CNIT 40: 2: DNS Protocol and Architecture
2 likes3,249 views
The document provides an overview of DNS (Domain Name System) architecture, including its history and design principles such as distributed storage and hierarchical data organization. It discusses the structure of DNS names, zones, types of DNS servers, and the process of DNS queries, including recursive and iterative queries. Additionally, it covers various resource record types, the concept of reverse DNS resolutions, and common server architectures based on company size.
CNIT 40: 2: DNS Protocol and Architecture
- 1. DNS Security Ch 2: DNS Overview: Protocol, Architecture, and Applications Updated 9-13-16
- 2. History • On ARPANet, host names were mapped to IP addresses in a hosts.txt file stored on a single master server • Other machines downloaded copies of hosts.txt periodically • Unix stored this information in /etc/hosts • This technique didn't scale well. DNS started in 1983.
- 3. Design Principles • Distributed storage – DNS data split across many servers – Smaller storage requirements for each server – Faster transfer of information – No single point of failure • Hierarchical organization of data – Allows local control of names and avoids name conflicts
- 4. DNS Name Structure • Up to four labels separated by dots form a Fully Qualified Domain Name (FQDN) hills.ccsf.edu Top Level Domain (TLD) Second Level Domain (SLD) {{ Host name {
- 6. DNS Zones • Data for a domain and all or some of its subdomains is called a zone • All of CCSF could be one zone, containing – ns.ccsf.edu – www.ccsf.edu – mail.ccsf.edu edu . www mailns ccsf
- 7. Domain Delegation • CCSF could have separate nameservers on each campus, each responsible for their own subdomain • The ccsf.edu parent domain would delegate responsibility for a subdomain to each campus, making many zones • URLs would be longer – www.ocean.ccsf.edu edu . mission evansocean ccsf ns www ns www ns www
- 8. DNS Clients, Servers, and Resolvers • DNS Client – A program like a Web browser using a domain name like www.ccsf.edu • DNS Server – Stores and serves DNS data • DNS Resolver – Software that accepts a query from a client, queries one or more DNS servers, and replies to the client
- 9. DNS Servers • Authoritative servers manage information about a domain – SOA (Start Of Authority) • Caching servers store data they copied from other servers – Not authoritative for any domain – Cache records have a Time To Live (TTL) – Specified by SOA for each record
- 10. DNS Queries • Recursive query – Server will find the answer, even if it has to query other servers to get it – Server will not respond with a referral to another server • Iterative query – If server does not have the answer, it will send a referral to another DNS server – Requester has to send another query to hunt for the answer
- 11. DNS Forwarder • Only external queries are sent to the forwarder in this example • Can reduce traffic through slow or expensive links • Because it can cache more records
- 12. DNS Resolvers • Receive requests from client applications • Query DNS servers • Can cache data • Stub resolver – Resolver connected to only one recursive server – Cannot follow referrals – Part of the operating system on the end device – Windows stub resolver caches – Linux stub resolver does not cache
- 13. Local DNS Server • Provided by Internet Service Provider (ISP) • Configured at the client manually or by DHCP
- 14. Typical Name Resolution Scenario ns ccsf Stub resolver in OS Local DNS Server Query: Where is www.ccsf.edu? Query: Where is the SOA for edu? Query: Where is the SOA for ccsf.edu? edu . Query: Where is www.ccsf.edu?
- 15. DNS Replication • Master server contains primary zone files • Slave servers have copies of the zone files • Zone transfer – The process of copying the files
- 16. Root Servers • Named . – "dot" • Has pointers to the top-level domains – com, biz, mil, net, and so on • If a DNS server has no data in the cache, e. g. after a reboot – Receives a recursive query – Is not the SOA for that domain – Must query root to find the TLD servers
- 17. Root Servers • Hundreds of servers • Dispersed around the world • 13 domain names – a.root-servers.net – b.root-servers.net – ... – m.root-servers.net
- 18. DNS Resource Record Types and Classes Each data element in DNS is called a Resource Record (RR)
- 19. Common RR Types A IPv4 address of host AAAA IPv6 address of host MX Mail exchange PTR Host name corresponding to IP address NS Host name of SOA name server CNAME Canonical Name: alias SOA Attributes of zone TXT General information
- 20. Common RR Types NAPTR Naming Authority Pointer SRV Service (for specific applications) SPF Sender Policy Framework (used to control spam) Also included in TXT records as a transitional mechanism DNSKEY, DS, RRSIG, NSEC for DNSSEC
- 21. dig any ccsf.edu
- 22. dig any ietf.org
- 24. Reverse DNS Resolutions • Start with IP address and query to find the domain name • Used to block spam email – If IP address of server doesn't have a valid domain name, or the domain name is blacklisted, the email is rejected
- 25. PTR Records for RDNS • Reverse the order of the IP address • Add "in-addr.arpa" • For example, 147.144.1.212 becomes • 212.1.144.147.in-addr.arpa.
- 27. Forward Lookup for Google
- 28. Reverse Lookup for Google
- 32. • Flags specify: – Query or Response – Forward or Reverse – Recursive or Iterative
- 33. Glue Records • If the SOA for ccsf.edu is ns3.ccsf.edu the system fails – Q: Where is www.ccsf.edu? – A: Ask ns3.ccsf.edu – Q: Where is ns3.ccsf.edu? – A: Ask ns3.ccsf.edu • To prevent this, each domain has a glue record in their top-level domain zone specifying the IP address of the SOA
- 34. Viewing Glue Records with dig 1. Find a .edu root NS server
- 35. 2. dig NS domain @root
- 36. IPv4 Glue for Google • 1. NS servers for .com
- 37. IPv4 Glue Records for Google
- 38. Bind Version Query • From c. 2015
- 39. Bind Version Query 9-13-16
- 40. ISC.ORG (Authors of Bind)
- 42. Small Company: Outsource DNS
- 43. Medium Company • Internal and external DNS servers • Only public servers like www and mx on external DNS servers
- 44. Large Company
- 46. Anycast