COMPUTER AND NETWORK
SYSTEM ADMINISTRATION
Summer 1996 - Lesson 15
Introduction to DNS
A. Introduction - The Domain Name Service (DNS)
1. host name to IP number mapping was originally done
by downloading a static file
2. the UNIX version of this file is /etc/hosts
3. the central file was maintained by the Stanford Research
Institute Network Information Center (SRI-NIC)
4. as the Internet grew this scheme became unworkable
- the size of the file became too large
- the load on SRI-NIC site became too heavy
- the file was always inconsistent with reality
- hostname collisions became frequent (anyone could
name their machine "whitehouse.gov" if they wanted to)
** Figure 1.1
A. Overview
1. In 1984 Paulk Mockapetris of USC designed the architecture
of DNS
2. the Domain Name Service is essentially a distributed database
managed by InterNIC.
3. Features
- local control: each segment is updated locally
- global access: each segment is available (almost) immediately
to the rest of the world upon update
- robustness: achieved through replication
- adequate performance: is achieved through caching
4. software
- servers: called name servers, contain information about
some segment of the network and make it available to
clients ("BIND" = "Berkely Internet Name Daemon",
includes "named", libraries, "nslookup", "dig", "host")
- client: resolvers, a set of library routines that
resolve names by accessing a server (originally
a separate library, like libresolv.a, now
usually part of libc.a)
C. Domain structure
** Figure 1.2
- similar to the structure of a hierarchical file system
- the root's name is the null label " " but is written
as a single dot "."
- each node represents a 'domain'
- every domain is named
- the full domain name is the sequence of labels from the
domain to the root, separated by periods
- unlike a file system pathname the name is read from leaf
to root (right to left rather than left to right)
delta.cs.fsu.edu
** Figure 1.3
D. Domain management
- each domain may be managed by a different organization
- the organization may divide itself into subdomains
- then delegate responsbility for maintaining them
- NIC manages the top-level domains
- example: NIC delegates the "fsu.edu" domain to ACNS
** Figure 1.4
E. Host names
1. each host on a network has a domain
2. the domain points to information about the host
3. this may include:
- an IP address
- mail routing information (different than for other
services)
- aliases which point to the real ("canonical") host name
** Figure 1.5
F. Name collisions
1. the possibility for name collisions is now greatly
reduced
** Figure 2.1
G. The domain name space
- there may be any number of branches at a node
- BINDs implementation limits the tree's depth
to 127
"davidsun.kuncicky.faculty.cs.fsu.edu.us.northamerica.earth.solarsystem.milky way....."
- each name may contain up to 63 characters
- the suggested length is < 12
- a domain name that is written relative to the root is
called a 'fully-qualified domain name' - FQDN
- names without trailing dots ("leading dots") are sometimes
interpreted as relative to some domain other than root
** Figure 2.2
- sibling nodes must have unique names
** Figure 2.3, 2.4
- the name of a domain is the domain name of the node at the
top of the domain (example purdue.edu)
- again, similar to a file system
** Figure 2.5
- a node is in multiple domains
- so, a domain is just a subtree of the domain name space ("subdomain")
H. Hosts
- where are the hosts?
- a domain name is just an index into the DNS database
- the 'hosts' are domain names that point to individual
machine information
- the hosts are realated 'logically' usually by geography
or organization
- they are NOT necessarily related by network or IP address
or hardware type
- you could have 10 different hosts on 10 different networks
in ten different countries all in the same domain (hp.com)
- nodes at the leaves of the tree usually represent individual hosts
** Figure 2.6
- interior nodes may point to both:
> host information
> subdomain information
- for example, "hp.com" is both the name of a domain and the name
of a machine that routes mail
I. The Domain name space
1. terms:
top-level domain: a child of root (edu)
first-level domain: a child of root (edu)
second-level domain: a child of 1st level domain
(fsu.edu)
2. naming rules
- there are not many rules imposed on the naming of domains
- there are some traditions at the top-level
- the original 7 top-level domains are:
com - commerical organizations
edu - educational organizations
gov - government bodies
mil - military organizations
net - networking organizations (nsf.net)
org - non-commercial organizations
int - international organizations
- what about the rest of the world?
- the Internet began as ARPANET, funded by Defense
Advanced Research Projects (the military)
- later was funded by the National Science Foundation
- no one anticipated the international success of the Internet
3. international names
- 2-letter designations are reserved for each country
- ex: DE - Germany
DK - Denmark
- each country may organize its domain space however it wishes
ex: Australia uses: edu.au and com.au
ex: Britain uses: co.uk - corporations
ac.uk - academic community
ex: USA uses states: fl.us
then cities: tlh.fl.us
J. Name servers
1. zones
- a program that stores information about the domain name space
is called a Domain Name Server
- a name server generally has complete information about some part
of the domain name space
- the subspace is called a 'zone'
- the server is said to have 'authority' for one or more zones
- what is the difference between a zone and a domain?
[ A domain may be composed of one or more zones, but not vice versa ]
** Figure 2.7
** Figure 2.8
** Figure 2.9
2. types of name servers
- primary master
> gets the data for its zones from flat data (text) files
- secondary master
> gets the data for its zone from another server
> it periodically updates its local data by copying the
primary master's files
> this is called a 'zone transfer'
- generally keep more than one name server for any given zone
> redundancy: fault tolerance
> load: localize it as much as possible
K. Resolvers
1. name service clients
- these are the clients that access name servers
- in BIND these are a set of library routines
- these are compiled into telnet, ftp, etc. so that
these programs will use DNS to resolve names ("gethostbyname()" and others)
2. duties of a simple resolver:
- this is called a 'stub resolver'
- querying a name server
- interpreting the response
- resend a response
- returning a reply to the program that it is servicing (ftp)
L. Resolution
1. how does the name server resolve names
- if the name is in the name server's zone then it can give
the resolver an immediate 'authoritative' response
- if not, then the name server must search the domain name space
for an answer
- it only needs one piece of information to get started: the location
of a root-level server
2. root name servers
- the root name servers are authoritative for the top-level domains
(edu, org, us, dk, etc.)
