

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IETF DPRIVE* („PRIVateExchange“)
•2014: „Let‘sdeal withthestubresolver
torecursorleg“
Most significantinformationleakage
1:few Relation –Authentication simple
„Don‘tattempttoboiltheocean“
•2018: Re-Chartering: Includes
„recursivetoauthoritative“
More complex: m:nconnections
(Authentication!)
Milestone forend of2019
*https://datatracker.ietf.org/wg/dprive/about/

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DNS over(D)TLS
IETF: DPRIVE / DNSOP / (TLS)

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Liste ofrelevant RFCs
•RFC 7626 –DNS Privacy Considerations(DPRIVE)
•RFC 7766 –TCP Transport forDNS (DNSOP)
•RFC 7816 –QNAME Minimization(DNSOP)
•RFC 7828 –EDNS keepalive(DNSOP)
•RFC 7858 –DNS overTLS (DPRIVE)
•RFC 8094 –DNS overDTLS (DPRIVE)
•RFC 7830 (+RFC 8467) –DNS Padding(DPRIVE)
•RFC 8310 –UsageProfiles
•RFC 8446 –TLS 1.3 (TLS)

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RFC 7626 –DNS Privacy Considerations
•Privacy aspects/ issuesin areasoftheDNS:
In theDNS message(Query Name, IP Adresse)
On theserver
On theWire
Re-Identificationbasedon patterns
•Killsthe„DNS ispublicanyways!“ argument
Website of„AlcoholicsAnonymous“ ispublic
The factthatsomeonevisitsthatwebsiteregularlyisdefinitely
privacyrelevant!
•Practicalexample(similar..)
drugstoremorningafterpillvienna16.at
(Browser searchrequestsleakingtotheDNS?)

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RFC 7766 –TCP Transport forDNS
•Goal: EstablishDNS overTCP als „firstclasscitizen“
•Features
Persistent connections(Client supposedtocloseconnections)
Connection re-use
Pipelining
Response Reordering
TCP Fast Open
Similar: „Happy Eyeballs“

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RFC 7816 –QNAME Minimization

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RFC 7828 –EDNS keepalive
•EDNS Option forSession Management
•ForTCP only!
•Clients: „Pleaseleaveconnectionopen forX seconds“
•Server: „Ok, leaveitopen forX seconds“ or„Please
closeconnectionnow!“

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RFC 7858 –DNS overTLS (DoT)
•New Port 853 / TCP
•„On thewire“ protocolisunmodified
•Authentification: Certificatesusw? -> RFC 8310
„Opportunistic“ vs. „Strict“
Chicken/Egg -> Bootstrapping des DoTServers wie?
•Doesnot changethe„path“ oftheDNS message
ExistingRecursiveNameserver cansimplyofferan additional,
encryptedchannel

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RFC 8094 –DNS overDTLS
•Port 853 / UDP
•„Same Samebut Different“
•Experimental!
Issueswithfragmentation
DTLS isnot widelyimplemented
•Performance advantageofUDP?
MostlybecauseTCP implementationusedtobeso „lousy“.

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EDNS(0) Padding
It‘srequiredforprivacy–but, why?

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EDNS(0) Padding–why?
•Encryption removes„direct“ accesstotheinformation
What‘sleftfortheAttacker?
•„PrettyBad Privacy –PitfallsofDNS Encryption“*
HayaShulman@ IETF 93
Applied Networking Research Price –IRTF
•Side Channel informationiskey!
Countermeasures
*https://www.ietf.org/proceedings/93/slides/slides-93-irtfopen-1.pdf

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ApplicationQueries–it‘sa stream
•A Pattern -Not just a singlequery/responsepair

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EncryptedDNS
•Streams still createsize/timing„patterns“

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Size basedCorrelation
•Comparewithknowncleartextpatterns
•Even workswitha subsetofmessagesizes

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IntroducingPadding
•Obfuscatesthesizepattern-> Hamperscorrelation
•More „hits“ -> lesslikelythatidentificationispossible

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RFC 7830 –EDNS(0) PaddingOption
•EDNS Option code12
https://tools.ietf.org/html/rfc7830

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DNS overHTTPS
An alternative encryptionscheme, drivenbybrowservendors

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Motivation –Browser Vendors
•(a) Browsers do a lotofDNS thesedays
Websites + assets(JS, Ads, Statistics…), CDNs
CertificateValidation (OCSP), SafeBrowsinglists, updates, …
More directcontrolovertheDNS API desired
•(b) Timing andavailabilityiscritical
„Happy Eyeballs“ –Slow orlousy(local) DNS serverscreatebad
userexperience
„Bad Hotel WiFi“ isoften„Bad Hotel DNS“…
•(c) DNS isusedforcensorship
Circumventinglocal(censoring) DNS serversprotectsFreedom of
Speech
Eg. Google Jigsaw
ttps://dnsserver.example.net/dns-query{?dns}

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IETF DoH* (DNS overHTTPs) group
•Founded2017
•2018: RFC 8484
GET orPOST
URI Templates (https://dnsserver.example.net/dns-
query{?dns})
Wire-Format: application/dns-message (identicalzu „normal“
DNS), oder JSON
HTTP Response-Code always2xx (ifsuccessful), nomatter
whichDNS responsecode
*https://datatracker.ietf.org/wg/doh/about/

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EffectsofencryptedDNS
The implicationsoftypicaloperational models

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„Plain“ DNS

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DNS overTLS

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DNS overHTTPS (typical)

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ConcernsregardingDoH
•4 Browser Vendors
•Fewbigpublicrecursorvendors(1.1.1.1, 8.8.8.8,
9.9.9.9)
•Market concentration/ Control?
Pre-configuredpublicrecursors
Example: Mozilla / Cloudflarediscussion
•Media echo (German only, sorry!)
https://Heise.de/-4203225.html(„Die DNS Gruft gehört
ausgelüftet“)
https://heise.de/-4205380.html(„Vom DNS, aktuellen Hypes,
Überwachung und Zensur“)

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Implementations
Server, Clients, Tools

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DoTClients
https://dnsprivacy.org/wiki/display/DP/DNS+Privacy+Implementation+Status

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DoTServer Software

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DoT(andDoH) publicrecursors
•Google DNS (8.8.8.8)
•Cloudflare(1.1.1.1)
•Quad9 (9.9.9.9)
•CleanBrowsing(various, withFilters)

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DoH
•Clients
Mozilla Firefox
Google Chrome
(plus testtools)
•Server Software
https://github.com/facebookexperimental/doh-proxy
https://github.com/curl/curl/wiki/DNS-over-HTTPS#doh-tools

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Android 9 –DNS overTLS bydefault
•UsesDNS overTLS ifavailable
on localnameserver
•Falls back tounencryptedDNS
ifunavailable

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ExecSummary
•DNS cannowbeencrypted, eithervia TLS orHTTPS
•DNS overHTTPs ismore„disruptive“ thanDNS overTLS
•Public recursorshaveimplementedeither(orboth)
But fewlocalprovidershaveimplementedit(seebelow:-/)
•Browser VendorsareimplementingDNS overHTTPs
Ongoingpolicydiscussionsaroundpre-configurationofrecursors
•Android 9 implementsDNS overTLS *bydefault*
Automaticallyusesitifavailable(seeabove:-/)
Google suggestingtoconfigure„dns.google“ manually
•Windows / MacOS –no„out ofthebox“ solutions–
„Stubby“

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nic.at GmbH
Jakob-Haringer-Str. 8/V · 5020 Salzburg · Austria
T [phone] · F -29
[email protected] · www.nic.at