computer networks

Introduction to the DNS system:

February 2003 slideset 1 Introduction to the DNS system Olaf M. Kolkman Okolkman@ripe.net

Purpose of naming:

February 2003 slideset 1 - 2 Purpose of naming Addresses are used to locate objects Names are easier to remember than numbers You would like to get to the address or other objects using a name DNS provides a mapping from names to resources of several types

Names and addresses in general:

February 2003 slideset 1 - 3 Names and addresses in general An address is how you get to an endpoint Typically, hierarchical (for scaling): 950 Charter Street, Redwood City CA, 94063 204.152.187.11, +1-650-381-6003 A “name” is how an endpoint is referenced Typically, no structurally significant hierarchy “David”, “Tokyo”, “itu.int”

Naming History:

February 2003 slideset 1 - 4 Naming History 1970’s ARPANET Host.txt maintained by the SRI-NIC pulled from a single machine Problems traffic and load Name collisions Consistency DNS reated in 1983 by Paul Mockapetris (RFCs 1034 and 1035), modified, updated, and enhanced by a myriad of subsequent RFCs

DNS:

February 2003 slideset 1 - 5 DNS A lookup mechanism for translating objects into other objects A globally distributed, loosely coherent, scalable, reliable, dynamic database Comprised of three components A “name space” Servers making that name space available Resolvers (clients) which query the servers about the name space

DNS Features: Global Distribution:

February 2003 slideset 1 - 6 DNS Features: Global Distribution Data is maintained locally, but retrievable globally No single computer has all DNS data DNS lookups can be performed by any device Remote DNS data is locally cachable to improve performance

DNS Features: Loose Coherency:

February 2003 slideset 1 - 7 DNS Features: Loose Coherency The database is always internally consistent Each version of a subset of the database (a zone) has a serial number The serial number is incremented on each database change Changes to the master copy of the database are replicated according to timing set by the zone administrator Cached data expires according to timeout set by zone administrator

DNS Features: Scalability:

February 2003 slideset 1 - 8 DNS Features: Scalability No limit to the size of the database One server has over 20,000,000 names Not a particularly good idea No limit to the number of queries 24,000 queries per second handled easily Queries distributed among masters, slaves, and caches

DNS Features: Reliability:

February 2003 slideset 1 - 9 DNS Features: Reliability Data is replicated Data from master is copied to multiple slaves Clients can query Master server Any of the copies at slave servers Clients will typically query local caches DNS protocols can use either UDP or TCP If UDP, DNS protocol handles retransmission, sequencing, etc.

DNS Features: Dynamicity:

February 2003 slideset 1 - 10 DNS Features: Dynamicity Database can be updated dynamically Add/delete/modify of any record Modification of the master database triggers replication Only master can be dynamically updated Creates a single point of failure

DNS Concepts:

February 2003 slideset 1 - 11 DNS Concepts Next slides are about concepts After this set of slides you should understand How the DNS is built Why it is built the way it is The terminology used throughout the course

Concept: DNS Names 1:

February 2003 slideset 1 - 12 Concept: DNS Names 1 The namespace needs to be made hierarchical to be able to scale. The idea is to name objects based on location (within country, set of organizations, set of companies, etc) unit within that location (company within set of company, etc) object within unit (name of person in company)

Concept: DNS Names 2 How names appear in the DNS:

February 2003 slideset 1 - 13 Concept: DNS Names 2 How names appear in the DNS Fully Qualified Domain Name (FQDN) WWW.RIPE.NET. labels separated by dots DNS provides a mapping from FQDNs to resources of several types Names are used as a key when fetching data in the DNS Note the trailing dot

Concept: Resource Records :

February 2003 slideset 1 - 14 The DNS maps names into data using Resource Records. More detail later www.ripe.net. … A 10.10.10.2 Concept: Resource Records Address Resource Resource Record

Concept: DNS Names 3:

February 2003 slideset 1 - 15 Concept: DNS Names 3 Domain names can be mapped to a tree. New branches at the ‘dots’ No restriction to the amount of branches. net com ripe www www edu isi tislabs • disi ws1 ws2 • • • • • • ftp sun google moon

Concept: Domains:

February 2003 slideset 1 - 16 Concept: Domains Domains are “namespaces” Everything below .com is in the com domain. Everything below ripe.net is in the ripe.net domain and in the net domain. net domain com domain ripe.net domain net com ripe www www edu isi tislabs • disi ws1 ws2 • • • • • • ftp sun moon google

Delegation:

February 2003 slideset 1 - 17 Delegation Administrators can create subdomains to group hosts According to geography, organizational affiliation or any other criterion An administrator of a domain can delegate responsibility for managing a subdomain to someone else But this isn’t required The parent domain retains links to the delegated subdomain The parent domain “remembers” who it delegated the subdomain to

