This document summarizes a presentation about DNS (Domain Name System) for developers. It discusses the basics of how DNS works to map domain names to IP addresses, different DNS record types like A, CNAME, and MX records, DNS zones, security considerations like cache poisoning and DNSSEC, using DNS for failover, load balancing, and CDNs, storing configuration data and doing service discovery in DNS, and some unconventional uses of DNS like tunneling HTTP and IP traffic over DNS. The presentation provides examples and demonstrations of these DNS concepts and techniques.

1. DNS
for Developers
Maarten Balliauw
@maartenballiauw

2. “Can we add a CNAME to
the DNS?”
Manager

3. “Sure, why?”
Me

4. “foo.bar.com
should redirect to
http://bar.com/foo.aspx”
Manager

6. Who am I?
Maarten Balliauw
Antwerp, Belgium
Developer Advocate, JetBrains
Founder, MyGet
AZUG
Focus on web
ASP.NET MVC, Azure, SignalR, ...
Former MVP Azure & ASPInsider
Big passion: Azure
http://blog.maartenballiauw.be
@maartenballiauw

7. Agenda
The 101 stuff
How the Internet works (the DNS part)
DNS zones
Security
DNS in application architecture
Failover, load balancing, CDN
Configuration and service discovery
DNS for fun and profit

8. How the Internet works
(the DNS part)

9. “Let’s Google!”

10. “Let’s Google!”
We need an IP address for www.google.com
Use Domain Name System (“phone book”)
Map www.google.com to 172.217.0.164 / 2a00:1450:4009:80f::2004
After which the browser will do its HTTP magic

11. “Let’s Google!”
Check operating system (hosts file, ...)
Check DNS cache
Ask home router
Check DNS cache at ISP, not in cache? Iterate!

12. “Let’s Google!”
Ask root servers where .com. lives
Ask .com. authoritative server where google.com. lives
Ask .google.com. authoritative server for www.google.com. IP address

13. Digging into the DNS
DEMO

14. DNS
2 types of servers
Authoritative
“Owns the domain”
Cache (recursor)
“Resolves the domain for you”

15. DNS
Designed in 1983 by Paul Mockapetris (University of California, Irvine)
Converts hostnames to IP addresses
Stores mail delivery information for a domain
Stores other information for a domain (TXT records)

16. How do I get a domain name?
TLD’s managed by separate organisations
Verisign (.com) - Canadian Internet Registration Authority (CIRA) (.ca) - DNS Belgium (.be) - EURid (.eu) - …
Rules!
Who can register a name?
Ownership change procedures
Disputes
Technical rules
Usually domain registration done by registrar
E.g. DNSimple - http://bit.ly/dns4developers

17. Hierarchical system
.
com
google
www
mail
org
example
www staff
www
ca
confoo
…

18. Root servers
ICANN’s 13 root servers http://root-servers.org/
Why only 13?
UDP packets limited to 512 bytes
Response with > 13 entries would be > 512 bytes
There are more: anycast

19. gTLD, ccTLD, iTLD, … servers
Delegation from root servers to gTLD, ccTLD, iTLD, … servers
List managed by IANA http://www.iana.org/domains/root/db
“Where does .tld live?”
.ca - https://www.iana.org/domains/root/db/ca.html
“any.ca-servers.ca”

20. Root servers are a convention!
Every OS has them, but they can be replaced
E.g. www.opennicproject.org
They have their own gTLD’s as well, e.g. .bit, .free, .null, .oss, …
Not widely used (?) as it’s an alternate realm
E.g. www.orsn.org Open Root Server Network
Mirrors ICANN root servers
Reduce over-dependence on the USA
“Independent mode” in case political situation requires it

21. Caches, caches everywhere!
“Let’s change the IP address for our webserver in the DNS”
Caches in recursive resolvers (e.g. at ISP’s)
https://www.whatsmydns.net/
Caches in OS
ipconfig /flushdns
Caches in application (e.g. in browser)
Restart browser
Lower TTL beforehand to make updating smoother

