1
The name and address space
Network Training WorkshopSan Jose, 13-20 June 1999
Track1: Host Based Internetworking
2
What is a domain name ?
Networking language
137.138.131.253hard to remember
Network Training Workshop
www.isoc.orgIntermediate language not easy to find
still difficult to rememberDNS: Domain Name Service
easy to find
3
What is DNS?
Domain Name Service Internet protocol Distributed data base system for objects in a computer
network Net-wide Database It Maps
name to address address to name
Set of tools to manage the service
4
IP addresses and names
Disadvantages of an address It is too long (192.162.16.201) It has no representation for the organization It is difficult to remember IP addresses can change
Names Human users prefer to use names instead of numbers A name can be associated to any device that has an IP
address Advantages of names
They make reference to an organization name: *.ibm.com They are easy to remember
5
Why use hierarchical names?
Internet hosts and other resources need globally unique names
Difficult to keep unstructured names unique would require a single list of all names
in useHierarchical names are much easier
to make unique
6
Domain Name Hierarchy
.
edu
Root domain
com gov mil net org ro fr at . . . . . . jp
ici rnc ase pub utt vsat
roearn ns std cs lmn dsp
ulise paul
ac co gv or . . .
uni-linz tuwien . . . . . . .
eunet
cc
univie
mat exp itc . . . . . .
phytia alpha chris
Top-Level-Domains
Second Level Domains
7
Domain name structures
The root of system is unnamed Top level domains are:
Within the United States of Americaedu - educational institutionscom - commercial institutionsnet - network support organizationsgov - government agenciesmil - military organizationsorg - organizations that don’t fit in any of the above
country code according to ISO-3316 (uk, de, pl, sk, mk, ...) Flat structure Based on generic categories (ac, co, gv, org, net, etc.) Based on geographical locations (waw.pl, poznan.pl, etc.)
8
ICANN
ICANN
Internet Corporation for Assigned Names and Numbers
ICANN is the overall authority for:
IP addresses; top-level-domains;
9
Domain Name Servers
Usually host names are used in network commands (telnet, ftp, mail, gopher clients, www clients, etc. );
Network connections always take place based on the IP address; The system always converts host names to addresses before the
network connection is made; How to associate names with addresses?
Host table Domain Name Server
10
Host Table
host table is a simple text file (/etc/hosts) which contains associations between an IP address and a list of host names.
Example: 130.65.240.254 carlos.t1.isocws.isoc.org
carlos Disadvantage
The table should contain all the Internet hosts ( x 1,000,000 entries);
The table is very large; Difficult to update when new hosts are connected to
Internet or IP addresses are changed
11
Domain Name Server (DNS)
Overcomes the weaknesses of the host table
It is a distributed database system
Host names are automatically disseminated to those who ask to have access
12
BIND - UNIX Name Service
In UNIX, DNS is implemented by the Berkeley Internet Name Domain (BIND) software;
It is a client/server software system; The client is called “resolver”; it generates queries for domain
name information that are sent to the server; The server is a daemon called named. The domain name is actually an argument used by the local
resolver; The resolver is responsible for hiding the distribution of data
among nameservers from the user; The resolver is part of the operating system.
13
DNS Operation
To establish a connection to a remote host, a user program sends a query to the resolover, asking the IP number of a specified hostname;
The resolver starts with knowledge of at least one nameserver; The resolver processes a user query by asking a known nameserver
for the information (IP for the remote host); If the nameserver has the answer, it will give it. The local nameservers knows the addresses of other nameservers; If the remote host is in a zone for which the local nameserver is not
authoritative, queries will be sent to other nameservers, starting with root servers to find out the delegated nameserver for the top-level-domain, the subdomain and so on;
14
DNS Operation (continue)
To minimize the answer time for the next query to the same host name, the nameserver caches the information obtained by a query; the information cached is valid a certain time period as defined when that zone was configured.
