DNSfor DevelopersMaarten Balliauw@maartenballiauw

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

“Sure, why?”Me

“foo.bar.comshould redirect tohttp://bar.com/foo.aspx”Manager

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

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

How the Internet works(the DNS part)

“Let’s Google!”

“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

“Let’s Google!”Check operating system (hosts file, ...)

Check DNS cache

Ask home router

Check DNS cache at ISP, not in cache? Iterate!

“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

Digging into the DNSDEMO

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

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)

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

Hierarchical system.

com

google

www

mail

orgexamp

lewww staff

www

ca

confoo

…

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

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”

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

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

DNS zones

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]“

DNS zone $ORIGIN example.com. ; designates the start of this zone file in the namespace$TTL 1h ; default expiration time of all resource recordsexample.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.comexample.com. IN NS ns.somewhere.example. ; another nameserverexample.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 nameexample.com. IN A 192.0.2.1 ; IPv4 address for example.com IN AAAA 2001:db8:10::1 ; IPv6 address for example.comns IN A 192.0.2.2 ; IPv4 address for ns.example.comwww IN CNAME example.com. ; www.example.com is an alias for example.commail IN A 192.0.2.3 ; IPv4 address for mail.example.commail2 IN A 192.0.2.4 ; IPv4 address for mail2.example.commail3 IN A 192.0.2.5 ; IPv4 address for mail3.example.com

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)

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

PTR “Reverse DNS” used for e.g. diagnostics tools like ping and traceroute

Email anti-spam uses this as well (check EHLO IP address)

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

Security

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 1hevil.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"/><!-- ... -->

DNS cache poisoningDEMO

DNSSEC (Domain Name System Security 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

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

DNS Amplification for DDoS

Attacker Victim

Open DNS resolver

Open DNS resolver

Open DNS resolver

Small, spoofed request (ANY)

(source IP = victim IP) Large, real re

sponse

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

DNS in application architecture

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?

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)

Azure Traffic ManagerDEMO

Content Delivery Network (CDN) Serve origin content from edge location close to the user

www.cdnreviews.com

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

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)

Configuration in DNSDEMO

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!

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

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 192.168.1.10146ce01._printer._tcp.local. SRV 515 46ce01.local_printer._tcp.local. PTR 46ce01._printer._tcp.local.

Service Discoverywith mDNS and DNS-SDDEMO

Abusing DNSFor fun and profit

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

Public hotspots

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

HTTP over DNSLocal browser

HoD client

HoD server

Target HTTP server

Browser uses local HoD client as proxy

HoD client tr

anslates request i

nto DNS query

Converts response back

HoD server makes upstream requestTranslates into DNS response(s)

HTTP over DNS on the InternetLocal browser

HoD client

HoD server

Target HTTP server

ISP nameserver

HTTP over DNSDEMO

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

Conclusion

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

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