22. DNS zones

23. DNS zone
“A Domain Name System (DNS) zone file is a text file that describes a
DNS zone. A DNS zone is a subset, often a single domain, of the
hierarchical domain name structure of the DNS.
The zone file contains mappings between domain names and IP
addresses and other resources, organized in the form of text
representations of resource records (RR).
A zone file may be either a DNS master file, authoritatively describing a
zone, or it may be used to list the contents of a DNS cache. [1]“

24. DNS zone
$ORIGIN example.com. ; designates the start of this zone file in the namespace
$TTL 1h ; default expiration time of all resource records
example.com. IN SOA ns.example.com. username.example.com. ( 2007120710 1d 2h 4w 1h )
example.com. IN NS ns ; ns.example.com is a nameserver for example.com
example.com. IN NS ns.somewhere.example. ; another nameserver
example.com. IN MX 10 mail.example.com. ; mail.example.com is the mailserver for example.com
@ IN MX 20 mail2.example.com. ; equivalent to above line, "@" represents zone origin
@ IN MX 50 mail3 ; equivalent to above line, but using a relative host name
example.com. IN A 192.0.2.1 ; IPv4 address for example.com
IN AAAA 2001:db8:10::1 ; IPv6 address for example.com
ns IN A 192.0.2.2 ; IPv4 address for ns.example.com
www IN CNAME example.com. ; www.example.com is an alias for example.com
mail IN A 192.0.2.3 ; IPv4 address for mail.example.com
mail2 IN A 192.0.2.4 ; IPv4 address for mail2.example.com
mail3 IN A 192.0.2.5 ; IPv4 address for mail3.example.com

25. DNS zone
Contains records describing a domain
Value + TTL
At the minimum: Start of Authority (SOA) record
“which server stores all the information about the website I want to look up”
Name of authoritative master name server
Email address of someone responsible for management of the name server
Expiration parameters
(serial #, slave refresh, slave retry time, slave expiration rime, cache duration or Time To Live)

26. DNS zone
Typical other records:
NS – Which are my nameservers? (or subdomain delegation)
A – IPv4 address pointer
AAAA – IPv6 address pointer
CNAME – Reference to another record (NOT A REDIRECT)
MX – Mail exchangers for the domain, with priorities
TXT – Textual value, often used to validate domain ownership/spam rules/…
SRV – Describes a service type and port

27. PTR
“Reverse DNS” used for e.g. diagnostics tools like ping and traceroute
Email anti-spam uses this as well (check EHLO IP address)

28. Zone transfer
Usually more than one nameserver for a zone
1 primary, other secondaries
No need to maintain zones on every slave!
Zone transfer
Primary knows secondary IP’s (we don’t want to transfer to just anyone)
Secondary knows zone name, queries primary over TCP (53) to replicate data
Uses SOA serial to check zone version & decide on update

29. Security

30. DNS cache poisoning
Consider this DNS zone…
Consider this web page…
Browser & OS cache ns1.google.com as 123.123.123.123.
$ORIGIN evil.com.
$TTL 1h
evil.com. IN SOA ns.evil.com. username.example.com. ( 2007120710 1d 2h 4w 1h )
evil.com. IN NS ns1.google.com.
ns1.google.com. IN A 123.123.123.123
<!-- ... -->
<img src="http://www.evil.com/image.gif"/>
<!-- ... -->

31. DNS cache poisoning
DEMO

32. DNSSEC (Domain Name SystemSecurity Extensions)
Set of extensions to DNS
Origin verification
Is the record really coming from the proper name server?
Adds signing support (and delegation)
Top-down the chain (root servers have DNSSEC, gTLD servers have DNSSEC, …)
Why did that demo work?
Custom resolver without DNSSEC  trust chain broken

33. DNS Amplification for DDoS
DNS recursion is awesome! (and often default)
Lots of DNS servers out there have recursion enabled for all
Lots of open resolvers out there
Saturate a victim’s network connection by using open DNS resolvers
UDP traffic has no source IP verification
Spoof source traffic

34. DNS Amplification for DDoS
Attacker Victim
Open DNS resolver
Open DNS resolver
Open DNS resolver

35. DNS Amplification for DDoS
Make sure to disable recursion
Or limit it to known, trusted networks
Use a DDoS filtering service
Akamai, CloudFlare, Verisign, ...
Use SPI firewall to verify packet origin

36. DNS in application
architecture

37. DNS failover / load balancing
Simple “round-robin”
www.example.local. IN A 192.168.0.1.
www.example.local. IN A 192.168.0.2.
www.example.local. IN A 192.168.0.3.
Most DNS servers return different IP as first item in list
Issues
What if one of the addresses is unreachable?
What if the order is cached at ISP?