Concept: Zones and Delegations:

February 2003 slideset 1 - 18 net domain Concept: Zones and Delegations Zones are “administrative spaces” Zone administrators are responsible for portion of a domain’s name space Authority is delegated from a parent and to a child ripe.net zone net zone disi.ripe.net zone net com ripe www www edu isi tislabs • disi ws1 ws2 • • • • • • ftp sun moon google

Concept: Name Servers:

February 2003 slideset 1 - 19 Concept: Name Servers Name servers answer ‘DNS’ questions. Several types of name servers Authoritative servers master (primary) slave (secondary) (Caching) recursive servers also caching forwarders Mixture of functionality

Concept: Name Servers authoritative name server:

February 2003 slideset 1 - 20 Concept: Name Servers authoritative name server Give authoritative answers for one or more zones. The master server normally loads the data from a zone file A slave server normally replicates the data from the master via a zone transfer master slave slave

Concept: Name Servers recursive server:

February 2003 slideset 1 - 21 Concept: Name Servers recursive server Recursive servers do the actual lookups; they ask questions to the DNS on behalf of the clients. Answers are obtained from authoritative servers but the answers forwarded to the clients are marked as not authoritative Answers are stored for future reference in the cache

Concept: Resolvers:

February 2003 slideset 1 - 22 Concept: Resolvers Resolvers ask the questions to the DNS system on behalf of the application. Normally implemented in a system library (e.g, libc) gethostbyname(char *name); gethostbyaddr(char *addr, int len, type);

Concept: Resolving process & Cache:

February 2003 slideset 1 - 23 Concept: Resolving process & Cache Resolver Question: www.ripe.net A www.ripe.net A ? Caching forwarder (recursive) root-server www.ripe.net A ? Ask net server @ X.gtld-servers.net (+ glue) gtld-server www.ripe.net A ? Ask ripe server @ ns.ripe.net (+ glue) ripe-server www.ripe.net A ? 192.168.5.10 192.168.5.10 Add to cache

Concept: Resource Records (more detail):

February 2003 slideset 1 - 24 Resource records consist of it’s name, it’s TTL, it’s class, it’s type and it’s RDATA TTL is a timing parameter IN class is widest used There are multiple types of RR records Everything behind the type identifier is called rdata Concept: Resource Records (more detail) Label ttl class type rdata www.ripe.net. 3600 IN A 10.10.10.2

Example: RRs in a zone file:

February 2003 slideset 1 - 25 Label ttl class type rdata ripe.net. 7200 IN SOA ns.ripe.net. olaf.ripe.net. ( 2001061501 ; Serial 43200 ; Refresh 12 hours 14400 ; Retry 4 hours 345600 ; Expire 4 days 7200 ; Negative cache 2 hours ) ripe.net. 7200 IN NS ns.ripe.net. ripe.net. 7200 IN NS ns.eu.net. pinkje.ripe.net. 3600 IN A 193.0.1.162 host25.ripe.net. 2600 IN A 193.0.3.25 Example: RRs in a zone file

Resource Record: SOA and NS:

February 2003 slideset 1 - 26 Resource Record: SOA and NS The SOA and NS records are used to provide information about the DNS itself. The NS indicates where information about a given zone can be found: The SOA record provides information about the start of authority, i.e. the top of the zone, also called the APEX. ripe.net. 7200 IN NS ns.ripe.net. ripe.net. 7200 IN NS ns.eu.net.

Resource Record: SOA:

February 2003 slideset 1 - 27 Resource Record: SOA Timing parameter Master server Contact address Version number net. 3600 IN SOA A.GTLD-SERVERS.net. nstld.verisign-grs.com. ( 2002021301 ; serial 30M ; refresh 15M ; retry 1W ; expiry 1D ) ; neg. answ. ttl

Concept: TTL and other Timers:

February 2003 slideset 1 - 28 Concept: TTL and other Timers TTL is a timer used in caches An indication for how long the data may be reused Data that is expected to be ‘stable’ can have high TTLs SOA timers are used for maintaining consistency between primary and secondary servers

Places where DNS data lives:

February 2003 slideset 1 - 29 Places where DNS data lives Registry DB Master Slave server Slave Cache server Changes in DNS do not propagate instantly! Not going to net if TTL>0 Might take up to refresh to get data from master Upload of zone data is local policy

To remember...:

February 2003 slideset 1 - 30 To remember... Multiple authoritative servers to distribute load and risk: Put your name servers apart from each other Caches to reduce load to authoritative servers and reduce response times SOA timers and TTL need to be tuned to needs of zone. Stable data: higher numbers

What have we learned What are we about to learn :

February 2003 slideset 1 - 31 What have we learned What are we about to learn We learned about the architecture: resolvers, caching forwarders, authoritative servers, timing parameters