1. Domain Name System (DNS)
G53ACC
11/17/2018
Domain Name System (DNS)
G53ACC
Chris Greenhalgh
School of Computer Science and IT
DNS

2. Contents Introduction DNS names and name space Resource records
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Contents
Introduction
DNS names and name space
Resource records
Applications
Administration and zones
Query patterns
Message format
Cacheing
Books: Comer Ch. 31
DNS

3. Domain Name System (DNS)
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Domain Name System (DNS)
DNS provides name-to-value mapping through:
hierarchical domain-based naming scheme
distributed database system
Primarily for mapping host names to IP addresses
E.g. as used in URLs
(other local solutions, e.g. Sun NIS, MS ActiveDirectory)
Defined in RFC1034 and RFC1035.
DNS

4. G53ACC
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DNS Name Space
The Internet is divided into several hundred top-level domains
Each domain covers many hosts
Each domain is partitioned into subdomains
these too are further partitioned, and so on.
These domains can be represented by a tree:
DNS

5. A Portion of the DNS Name Tree
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A Portion of the DNS Name Tree
com
edu
gov
mil
org uk
mit ac co
ftp
nottingham
warwick
demon cs
ccc
much
DNS

6. Names A domain is named by the path from it to the (unnamed) root.
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Names
A domain is named by the path from it to the (unnamed) root.
The components are separated by “dots”.
e.g. a Nottingham CS machine might be much.cs.nottingham.ac.uk
Hierarchical, so
much at CS (much.cs.nott.ac.uk) is not much at Warwick (much.warwick.ac.uk)
DNS

7. Names Restrictions Domain names are case insensitive
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Names Restrictions
Domain names are case insensitive
edu and EDU mean the same thing
Component names can be up to 63 characters long
Full path names must not exceed 255 characters
DNS

8. G53ACC
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DNS Resource Records
Each domain has a set of “resource records” associated with it.
For a single host, the most common resource record is just its IP address
but many other kinds of resource records exist…
When a query is directed to a domain, the result is the relevant resource record(s) associated with that name
DNS

9. Resource Record structure
Each resource record comprises:
Owner = domain name
Type = entry type, 16 bits, e.g. A, CNAME, …
Class = protocol or protocol family, 16 bits, e.g. IN (the INternet)
TTL = time, 32 bits, seconds, that this resource record can be cached
RDATA = type-specific information, e.g. IP address for type A record

10. Resource record Types Type Meaning Value
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Resource record Types
Type Meaning Value
SOA Start of Authority Parameters for this zone
A IP address of a host 32bit integer
MX Mail Exchange Priority, domain
willing to accept
CNAME Canonical Name Domain Name
PTR Pointer Alias for an IP address
HINFO Host Description CPU and OS in ASCII
(security risk – rarely used)
TXT Text Uninterpreted ASCII text
DNS

11. A Simple Example Resource Records for Much
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A Simple Example
Resource Records for Much
(Class (‘IN’) and TTL are nameserver defaults)
much A
MX 1 marian
MX 2 pat
MX 5 nfs-relay.ac.uk
HINFO Sparc UNIX
DNS

12. Applications (1) Host Naming ASCII names Numerical addresses
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Applications (1)
Host Naming
ASCII names
e.g. marian.cs.nott.ac.uk
human readable
memorable
Numerical addresses
e.g
needed by network (e.g. for routing)
DNS host name = domain name, ‘A’ record contains numerical IP address
DNS

13. Applications (2) Host aliases
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Applications (2)
Host aliases
‘CNAME’ records, e.g. ‘ –> ‘pat.cs.nott.ac.uk’
Note: a node with a CNAME record should only have a CNAME record –
All other queries are answered using records for the pointed-to domain name
‘PTR’ records, which just point to another domain name
Other queries not automatically forwarded
E.g. reverse lookup…
DNS

14. Applications (3) Reverse lookup: IP address to host name
Special domain IN-ADDR.ARPA is root of tree with (reversed) dot-separated IP addresses as nodes/sub-nodes.
PTR records identify host domain name records
IN-ADDR.ARPA => (PTR) picasso.mrl.nottingham.ac.uk
picasso.mrl.nottingham.ac.uk => (A)
Why?
for security (delegation of sub-zones follows IP address allocation)
Is this really the machine which it says it is?
Is it entitled to use this IP address?

