DNS over anything, encrypted

DNS Privacy

DNS is metadata

• IETF started the DPRIVE (DNS Privacy Working Group) after the Snowden revelations
• RFC 7672 DNS Privacy Considerations
• current focus of DPRIVE is the client to resolver channel
• creating protocols that are stealthy sometimes painfully collides with clean protocol design

QNAME Minimization

• DNS protocol has been implemented quite "chatty"
• not required by the DNS protocol
• RFC 7816 DNS Query Name Minimisation to Improve Privacy

original DNS queries 1/6

original DNS queries 2/6

original DNS queries 3/6

original DNS queries 4/6

original DNS queries 5/6

original DNS queries 6/6

QNAME minimized queries

Implementations

• Unbound
• Knot-Resolver

Test

testing if the resolver path supports QNAME minimization

shell$ dig txt qnamemintest.internet.nl +short
a.b.qnamemin-test.internet.nl.
"HOORAY - QNAME minimisation is enabled on your resolver :)!"

DNS over TLS

• RFC 7858 Specification for DNS over Transport Layer Security (TLS)
• DNS wireformat over TLS over TCP
• Port 853 (TCP)
• encryption and authentication

Performance

• performance of DNS-over-TLS can be quite good
• for existing sessions with TLS 1.3 as good as DNS-over-UDP
  • pipelining
  • TCP fast open
  • 0-RTT resume
• current implementations are not optimized

Implementations

• Client
  • Unbound (as local forwarder)
  • Stubby (getdnsapi)
  • dnsfwd
• Server
  • Unbound (as remote resolver)
  • Knot
  • any DNS server via stunnel

Developments

• TLS 1.3 deployment stalled because of misbehaving middleboxes (BlueCoat)
• controversial in the IETF, but useful: multiplexing HTTPS and DNS on port 443 (https://gitlab.com/dkg/hddemux)

DNS over DTLS

• RFC 8094 DNS over Datagram Transport Layer Security (DTLS)
• DNS wireformat over TLS over UDP
• Port 853 (UDP)
• encryption and authentication

DNS over DTLS issues

• adversary can block DNS queries
• resource exhaustion attacks against DNS server possible
• no known implementations

DNS over HTTP(S)

• IETF Internet Draft DNS Queries over HTTPS https://tools.ietf.org/html/draft-hoffman-dns-over-https
• DNS HTTP-Format over HTTPS over TCP
• Port 443 (HTTP/2)
• URL: https://server/.well-known/dns-query
• base64url encoded DNS data, Content-Header application/dns-udpwireformat
• encryption and authentication

Benefits

• HTTPS might be the only option in highly firewalled networks
• easy to implement for (Web-)Developers (JavaScript etc)

Additional documents

• Representing DNS Messages in JSON https://tools.ietf.org/html/draft-hoffman-dns-in-json
• DNS Messages in XML (expired) https://tools.ietf.org/html/draft-mohan-dns-query-xml

Similar Implementations

• DNS over JSON over HTTPS over TCP

  • Google DNS Server-Side

  https://developers.google.com/speed/public-dns/docs/dns-over-https

  • dingo - A DNS client in Go that supports Google DNS over HTTPS

  https://github.com/pforemski/dingo

  • CoreDNS https://coredns.io/2016/11/26/dns-over-https/

DNS-Wireformat over HTTPS

• DNS wireformat over HTTP(S) over TCP
• Internet Draft DNS wire-format over HTTP https://tools.ietf.org/html/draft-ietf-dnsop-dns-wireformat-http
• Port 80 or 443 (HTTP 1.1 or HTTP/2)
• URL: https://server/.well-known/dns-wireformat
• Content-Header application/dns-wireformat
• may provide encryption and authentication
• DNS wire-format data is wrapped with a HTTP header and transmitted on port 80 or 443

DNS over QUIC

• DNS over QUIC over UDP
• Specification of DNS over Dedicated QUIC Connections https://tools.ietf.org/html/draft-huitema-quic-dnsoquic

what is QUIC

• modern TCP replacement from Google, now standardized in the IETF
  • uses UDP, implements TCP features
  • usually implemented in applications, not OS kernel
  • includes TLS 1.3
  • 0-RTT
• performance in-par with DNS-over-UDP
• QUIC Documents https://tools.ietf.org/wg/quic/

DNS-over-X comparison

DNS over opportunistic IPSec

• DNS over UDP or TCP over IPSec
• DNS queries will be tunneled via IPSec
• provides encryption (but only limited authentication)
• Unauthenticated Opportunistic IPsec https://libreswan.org/wiki/HOWTO:_Unauthenticated_Opportunistic_IPsec

Implementations

• LibreSWAN and Unbound (IPSec Module)

additional work

• make IPSec work in case of heavy firewalling: RFC 8229 TCP Encapsulation of IKE and IPsec Packets
• allows IPSec to work on Port 443 (multiplexed with HTTPS)

DNS over DNSCrypt

• DNS over DNSCrypt over UDP or TCP
• DNSCrypt is a DNS privacy solution originally developed by OpenDNS (now Cisco)
• encryption and authentication
• protocol is open source, but somewhat underdocumented
• client operates a DNS proxy that tunnels DNS over DNSCrypt
• some, but not all DNSCrypt resolver support DNSSEC

DNS over Tor

• DNS over Tor over TCP
• Port 9053
• Tor client proxies the DNS queries through a tor circut
• only A/AAAA-Records supported (no TXT, MX, SOA …)
• no DNSSEC, rogue Tor exit node can spoof DNS traffic
• > 30 % of Tor exit nodes use Google public DNS

Padding of DNS data

• DNS query/responses are small data chunks
• traffic analysis might be acute when dealing with DNS queries
• the IETF is working on padding schemes for DNS to make traffic analysis more difficult
  • RFC 7830 The EDNS(0) Padding Option
  • Padding Policy for EDNS(0) https://tools.ietf.org/html/draft-ietf-dprive-padding-policy

Performance

Alexa Top 1000 domains

1000 DNS-Queries from office network

Reading the crystal ball

My totally un-scientific predictions, based on the wisdom of my stomach

• DNS-over-TLS will be implemented, but will have a hard time getting traction
• DNS-over-QUIC will be in Chrome, Opera, Android -> queries towards Google DNS. Fast but not private.
• ISPs (large and small) will loose control over DNS resolution
  • good: no DNS interception
  • bad: no fast local DNS resolution
  • bad: difficult to implement local content caches
• Web-Applications will use DNS-over-HTTPS (but mostly via Google)

Links

Links from this presentation can be found at http://talks.strotmann.de/view/welcome-visitors/view/infosec-stammtisch-ms-dns-over-anything-encrypted