- they can point you to the name servers for each of the top-level
domains
- they, in turn can point you to their subdomains, etc. until the
name is resolved
- this scheme puts a lot of importance on the root-level servers
** Figure 2.10
- example of name resolution via "nslookup"
nslookup
> set debug
> ftp.es.net
Server: dns.scri.fsu.edu
Address: 144.174.128.17
------------
Got answer:
HEADER:
opcode = QUERY, id = 3, rcode = NXDOMAIN
header flags: response, authentic answer, want recursion, recursion available
questions = 1, answers = 0, authority records = 1, additional = 0
QUESTIONS:
ftp.es.net.scri.fsu.edu, type = A, class = IN
AUTHORITY RECORDS:
-> scri.fsu.edu
ttl = 86400 (1 day)
origin = dns.scri.fsu.edu
mail address = hays.mailer.scri.fsu.edu
serial = 1996061302
refresh = 3600 (1 hour)
retry = 900 (15 mins)
expire = 3600000 (41 days 16 hours)
minimum ttl = 86400 (1 day)
------------
------------
Got answer:
HEADER:
opcode = QUERY, id = 4, rcode = NOERROR
header flags: response, want recursion, recursion available
questions = 1, answers = 2, authority records = 4, additional = 6
QUESTIONS:
ftp.es.net, type = A, class = IN
ANSWERS:
-> ftp.es.net
canonical name = nic3.es.net
ttl = 3556 (59 mins 16 secs)
-> nic3.es.net
internet address = 198.128.3.83
ttl = 3600 (1 hour)
AUTHORITY RECORDS:
-> ES.net
nameserver = NS1.es.net
ttl = 168588 (1 day 22 hours 49 mins 48 secs)
-> ES.net
nameserver = DNS-EAST.es.net
ttl = 168588 (1 day 22 hours 49 mins 48 secs)
-> ES.net
nameserver = DNS-WEST.NERSC.GOV
ttl = 168588 (1 day 22 hours 49 mins 48 secs)
-> ES.net
nameserver = IGW.NERSC.GOV
ttl = 168588 (1 day 22 hours 49 mins 48 secs)
ADDITIONAL RECORDS:
-> NS1.es.net
internet address = 198.128.2.10
ttl = 168588 (1 day 22 hours 49 mins 48 secs)
-> DNS-EAST.es.net
internet address = 134.55.6.130
ttl = 168588 (1 day 22 hours 49 mins 48 secs)
-> DNS-WEST.NERSC.GOV
internet address = 128.55.128.191
ttl = 168588 (1 day 22 hours 49 mins 48 secs)
-> DNS-WEST.NERSC.GOV
internet address = 128.55.32.12
ttl = 168588 (1 day 22 hours 49 mins 48 secs)
-> IGW.NERSC.GOV
internet address = 128.55.32.9
ttl = 3600 (1 hour)
-> IGW.NERSC.GOV
internet address = 128.55.128.151
ttl = 3600 (1 hour)
------------
Non-authoritative answer:
Name: nic3.es.net
Address: 198.128.3.83
Aliases: ftp.es.net
3. recursion
- note that the first name server made multiple requests
- the others simply referred the first server to another machine
- the local server was responding to a 'recursive query'
- this is because the local 'dumb' resolver is not smart enough to
follow any referrals
- a recursive query places most of the work on a single name server
- when a recusive query is made the name server is obliged to go find the
answer or return an error message
4. iteration
- iterative resolution is the method used when a name server
receives an 'iterative query'
- in this case, if the server does not have the authoritative
answer it returns:
> a cached answer (we'll talk about this in a minute)
> the names and addresses of several servers that are closer
to the answer
** Figure 2.11
nslookup
> server ns1.es.net
> ftp.es.net
Server: ns1.es.net
Address: 198.128.2.10
Name: nic3.es.net
Address: 198.128.3.83
Aliases: ftp.es.net
M. Mapping addresses to names
1. what if you have an IP number and want to find the host name?
- this is usefule to make output more readable
- used for some security checks
2. this was easy with the old /etc/hosts tables
3. the DNS data is indexed by name
- could do an exhuastive search
4. the clever solution
- create a part of the domain name space that uses
addresses as names
- this is: in-addr.arpa
** Figure 2.12
- for example type:
nslookup
>set type=any
>10.121.186.128.in-addr.arpa
returns:
nu.cs.fsu.edu
As we saw earlier, though, newer "nslookup" versions will automatically
build the reverse name lookup (like on Linux):
nslookup
> 128.186.121.10
Server: dns.scri.fsu.edu
Address: 144.174.128.17
Name: nu.cs.fsu.edu
Address: 128.186.121.10
N. Caching
1. each time a local name server processes a recursive query
it learns a lot of information
2. this is cached which speeds up successive queries
** Figure 2.14
3. example:
- say our server has already looked up the address of
eecs.berkeley.edu
- this means it has cached the name servers for both:
eecs.berkeley.edu
berkeley.edu
- if we now make a query for baobab.cs.berkeley.edu
the local server can skip the root-level query and
go right to berkeley.edu
4. time to live (TTL)
- TTL is the amount of time that information is cached
before it is discarded
- the trade-off is between consistency and performance