Transcript csci5211: Computer Networks and Data Communications

Internet Engineering Course
DHCP, DNS
Introduction

Client administration:
◦ IP address management:
 They need to ease the process of joining the network and
they do not want users to do any special configuration
 They want to network boot their workstations
 i.e. Diskless workstations or remote OS installation (acquiring the
network setting during boot process)
 Solution: Deploy a DHCP server
◦ Machine names management:
 They need to be able to name machines and access them
by names instead of IP addresses
 Solution: Deploy A DNS server

So we discuss on DHCP and DNS in this
session
DHCP
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Provides configuration parameters specific to the DHCP client host to
participate on an IP network
Methods of IP allocation
◦ Manual
 Only requesting clients with a MAC address listed in the table (MAC-IP
pairs) get the IP address according to the table
◦ Automatic
 DHCP server permanently assigns to a requesting client a free IPaddress from a range given by the administrator
◦ Dynamic
 The only method which provides dynamic re-use of IP addresses
 The request-and-grant process uses a lease concept with a controllable
time period.
DHCP cont.
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DHCP server can provide optional configuration
◦ e.g. Subnet Mask, Name Server, …
◦ RFC 2132 defines DHCP options Usage
DHCP relay agent (mostly in network routers/high-end switches)
◦ Relays DHCP Discover broadcasts from a LAN without DHCP
to a network which has one
Usage
◦ Cable Internet providers
◦ DSL providers
◦ broadband ISP networks
◦ Office networks
◦ public internet access
◦ Places where there are mobile nodes that want to access the
net
DHCP Implementations
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Microsoft introduced DHCP on their servers with Windows NT
3.5 in late 1994
◦ DHCP did not originate from Microsoft
Internet Software Consortium published DHCP for Unix variants
◦ Version 1.0.0 released on December 6, 1997
◦ Version 2.0 on June, 1999 – A more RFC-compliant one
Novell included a DHCP server in NetWare OS since v. 5, 1999
◦ It integrates with Novell eDirectory
Cisco since Cisco IOS 12.0 in February 1999
Sun added DHCP support in Solaris 8, July 2001
BOOTP
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BOOTstrap Protocol (RFC 951)
UDP
Used to obtain IP address automatically
◦ Usually in booting process of computers or OSs
 Diskless workstations