Finally, the nameserver delegated for the zone containing the remote host is reached and the IP addresses is obtained and returned to the host which initiated the first query;
15
The resolver/server model (diagram)
AuthoritativeNameserver
Recursive Nameserver
CACHE
Resolver Resolver
16
Resolver Configuration
Configuration file name /etc/resolv.conf.
Designates the name servers on the network that should be sent queries.
The resolver will try to contact a name server The resolv.conf file contains directives
; comment # another comment search search-list nameserver server-address ; multiple
lines
17
Example - Resolver Configuration
search t1.isocws.isoc.org
nameserver 130.65.37.128
nameserver 130.65.37.215
18
Checking DNS using nslookup
nslookup commands:server <nameserver> ; set the server to be queriedset type = NS ;queries NS resourcesset type = SOA ;queries SOA resourcesset type = A ;queries A resourcesset type = MX ;queries MX resourcesset type = CNAME ;queries CNAME resourcesset type = PTR ;queries PTR resourcesset type = ANY ;queries ANY resourcesls <domain> ;lists the <domain> zonels <domain> > <file-name> ;gets the zone <domain> into the
file<file-name>
19
Checking DNS using dig
Dig Tool to manage DNS settings Syntax is:
dig [domain] @nameserver [query-type]
20
Exercise 1. Checking DNS using nslookup
nslookup commands:server <nameserver> ; set the server to be queriedset type = NS ;queries NS resourcesset type = SOA ;queries SOA resourcesset type = A ;queries A resourcesset type = MX ;queries MX resourcesset type = CNAME ;queries CNAME resourcesset type = PTR ;queries PTR resourcesset type = ANY ;queries ANY resources
ls <domain> ;lists the <domain> zonels <domain> > <file-name> ;gets the zone <domain> into <file-name>view <file-name> ;views the <file-name> file
#nslookup>set type=any>www.t1.isocws.isoc.org.>org.>ls isoc.org > file1>view file1
You can check other domains, known to you.
21
Exercise 2. Checking DNS using dig
dig command:# dig [zone] @nameserver [query-type]
Exercises # dig @ns.t1.isocws.isoc.org t1.isocws.isoc.org A # dig @ns.t1.isocws.isoc.org t1.isocws.isoc.org NS # dig @ns.t1.isocws.isoc.org isocws.isoc.org MX
What information does give you?
You can check other domains, known to you.
22
Remarks
DNS is essential for a proper operation of the network; If DNS fails, connections to any remote host are no more
possible; To allow network operation in case of failures of a nameserver,
each nameserver (called primary nameserver should have at least another nameserver (called secondary nameserver) which holds a copy of the zone file;
The secondary nameserver periodically queries the primary nameserver whether the zone was changed. In case of the change, it is transferred into its cache and also stored in a local file, which will be used next time at starting up as initial data;
When data about a new host should be added to the DNS database, or a change of address/name should be done, the entry is done only on the primary nameserver; the secondary nameserver will get it automatically.
23
Reverse Lookup
When a source host establishes a connection to a destination host, the TCP/IP packets carry out only IP addresses of the source host;
For authentication, access rights or accounting information, the destination host wants to know the name of the source host;
For this purpose, a special domain “in-addr.arpa” is used; The reverse name is obtained by reversing the IP number and
adding the name “in-addr.arpa”; Example: address: 130.65.240.254
reverse name: 254.240.65.130.in-addr.arpa Reverse domains form a hierarchical tree and are treated as any
other Internet domain.
24
Reverse Domain Hierarchy
187 188 189 190 191 192 193 194 195 . . . . . .
157 158 159 160 165 166 167 168 . . .
.in-addr
.arpa
162161 163 164
161514 1312 17 18 19 20 21
31 2 4 5
25
Hierarchy of nameservers
Root nameserver - delegates nameservers for top-level-domains;
Nameserver for each top level domain - holds information about the top level domain, delegates nameservers for subdomains;
Nameserver for each subdomainNameserver for each reverse domain
26
Requirements for a nameserver
A query should be resolved as fast as possible; It should be available 24 hours a day; It should be reachable via fast communication lines; It should be located in the central in the network
topology; It should run robust, without errors and interrupts.