15. Email addresses (Internet) for example, mjr@cs.nott.ac.uk
Applications (4)
addresses (Internet)
for example,
MX record - Mail eXchanger –> host to deliver mail to for the given address
for cs.nott.ac.uk the answer is (with priority, low best): MX pat.cs.nott.ac.uk. MX marian.cs.nott.ac.uk. MX pat.ccc.nottingham.ac.uk.

16. Administration: Name Allocation
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Administration: Name Allocation
Naming follows organisational boundaries, not physical networks
Departments sharing the same LAN can have distinct domains
Hosts in departments split over several buildings can have the same domain
ac.uk …
nottingham
warwick … cs
ccc …
much …
DNS

17. Administration: Domain Allocation
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Administration: Domain Allocation
Each domain controls how it allocates domains underneath it.
To create a new domain, permission is required of the domain in which it will be included
e.g. to create psychology.nott.ac.uk needs permission from nott.ac.uk
A new domain can create sub-domains without needing external permission
Top-level domains are administered by registries, e.g. interNIC for .com, .org, .net, .edu
DNS

18. Administration: Zones
A zone is a fully connected subset of the domain name space which is administered as a single unit.
A zone will be a single sub-tree, less those sub-sub-trees which have been delegated to other administrative zones.
A zone must have at least two name servers, and must identify all sub-zone name servers.
Root zone information is cached by hand at most/all name servers.

19. A Portion of the DNS Name Space with Possible Zones
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A Portion of the DNS Name Space with Possible Zones uk ac co
nott
warwick
demon cs
ccc
mrl
DNS

20. DNS Protocol Operation
Simple request-response protocol over UDP…
Client forms request
uses UDP
sends to default server (e.g. from /etc/resolv.conf)
Server receives request
sends response back to client
may make additional (recursive) queries to other servers (if permitted by query flag RD)…

21. DNS Message Format (i) [bytes 0,1] Client query identification no.
[2,3] Flags, including:
QR (1 bit) query (0), response (1)
AA (1 bit) authoritative answer, i.e. by DNS server responsible for the domain in question.
RD (1 bit) recursion desired
RA (1 bit) recursion available (from server)
RCODE (4 bits) ok/error response (from server)

22. DNS Message Format (ii)
[4,5] number of questions
[6,7] number of answers
[8,9] number of authority records
[10,11] number of additional records
Followed by questions, answers, authority records, and additional records in turn.
Question comprises domain name, type (can be ‘any’) and class.
Answers and other records comprise standard resource records (see earlier)
Standard encodings and compression scheme using for domain names defined in RFC.

23. DNS Recursive Query (normal)
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DNS Recursive Query (normal)
.com
DNS
.foo.com
Client
DNS
DNS
wu.foo.com?
(A records, INternet)
marian: .cs.nott.ac.uk
1. Client queries default (local) DNS server.
2. If server cannot resolve query, it in turn queries a DNS server
which is responsible for the root of that domain.
3. That server may pass the request on to another DNS server
responsible for a more specific sub-domain. And so on.
DNS

24. DNS Iterative Query 4. 3. 2. 1. 1. Query sent to local DNS server
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DNS Iterative Query 4.
DNS 3.
Client
DNS 2. 1.
DNS
1. Query sent to local DNS server
2. Local server cannot resolve it, but returns address of DNS to
query next (server with authority for domain)
3. Client queries authority DNS server
4. That server returns results of query, or goto step 2.
DNS

25. Response Caching
DNS servers (and clients) can cache responses to previous recursive queries
gives a “non-authoritative” answer
the server in not responsible for that domain but has a recent response
Needed for scalability, e.g. avoid accessing US for every .com domain name request
(also uses replication of servers)
Can optionally cache negative responses (e.g. common typos or defunct domains)

26. Tools Command-line dig nslookup … Web sites Various…
E.g. “dig any cs.nott.ac.uk”
nslookup …
Web sites
Various…
Providing access to similar facilities (and more)