Historically used for UNIX-like diskless workstations
◦ Also obtains the locations of the boot image
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Also can be used for installing a pre-configured OS
Protocol became embedded in the BIOS of some
NICs
◦ Allowing direct network booting without need for a floppy
BOOTP cont.
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Recently is used for booting a Windows OS
in diskless standalone media center PCs
DHCP is a more advanced protocol base on
BOOTP
◦ Far more complex to implement than BOOTP
◦ Most DHCP servers also offer BOOTP support
◦ Duration based leases is the fundamental addition
in DHCP
 Dynamic in DHCP is for this
DHCP Anatomy
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Uses the same IANA assigned ports as BOOTP
◦ 67/udp for the server, 68/udp for the client
DHCP Messages
◦ Discover
 Client broadcasts on the local physical subnet to find servers
 UDP packet (broadcast dest. 255.255.255.255)
 Also request last-known IP address (optional parameter)
◦ Offer
 Server determines the configuration based on the client’s MAC addr.
 Server specifies the IP address and put optional parameters
◦ Request
 Client selects a configuration out the DHCP Offer packet and
broadcasts it again
◦ Acknowledge
 Server acknowledges the request and sends the ack to the client
DHCP Anatomy
DHCP Anatomy cont.
◦ Inform
 Client requests more information than the server
sent with the DHCPACK, or to repeat data for a
particular application (e.g. to obtain web proxy
settings by a browser)
◦ Release
 Client requests the server to release the DHCP
and the client unconfigures its IP address
 Sending this message is not mandatory (unplug or
…)
RARP
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ARP
◦ Address Resolution Protocol
◦ Resolve a hardware address from a given IP address
◦ Try arp command in both Windows and Linux
RARP
◦ Reverse Address Resolution Protocol (RFC 903)
◦ Complement of ARP
◦ Resolve an IP address from a given hardware address
◦ Needs manual configuration on a central server
 Not scalable
◦ Obsolete by BOOTP and the more modern DHCP
◦ Try rarp command in Linux (if supported by Kernel), and RARP
daemon - RARPd
DNS
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Domain Name System (RFC 1034, 1035)
◦ RFC 1034 and 1035 made RFC 882, 883 obsolete
A system that stores info associated with
domain names in a distributed database on
networks (such as Internet)
 Many types of information for the domain
are provided by DNS
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◦ Most important, IP address associated with
domain name
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Mainly UDP
◦ TCP only when response data size exceeds 512
bytes or for things like zone transfer
DNS is Decentralized
No single point of failure
 Less traffic volume
 Easier maintenance
 Scalable
 Less distant (delay) issues
 Delegation
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Resolvers
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Clients that access name servers
◦ Querying a name server
◦ Interpreting responses
◦ Returning the information to the programs that
requested it
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In BIND, the resolver is just a set of library
routines that is linked into programs
◦ Not even a separate process
◦ Most of the overhead of finding an answer to the
query is placed on the name server
◦ The DNS specs call this kind of resolver a stub
resolver
Types of DNS Servers
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Primary master
◦ Reads the data for the zone from a file on its host
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Secondary master (Slave)
◦ Gets the zone data from another ns that is authoritative for the
zone (master server)
◦ Often, master server is the zone’s primary master
 Not always the case
 Secondary master may get the info from another secondary server
◦ Zone transfer
 Contacting master ns and if necessary pulling the zone data
◦ Redundancy
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Both
◦ An authoritative ns may be master for some of its zones and be
slave for some others
◦ It’s imprecise to call an ns, master or slave!
DNS Applications
Attach IP addresses to domain names
(ease of use)
 Many to many mapping
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◦ Virtual Hosting
Sender Policy Framework
 Makes it possible for people to assign
authoritative names, without needing to
communicate with a central registrar
 Load balancing between hosts
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DNS History
Idea in ARPAnet
 Originally, each computer retrieved a file called
HOSTS.TXT which contained the mappings
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◦ Hosts file exists today (Looked up before querying
DNS)
 /etc/hosts, C:\WINDOWS\system32\drivers\etc\hosts
◦ Limitations
 Not scalable
 Each time a given computer’s address changed, all computers
should update their Hosts file
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DNS invented by Paul Mockapetris in 1983
◦ First implementation was called JEEVES by himself
Parts of a Domain Name
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Domain name consists of two or more parts
separated by dots (eng.ui.ac.ir for example)
◦ Rightmost label: Top-level domain (ir)
◦ Each label to the left specifies a subdomain of the
domain above it.
 ac is a subdomain of the ir domain
 ui is a subdomain of the ac.ir domain
 Theoretical limits: 127 level, each level 63 chars, total
domain name 255 chars
◦ A domain name with one or more IP addresses is
called a hostname (eng.ui.ac.ir, ui.ac.ir but not
ac.ir)
A Distributed Hierarchical Database
Root Servers (13 root
servers worldwide)
 TLD Servers (.com,
.org, .net, .uk, .ir, …)
 Authoritative DNS
Servers (organization’s
DNS server)
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Distributed, Hierarchical Database
Root DNS Servers
com DNS servers
ir DNS servers
TLD
edu DNS servers Servers
co.ir
poly.edu
ac.ir
umass.edu
yahoo.com amazon.com
DNS servers DNS servers DNS servers DNS servers DNS servers DNS servers
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Root servers and TLD servers typically do not contain
hostname to IP mappings; they contain mappings for
locating authoritative servers.
Local DNS Server
Does not belong to hierarchy
 Also called default name server
 Acts as a proxy (forwarder), forwards
query into hierarchy
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◦ Caches the results if of interest
DNS Queries
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Recursive
◦ Contacted name server should recurs and find
the mapping for the requesting host
◦ Heavy load on the servers
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Iterative
◦ Contacted server replies with the name of the
server to contact
◦ An ns provides the name of the next ns
 Bootstrapping problem (another query is required and
…)
 So the IP of the next ns is provided
 Glue record
DNS Queries
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Recursive query
example
root DNS server
2
3
7
6
TLD DNS server
local DNS server
Ns.ui.ac.ir
1
5
4
8
authoritative DNS server
dns.cs.umass.edu
requesting host
gaia.cs.umass.edu
DNS Queries
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Iterative query
example
root DNS server
2
3
TLD DNS server
4
5
local DNS server
ns.ui.ac.ir
1
8
7
6
authoritative DNS server
dns.cs.umass.edu
requesting host
gaia.cs.umass.edu
DNS Caching and Updating Records
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Once a name server learns mapping, it caches it
◦ It’ll expire (TTL defined by the authoritative
server)
◦ TLD servers typically cached in local name
server
 Root name servers not often visited
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Update/Notify Mechanisms
◦ RFC 2136
DNS records
DNS: distributed db storing resource records (RR)
RR format: (name,
 Type=A
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name is hostname
value is IP address
Type=NS
value, type, ttl)
 Type=CNAME

name is alias name for some
“canonical” (the real) name
www.ibm.com is really
servereast.backup2.ibm.com
 value is canonical name
◦ name is domain (e.g. foo.com)
◦ value is IP address of
authoritative name server for  Type=MX
this domain
 value is name of mail server
associated with name
Legal Users of Domains
Registrant
 Administrative contact
 Technical contact
 Billing contact
 Name servers
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Try whois in Linux and see these
information for different hosts
DNS Software
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BIND (Berkeley Internet Name Daemon) – full
featured, most popular, de facto Internet standard
Djbdns (Daniel J. Bernstein's DNS) – composed of
several small-footprint components
MaraDNS – UDP only
VitalQIP (Lucent Technologies)
Adonis DNS Management Appliance (BlueCat
Networks Inc)
NSD (Name Server Daemon) – small footprint, UDP
only, authoritative only
PowerDNS
Microsoft DNS (in the server editions of Windows
2000 and Windows 2003)
References
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Wikipedia, the free encyclopedia
◦ http://en.wikipedia.org/wiki/Domain_Name_Syste
m
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Computer Networking: A Top Down
Approach Featuring the Internet, 3rd edition,
Jim Kurose, Keith Ross, Addison-Wesley, July
2004
DNS and BIND, 3rd edition, Cricket Liu, Paul
Albitz, O’Reilly, September 1998
BIND9 Administrator Reference Manual