27
Type of servers
DNS server primary for some zones secondary for others,
Caching Only Server All servers & not authoritative for any zone
Forwarding Server always forwards queries it cannot satisfy from its cache, to a
fixed list of forwarding servers; the queries to the forwarding servers are recursive queries.
28
Files
Name server uses several files to load its data base This section covers the files and their formats
needed for named Boot File
file - first read when named starts up tells the server what type of server it is, which
zones it has authority over and where to get its initial data.
The default location /etc/named.confCan be changed by setting the BOOTFILE variable when
you compile named or by specifying the location on the command line when named is started up.
29
Domain data files
Two standard files for specifying the data for a domain mapping addresses in names mapping name in addresses
These files use the Standard Resource Record Format (RR)
Note: The file names are arbitrary; many network administrators
prefer to name their zone files after the domains they contain, especially in the average case which is where a given server is primary and/or secondary for many different zones.
30
Forward mapping File
This file contains all the data about the machines in this zone.
The location of this file is specified in the boot file.
Declare in named.conf file
31
Reverse mapping File
This file specifies the IN-ADDR.ARPA domainDeclared in named.conf fileThis is a special domain for allowing address
to name mapping. Special domain formed to allow inverse
mappingThe IN-ADDR.ARPA domain has four labels
preceding it. (4 octets of an Internet address) E.g. Internet address 128.32.0.4 domain 4.0.32.128.IN-ADDR.ARPA.
32
Standard Resource Record Format (RR)
The records in the name server data files are called resource records. The Standard Resource Record Format (RR) is specified in RFC1035.
General description of these records: {name} {ttl} addr-class Record Type Record
Specific data the name may be left blank The second field is an optional time to live field. (default
ttl is specified in the Start Of Authority resource record). The third field is the address class currently, only one
class is supported: IN
33
IP address for a host
A recordOwner is host nameData is IP address
; IP address of infoserver.xyz.comwww.dns.net. 86400 IN A
196.34.13.9
34
Information needed by the DNS infrastructure itself
SOA record Each zone has exactly one SOA record
NS records Each zone has several nameservers that
are listed as having authoritative information about domains in the zone
One NS record for each such nameserver
35
SOA record
Every zone has exactly one SOA record
The domain name at the top of the zone owns the SOA record
Data portion of SOA record contains: MNAME - name of master nameserver RNAME - email address of zone administrator The Five Magic Numbers
SERIAL - serial numberREFRESH RETRY EXPIRE MINIMUM - timing parameters
36
NS record
Each zone has several listed nameserversOne NS record for each listed nameserver
primary and secondarythe data portion of each NS record contains
the domain name of a nameserverDoes not contain IP address
Get that from an A record for the nameserver
37
SOA and NS record example
; owner TTL class type dataxyz.com. 86400 IN SOA ns1.xyz.com. fred.xyz.com. ( 199906141 ; serial 21600 ; refresh 3600 ; retry 2600000 ; expire 900 ) ; minimumxyz.com. 86400 IN NS ns1.xyz.com.xyz.com. 86400 IN NS ns2.xyz.com.xyz.com. 86400 IN NS server.where.example.
38
SOA and NS example using some shortcuts
$ORIGIN xyz.com.$TTL 86400; owner TTL class type data@ IN SOA ns1.xyz.com. fred.xyz.com. ( 199710161 ; serial 21600 ; refresh 3600 ; retry 2600000 ; expire 900 ) ; minimum NS ns1 NS ns2 NS server.where.example.
39
More RRs above and below zone cuts
RRs in the child zone (below the cut) SOA and NS records (authoritative)
RRs in the parent zone (above the cut) NS records (should be identical to those
in the child zone)glue records
the child zone’s nameservers sometimes need A records in the parent zone
40
Zone cut example - RRs in the child zone
parent is COM zone; child is XYZ.COM zone child zone has SOA and NS records, and A
records for hostsxyz.com. IN SOA xxx xxx xxx xxx xxx xxx xxx NS ns1.xyz.com. NS another.where.edu. ns1.xyz.com. A 193.226.0.3; the xyz.com zone does not have an A record; for another.where.edu.