38. DNS failover / load balancing
Intelligent DNS server
e.g. Azure Traffic Manager / Amazon Route 53
Scenarios
Round-robin
Failover
Performance
Issues
What if one of the addresses is unreachable?  monitoring of endpoints
What if the order is cached at ISP?  low TTL (still gaps)

39. Azure Traffic Manager
DEMO

40. Content Delivery Network (CDN)
Serve origin content from edge location close to the user
www.cdnreviews.com

41. Content Delivery Network (CDN)
Serve origin content from edge location close to the user
Intelligent DNS approach
Check user IP address location, return DNS record closer to the user
Try nslookup myget-2e16.kxcdn.com
Use IP Anycast
Advertise the same IP for edge server in different networks
No logic needed in DNS
The DNS root servers use this as well

42. Configuration in DNS
Typical application configuration
Key/value pairs
Hierarchy
 Store as DNS records (TXT?)
Typically multiple environments
One special DNS server per environment
One master to which we can recurse (e.g. shared settings)

43. Configuration in DNS
DEMO

44. Configuration in DNS
Alternative: store just the hostnames per environment
api.app.local  different IP per environment
Downside to configuration in DNS
Still need to maintain “the phone book” when changes occur
Not very flexible with dynamic resources...
Caches, CACHES!

45. Service discovery
“Detect services on various devices on a network of computers with minimal
configuration.”
UPnP
Service Location Protocol (SLP)
Zero Configuration Networking (Zeroconf)
Simple way to find and list services without maintaining a directory
Every service announces itself

46. Service discovery
Multicast DNS (mDNS)
224.0.0.251 port 5353 - every machine on the network listens
DNS Service Discovery (DNS-SD)
Works with mDNS and DNS
SRV (name + type, port, hostname)
PTR (pointer)
A (service IP)
TXT (additional information)
You are probably already using this today!
Printer, Apple Bonjour, Office365, …
46ce01.local. A 192.168.1.101
46ce01._printer._tcp.local. SRV 515 46ce01.local
_printer._tcp.local. PTR 46ce01._printer._tcp.local.

47. Service Discovery
with mDNS and DNS-SD
DEMO

48. Abusing DNS
For fun and profit

49. Public hotspots
Connect to wifi
Captive portal
Usually intercepts HTTP(S) only
Usually allows DNS lookups

50. Public hotspots

51. HTTP over DNS
Custom client and server
Server
Identify client
Fetch upstream data and make it available as DNS records
Client
Expose itself as a local proxy
Make DNS lookups with custom server
Things to be aware of…
UDP packet size, maximum length of records, maximum # of records
Encrypt transport

52. HTTP over DNS
Local browser
HoD client
HoD server
Target HTTP server
Browser uses local HoD client as proxy HoD server makes upstream request
Translates into DNS response(s)

53. HTTP over DNS on the Internet
Local browser
HoD client
HoD server
Target HTTP server
ISP nameserver

54. HTTP over DNS
DEMO

55. IP over DNS
Same idea as HTTP over DNS: tunnel traffic
http://code.kryo.se/iodine/
More elaborate protocol:
User identification
Auto-optimize UDP packet size
Compression

56. Conclusion

57. Conclusion
DNS is a hierarchical system
Built in 1983, flexible and widely used
Record types
DNSSEC
Application architecture
Failover, load balancing, CDN
Configuration and service discovery
Fun

58. Thank you!
http://blog.maartenballiauw.be
@maartenballiauw