41
Zone cut example - RRs in the parent zone
parent is COM zone; child is XYZ.COM zone parent zone has its own SOA and NS records, plus
copies of child zone’s NS records, plus glue recordsCOM. IN SOA xxx xxx xxx xxx xxx xxx xxx
NS xxxxxxx
NS yyyyyyy
xyz.com. NS ns1.xyz.com.
NS another.where.ro.
ns1.xyz.com. A 192.0.2.3
; the com zone does not have an A record
; for another.elsewhere.edu.
42
Hostname for an IP address
PTR record Owner is IP address, mapped into the in-
addr.arpa domain Data is name of host with that IP address
; host name for IP address 192.0.3.23.0.226.193.in-addr.arpa. PTR
infoserver.ici.ro.
43
Information about mail routing
MX record Owner is name of email domain Data contains preference value, and name
of host that receives incoming email
; send xyz.com’s email to mailserver or backupserver
xyz.com. MX 0 mail.xyz.ro.
xyz.com. MX 10 backup.xyz.ro.
44
Free form text
TXT record Owner is any domain name Data is any text associated with the
domain name Very few conventions about how to use it
.ro. TXT “George Macri - Tech-contact”
45
Alias to canonical name mapping
CNAME record Owner is non-canonical domain name (alias) Data is canonical domain name
; ftp.rnc.ro is an alias; info.ici.ro is the canonical nameftp.rnc.ro. CNAME info.ici.ro.
46
BIND 8 Highlights
DNS Dynamic Updates (RFC 2136) DNS Change Notification (RFC 1996) Completely new configuration syntax Flexible, categorized logging system IP-address-based access control for queries, zone
transfers, and updates that may be specified on a zone-by-zone basis
More efficient zone transfers Improved performance for servers with thousands of
zones The server no longer forks for outbound zone transfers Many bug fixes
47
Statements
A BIND 8 configuration consists of statements and comments. Statements end with a semicolon.
Statements supported: acl
defines a named IP address matching list, for access control and other uses include
includes a file key
specifies key information for use in authentication and authorization logging
specifies what the server logs, and where the log messages are sent options
controls global server configuration options and sets defaults for other statements
server zone
48
Example Config File
/* A simple BIND 8 configuration */options { directory "/var/named";};
};zone ”rnc.ro" in { type master; file "master/isc.org";};
49
Example Config File (cont.)
zone ”ici.ro" in { type slave; file "slave/vix.com"; masters { 192.162.16.31; };};zone "." in { type hint; file "named.cache";};zone "0.0.127.in-addr.arpa" in { type master; file "master/127.0.0";};
50
Domain Management
Starting namedNamed signaling processingDebugging DNS
Toolsnslookupdighost
51
Questions
52
REFERENCES
Christian Huitema, Routing in the Internet, Prentice Hall, ISBN 013-132192-7, 1996
Kevin Dowd, “Getting Connected, Internet at 56K and Up”, O’Reilly & Associates, Inc., Bonn, 1996
Network Technology Workshop, Tracks, 1996-1998 Craig Hunt, “TCP/IP Network Administration, O’Reilly & Associates,
Inc., Sebastopol, 1993 Internetworking Technology Overview, Cisco Systems, Inc., 1993 Book text of the 4th Network Seminar and Intensive Course for
Scientists and Network Managers from Central Europe, Feb. 1993, Vienna University Computer Center
E. Comer, “Internetworking with TCP/IP”, Vol I, Principles, Protocols and Architecture, Prentice Hall, Englewood Cliffs, New Jersey, 1991.
William Stallings, Data and Computer Communications, Macmillan Publishing Company, New York, 1985.
Top Related